Commit | Line | Data |
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f22eee08 | 1 | \input texinfo |
c8072296 | 2 | @setfilename ld.info |
b4d4e8e3 | 3 | @syncodeindex ky cp |
7f9ae73e | 4 | @include configdoc.texi |
8de26d62 | 5 | @c (configdoc.texi is generated by the Makefile) |
3964a18a | 6 | @include ldver.texi |
ec40bbb8 DM |
7 | |
8 | @c @smallbook | |
1c48127e RP |
9 | |
10 | @ifinfo | |
11 | @format | |
12 | START-INFO-DIR-ENTRY | |
f9d3d71a | 13 | * Ld: (ld). The GNU linker. |
1c48127e RP |
14 | END-INFO-DIR-ENTRY |
15 | @end format | |
16 | @end ifinfo | |
17 | ||
b4d4e8e3 | 18 | @ifinfo |
3964a18a | 19 | This file documents the @sc{gnu} linker LD version @value{VERSION}. |
b4d4e8e3 | 20 | |
f4175166 | 21 | Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 1998 Free Software Foundation, Inc. |
b4d4e8e3 RP |
22 | |
23 | Permission is granted to make and distribute verbatim copies of | |
24 | this manual provided the copyright notice and this permission notice | |
25 | are preserved on all copies. | |
26 | ||
d76ae847 RP |
27 | Permission is granted to copy and distribute modified versions of this |
28 | manual under the conditions for verbatim copying, provided also that | |
29 | the entire resulting derived work is distributed under the terms of a | |
30 | permission notice identical to this one. | |
31 | ||
32 | Permission is granted to copy and distribute translations of this manual | |
33 | into another language, under the above conditions for modified versions. | |
34 | ||
b4d4e8e3 RP |
35 | @ignore |
36 | Permission is granted to process this file through Tex and print the | |
37 | results, provided the printed document carries copying permission | |
38 | notice identical to this one except for the removal of this paragraph | |
39 | (this paragraph not being relevant to the printed manual). | |
40 | ||
41 | @end ignore | |
b4d4e8e3 | 42 | @end ifinfo |
2c5c0674 RP |
43 | @iftex |
44 | @finalout | |
b4d4e8e3 | 45 | @setchapternewpage odd |
246504a5 | 46 | @settitle Using LD, the GNU linker |
f22eee08 | 47 | @titlepage |
246504a5 | 48 | @title Using ld |
c8072296 | 49 | @subtitle The GNU linker |
f22eee08 | 50 | @sp 1 |
cb70c872 | 51 | @subtitle @code{ld} version 2 |
3964a18a | 52 | @subtitle Version @value{VERSION} |
c653b370 | 53 | @author Steve Chamberlain |
ed1cc83d ILT |
54 | @author Ian Lance Taylor |
55 | @author Cygnus Solutions | |
b4d4e8e3 RP |
56 | @page |
57 | ||
58 | @tex | |
b4d4e8e3 | 59 | {\parskip=0pt |
ed1cc83d ILT |
60 | \hfill Cygnus Solutions\par |
61 | \hfill ian\@cygnus.com, doc\@cygnus.com\par | |
ec40bbb8 | 62 | \hfill {\it Using LD, the GNU linker}\par |
d4e5e3c3 | 63 | \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par |
b4d4e8e3 RP |
64 | } |
65 | \global\parindent=0pt % Steve likes it this way. | |
66 | @end tex | |
67 | ||
f22eee08 | 68 | @vskip 0pt plus 1filll |
f4175166 | 69 | Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 1998 Free Software Foundation, Inc. |
f22eee08 RP |
70 | |
71 | Permission is granted to make and distribute verbatim copies of | |
72 | this manual provided the copyright notice and this permission notice | |
73 | are preserved on all copies. | |
74 | ||
75 | Permission is granted to copy and distribute modified versions of this | |
76 | manual under the conditions for verbatim copying, provided also that | |
77 | the entire resulting derived work is distributed under the terms of a | |
78 | permission notice identical to this one. | |
79 | ||
80 | Permission is granted to copy and distribute translations of this manual | |
81 | into another language, under the above conditions for modified versions. | |
f22eee08 | 82 | @end titlepage |
2c5c0674 | 83 | @end iftex |
b4d4e8e3 | 84 | @c FIXME: Talk about importance of *order* of args, cmds to linker! |
f22eee08 | 85 | |
f22eee08 | 86 | @ifinfo |
ec40bbb8 DM |
87 | @node Top |
88 | @top Using ld | |
3964a18a | 89 | This file documents the @sc{gnu} linker ld version @value{VERSION}. |
f22eee08 | 90 | |
2c5c0674 | 91 | @menu |
2d59b2c3 RP |
92 | * Overview:: Overview |
93 | * Invocation:: Invocation | |
af54556a | 94 | * Scripts:: Linker Scripts |
ec40bbb8 | 95 | @ifset GENERIC |
2d59b2c3 | 96 | * Machine Dependent:: Machine Dependent Features |
ec40bbb8 DM |
97 | @end ifset |
98 | @ifclear GENERIC | |
99 | @ifset H8300 | |
100 | * H8/300:: ld and the H8/300 | |
101 | @end ifset | |
f9d3d71a ILT |
102 | @ifset Hitachi |
103 | * Hitachi:: ld and other Hitachi micros | |
104 | @end ifset | |
ec40bbb8 DM |
105 | @ifset I960 |
106 | * i960:: ld and the Intel 960 family | |
107 | @end ifset | |
108 | @end ifclear | |
109 | @ifclear SingleFormat | |
2d59b2c3 | 110 | * BFD:: BFD |
ec40bbb8 DM |
111 | @end ifclear |
112 | @c Following blank line required for remaining bug in makeinfo conds/menus | |
113 | ||
9fde46a4 | 114 | * Reporting Bugs:: Reporting Bugs |
2d59b2c3 RP |
115 | * MRI:: MRI Compatible Script Files |
116 | * Index:: Index | |
2c5c0674 | 117 | @end menu |
ec40bbb8 | 118 | @end ifinfo |
2c5c0674 | 119 | |
ec40bbb8 | 120 | @node Overview |
f22eee08 RP |
121 | @chapter Overview |
122 | ||
c653b370 | 123 | @cindex @sc{gnu} linker |
2c5c0674 | 124 | @cindex what is this? |
246504a5 | 125 | @code{ld} combines a number of object and archive files, relocates |
ec40bbb8 DM |
126 | their data and ties up symbol references. Usually the last step in |
127 | compiling a program is to run @code{ld}. | |
f22eee08 | 128 | |
246504a5 | 129 | @code{ld} accepts Linker Command Language files written in |
2c5c0674 | 130 | a superset of AT&T's Link Editor Command Language syntax, |
b4d4e8e3 | 131 | to provide explicit and total control over the linking process. |
f22eee08 | 132 | |
ec40bbb8 | 133 | @ifclear SingleFormat |
246504a5 RP |
134 | This version of @code{ld} uses the general purpose BFD libraries |
135 | to operate on object files. This allows @code{ld} to read, combine, and | |
b4d4e8e3 RP |
136 | write object files in many different formats---for example, COFF or |
137 | @code{a.out}. Different formats may be linked together to produce any | |
d4e5e3c3 | 138 | available kind of object file. @xref{BFD}, for more information. |
ec40bbb8 | 139 | @end ifclear |
f22eee08 | 140 | |
c653b370 | 141 | Aside from its flexibility, the @sc{gnu} linker is more helpful than other |
2c5c0674 RP |
142 | linkers in providing diagnostic information. Many linkers abandon |
143 | execution immediately upon encountering an error; whenever possible, | |
246504a5 | 144 | @code{ld} continues executing, allowing you to identify other errors |
2c5c0674 RP |
145 | (or, in some cases, to get an output file in spite of the error). |
146 | ||
ec40bbb8 | 147 | @node Invocation |
2c5c0674 RP |
148 | @chapter Invocation |
149 | ||
c653b370 | 150 | The @sc{gnu} linker @code{ld} is meant to cover a broad range of situations, |
2c5c0674 | 151 | and to be as compatible as possible with other linkers. As a result, |
ec40bbb8 | 152 | you have many choices to control its behavior. |
2c5c0674 | 153 | |
ec40bbb8 | 154 | @ifset UsesEnvVars |
2c5c0674 | 155 | @menu |
2d59b2c3 RP |
156 | * Options:: Command Line Options |
157 | * Environment:: Environment Variables | |
2c5c0674 | 158 | @end menu |
f22eee08 | 159 | |
ec40bbb8 | 160 | @node Options |
2c5c0674 | 161 | @section Command Line Options |
ec40bbb8 | 162 | @end ifset |
2c5c0674 RP |
163 | |
164 | @cindex command line | |
165 | @cindex options | |
67afbcea DE |
166 | The linker supports a plethora of command-line options, but in actual |
167 | practice few of them are used in any particular context. | |
2c5c0674 | 168 | @cindex standard Unix system |
246504a5 | 169 | For instance, a frequent use of @code{ld} is to link standard Unix |
b4d4e8e3 RP |
170 | object files on a standard, supported Unix system. On such a system, to |
171 | link a file @code{hello.o}: | |
ec40bbb8 | 172 | |
c653b370 | 173 | @smallexample |
ec40bbb8 | 174 | ld -o @var{output} /lib/crt0.o hello.o -lc |
c653b370 | 175 | @end smallexample |
ec40bbb8 | 176 | |
d76ae847 | 177 | This tells @code{ld} to produce a file called @var{output} as the |
b4d4e8e3 | 178 | result of linking the file @code{/lib/crt0.o} with @code{hello.o} and |
ec40bbb8 DM |
179 | the library @code{libc.a}, which will come from the standard search |
180 | directories. (See the discussion of the @samp{-l} option below.) | |
f22eee08 | 181 | |
246504a5 | 182 | The command-line options to @code{ld} may be specified in any order, and |
67afbcea DE |
183 | may be repeated at will. Repeating most options with a different |
184 | argument will either have no further effect, or override prior | |
ec40bbb8 | 185 | occurrences (those further to the left on the command line) of that |
67afbcea DE |
186 | option. Options which may be meaningfully specified more than once are |
187 | noted in the descriptions below. | |
f22eee08 | 188 | |
2c5c0674 | 189 | @cindex object files |
67afbcea DE |
190 | Non-option arguments are objects files which are to be linked together. |
191 | They may follow, precede, or be mixed in with command-line options, | |
192 | except that an object file argument may not be placed between an option | |
193 | and its argument. | |
f22eee08 | 194 | |
7f9ae73e RP |
195 | Usually the linker is invoked with at least one object file, but you can |
196 | specify other forms of binary input files using @samp{-l}, @samp{-R}, | |
197 | and the script command language. If @emph{no} binary input files at all | |
198 | are specified, the linker does not produce any output, and issues the | |
199 | message @samp{No input files}. | |
2c5c0674 | 200 | |
0b3499f6 ILT |
201 | If the linker can not recognize the format of an object file, it will |
202 | assume that it is a linker script. A script specified in this way | |
203 | augments the main linker script used for the link (either the default | |
204 | linker script or the one specified by using @samp{-T}). This feature | |
205 | permits the linker to link against a file which appears to be an object | |
206 | or an archive, but actually merely defines some symbol values, or uses | |
cc28f8fb ILT |
207 | @code{INPUT} or @code{GROUP} to load other objects. Note that |
208 | specifying a script in this way should only be used to augment the main | |
209 | linker script; if you want to use some command that logically can only | |
210 | appear once, such as the @code{SECTIONS} or @code{MEMORY} command, you | |
211 | must replace the default linker script using the @samp{-T} option. | |
af54556a | 212 | @xref{Scripts}. |
0b3499f6 | 213 | |
d4e5e3c3 DM |
214 | For options whose names are a single letter, |
215 | option arguments must either follow the option letter without intervening | |
f22eee08 RP |
216 | whitespace, or be given as separate arguments immediately following the |
217 | option that requires them. | |
218 | ||
d4e5e3c3 DM |
219 | For options whose names are multiple letters, either one dash or two can |
220 | precede the option name; for example, @samp{--oformat} and | |
9fde46a4 | 221 | @samp{--oformat} are equivalent. Arguments to multiple-letter options |
d4e5e3c3 DM |
222 | must either be separated from the option name by an equals sign, or be |
223 | given as separate arguments immediately following the option that | |
224 | requires them. For example, @samp{--oformat srec} and | |
225 | @samp{--oformat=srec} are equivalent. Unique abbreviations of the names | |
226 | of multiple-letter options are accepted. | |
227 | ||
f22eee08 | 228 | @table @code |
67afbcea DE |
229 | @kindex -a@var{keyword} |
230 | @item -a@var{keyword} | |
231 | This option is supported for HP/UX compatibility. The @var{keyword} | |
232 | argument must be one of the strings @samp{archive}, @samp{shared}, or | |
233 | @samp{default}. @samp{-aarchive} is functionally equivalent to | |
234 | @samp{-Bstatic}, and the other two keywords are functionally equivalent | |
235 | to @samp{-Bdynamic}. This option may be used any number of times. | |
236 | ||
ec40bbb8 | 237 | @ifset I960 |
2c5c0674 RP |
238 | @cindex architectures |
239 | @kindex -A@var{arch} | |
b4d4e8e3 | 240 | @item -A@var{architecture} |
67afbcea DE |
241 | @kindex --architecture=@var{arch} |
242 | @itemx --architecture=@var{architecture} | |
246504a5 RP |
243 | In the current release of @code{ld}, this option is useful only for the |
244 | Intel 960 family of architectures. In that @code{ld} configuration, the | |
1c48127e RP |
245 | @var{architecture} argument identifies the particular architecture in |
246 | the 960 family, enabling some safeguards and modifying the | |
d76ae847 RP |
247 | archive-library search path. @xref{i960,,@code{ld} and the Intel 960 |
248 | family}, for details. | |
b4d4e8e3 | 249 | |
246504a5 | 250 | Future releases of @code{ld} may support similar functionality for |
b4d4e8e3 | 251 | other architecture families. |
ec40bbb8 | 252 | @end ifset |
b4d4e8e3 | 253 | |
ec40bbb8 | 254 | @ifclear SingleFormat |
2c5c0674 RP |
255 | @cindex binary input format |
256 | @kindex -b @var{format} | |
67afbcea | 257 | @kindex --format=@var{format} |
2c5c0674 | 258 | @cindex input format |
2c5c0674 | 259 | @cindex input format |
c653b370 | 260 | @item -b @var{input-format} |
67afbcea | 261 | @itemx --format=@var{input-format} |
1fb57a5d RP |
262 | @code{ld} may be configured to support more than one kind of object |
263 | file. If your @code{ld} is configured this way, you can use the | |
264 | @samp{-b} option to specify the binary format for input object files | |
265 | that follow this option on the command line. Even when @code{ld} is | |
266 | configured to support alternative object formats, you don't usually need | |
267 | to specify this, as @code{ld} should be configured to expect as a | |
268 | default input format the most usual format on each machine. | |
269 | @var{input-format} is a text string, the name of a particular format | |
270 | supported by the BFD libraries. (You can list the available binary | |
67afbcea | 271 | formats with @samp{objdump -i}.) |
1fb57a5d | 272 | @xref{BFD}. |
2c5c0674 RP |
273 | |
274 | You may want to use this option if you are linking files with an unusual | |
ec40bbb8 | 275 | binary format. You can also use @samp{-b} to switch formats explicitly (when |
2c5c0674 | 276 | linking object files of different formats), by including |
ec40bbb8 | 277 | @samp{-b @var{input-format}} before each group of object files in a |
2c5c0674 RP |
278 | particular format. |
279 | ||
280 | The default format is taken from the environment variable | |
ec40bbb8 DM |
281 | @code{GNUTARGET}. |
282 | @ifset UsesEnvVars | |
283 | @xref{Environment}. | |
284 | @end ifset | |
af54556a ILT |
285 | You can also define the input format from a script, using the command |
286 | @code{TARGET}; see @ref{Format Commands}. | |
ec40bbb8 | 287 | @end ifclear |
2c5c0674 | 288 | |
2d59b2c3 | 289 | @kindex -c @var{MRI-cmdfile} |
67afbcea | 290 | @kindex --mri-script=@var{MRI-cmdfile} |
2d59b2c3 RP |
291 | @cindex compatibility, MRI |
292 | @item -c @var{MRI-commandfile} | |
67afbcea | 293 | @itemx --mri-script=@var{MRI-commandfile} |
2d59b2c3 RP |
294 | For compatibility with linkers produced by MRI, @code{ld} accepts script |
295 | files written in an alternate, restricted command language, described in | |
d76ae847 | 296 | @ref{MRI,,MRI Compatible Script Files}. Introduce MRI script files with |
ec40bbb8 | 297 | the option @samp{-c}; use the @samp{-T} option to run linker |
d76ae847 | 298 | scripts written in the general-purpose @code{ld} scripting language. |
8ddef552 DM |
299 | If @var{MRI-cmdfile} does not exist, @code{ld} looks for it in the directories |
300 | specified by any @samp{-L} options. | |
b4d4e8e3 | 301 | |
2c5c0674 RP |
302 | @cindex common allocation |
303 | @kindex -d | |
2c5c0674 | 304 | @kindex -dc |
2c5c0674 | 305 | @kindex -dp |
c653b370 ILT |
306 | @item -d |
307 | @itemx -dc | |
b4d4e8e3 RP |
308 | @itemx -dp |
309 | These three options are equivalent; multiple forms are supported for | |
af54556a ILT |
310 | compatibility with other linkers. They assign space to common symbols |
311 | even if a relocatable output file is specified (with @samp{-r}). The | |
312 | script command @code{FORCE_COMMON_ALLOCATION} has the same effect. | |
313 | @xref{Miscellaneous Commands}. | |
b4d4e8e3 | 314 | |
2c5c0674 RP |
315 | @cindex entry point, from command line |
316 | @kindex -e @var{entry} | |
67afbcea | 317 | @kindex --entry=@var{entry} |
f22eee08 | 318 | @item -e @var{entry} |
67afbcea | 319 | @itemx --entry=@var{entry} |
f22eee08 | 320 | Use @var{entry} as the explicit symbol for beginning execution of your |
e2ebb8ed JL |
321 | program, rather than the default entry point. If there is no symbol |
322 | named @var{entry}, the linker will try to parse @var{entry} as a number, | |
323 | and use that as the entry address (the number will be interpreted in | |
324 | base 10; you may use a leading @samp{0x} for base 16, or a leading | |
325 | @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults | |
326 | and other ways of specifying the entry point. | |
f22eee08 | 327 | |
c653b370 | 328 | @cindex dynamic symbol table |
7ec9d825 | 329 | @kindex -E |
9fde46a4 | 330 | @kindex --export-dynamic |
7ec9d825 | 331 | @item -E |
9fde46a4 | 332 | @itemx --export-dynamic |
67afbcea | 333 | When creating a dynamically linked executable, add all symbols to the |
5a59e34d ILT |
334 | dynamic symbol table. The dynamic symbol table is the set of symbols |
335 | which are visible from dynamic objects at run time. | |
336 | ||
337 | If you do not use this option, the dynamic symbol table will normally | |
338 | contain only those symbols which are referenced by some dynamic object | |
339 | mentioned in the link. | |
340 | ||
341 | If you use @code{dlopen} to load a dynamic object which needs to refer | |
342 | back to the symbols defined by the program, rather than some other | |
343 | dynamic object, then you will probably need to use this option when | |
344 | linking the program itself. | |
c653b370 | 345 | |
de220cbd ILT |
346 | @kindex -f |
347 | @kindex --auxiliary | |
348 | @item -f | |
349 | @itemx --auxiliary @var{name} | |
350 | When creating an ELF shared object, set the internal DT_AUXILIARY field | |
351 | to the specified name. This tells the dynamic linker that the symbol | |
352 | table of the shared object should be used as an auxiliary filter on the | |
353 | symbol table of the shared object @var{name}. | |
354 | ||
cc28f8fb ILT |
355 | If you later link a program against this filter object, then, when you |
356 | run the program, the dynamic linker will see the DT_AUXILIARY field. If | |
357 | the dynamic linker resolves any symbols from the filter object, it will | |
358 | first check whether there is a definition in the shared object | |
359 | @var{name}. If there is one, it will be used instead of the definition | |
360 | in the filter object. The shared object @var{name} need not exist. | |
361 | Thus the shared object @var{name} may be used to provide an alternative | |
362 | implementation of certain functions, perhaps for debugging or for | |
363 | machine specific performance. | |
364 | ||
5a59e34d ILT |
365 | This option may be specified more than once. The DT_AUXILIARY entries |
366 | will be created in the order in which they appear on the command line. | |
367 | ||
2c5c0674 | 368 | @kindex -F |
de220cbd ILT |
369 | @kindex --filter |
370 | @item -F @var{name} | |
371 | @itemx --filter @var{name} | |
372 | When creating an ELF shared object, set the internal DT_FILTER field to | |
373 | the specified name. This tells the dynamic linker that the symbol table | |
cc28f8fb ILT |
374 | of the shared object which is being created should be used as a filter |
375 | on the symbol table of the shared object @var{name}. | |
376 | ||
377 | If you later link a program against this filter object, then, when you | |
378 | run the program, the dynamic linker will see the DT_FILTER field. The | |
379 | dynamic linker will resolve symbols according to the symbol table of the | |
380 | filter object as usual, but it will actually link to the definitions | |
381 | found in the shared object @var{name}. Thus the filter object can be | |
382 | used to select a subset of the symbols provided by the object | |
383 | @var{name}. | |
de220cbd ILT |
384 | |
385 | Some older linkers used the @code{-F} option throughout a compilation | |
ec40bbb8 | 386 | toolchain for specifying object-file format for both input and output |
de220cbd ILT |
387 | object files. The @sc{gnu} linker uses other mechanisms for this |
388 | purpose: the @code{-b}, @code{--format}, @code{--oformat} options, the | |
389 | @code{TARGET} command in linker scripts, and the @code{GNUTARGET} | |
390 | environment variable. The @sc{gnu} linker will ignore the @code{-F} | |
391 | option when not creating an ELF shared object. | |
2c5c0674 | 392 | |
a1d393cf ILT |
393 | @kindex --force-exe-suffix |
394 | @item --force-exe-suffix | |
395 | Make sure that an output file has a .exe suffix. | |
396 | ||
397 | If a successfully built fully linked output file does not have a | |
398 | @code{.exe} or @code{.dll} suffix, this option forces the linker to copy | |
399 | the output file to one of the same name with a @code{.exe} suffix. This | |
400 | option is useful when using unmodified Unix makefiles on a Microsoft | |
401 | Windows host, since some versions of Windows won't run an image unless | |
402 | it ends in a @code{.exe} suffix. | |
403 | ||
2c5c0674 | 404 | @kindex -g |
b4d4e8e3 | 405 | @item -g |
ec40bbb8 | 406 | Ignored. Provided for compatibility with other tools. |
b4d4e8e3 | 407 | |
8ddef552 | 408 | @kindex -G |
67afbcea | 409 | @kindex --gpsize |
8ddef552 DM |
410 | @cindex object size |
411 | @item -G@var{value} | |
67afbcea | 412 | @itemx --gpsize=@var{value} |
8ddef552 | 413 | Set the maximum size of objects to be optimized using the GP register to |
67afbcea DE |
414 | @var{size}. This is only meaningful for object file formats such as |
415 | MIPS ECOFF which supports putting large and small objects into different | |
416 | sections. This is ignored for other object file formats. | |
8ddef552 | 417 | |
71b012a8 RH |
418 | @kindex --gc-sections |
419 | @cindex garbage collection | |
420 | @item --gc-sections | |
421 | Enable garbage collection of unused input sections. It is ignored on | |
422 | targets that do not support this option. This option is not compatible | |
423 | with @samp{-r}, nor should it be used with dynamic linking. | |
424 | ||
67afbcea DE |
425 | @cindex runtime library name |
426 | @kindex -h@var{name} | |
427 | @kindex -soname=@var{name} | |
428 | @item -h@var{name} | |
429 | @itemx -soname=@var{name} | |
430 | When creating an ELF shared object, set the internal DT_SONAME field to | |
431 | the specified name. When an executable is linked with a shared object | |
432 | which has a DT_SONAME field, then when the executable is run the dynamic | |
433 | linker will attempt to load the shared object specified by the DT_SONAME | |
434 | field rather than the using the file name given to the linker. | |
de87cdb4 | 435 | |
2c5c0674 RP |
436 | @kindex -i |
437 | @cindex incremental link | |
f22eee08 | 438 | @item -i |
ec40bbb8 | 439 | Perform an incremental link (same as option @samp{-r}). |
f22eee08 | 440 | |
2c5c0674 | 441 | @cindex archive files, from cmd line |
de87cdb4 | 442 | @kindex -l@var{archive} |
67afbcea DE |
443 | @kindex --library=@var{archive} |
444 | @item -l@var{archive} | |
445 | @itemx --library=@var{archive} | |
446 | Add archive file @var{archive} to the list of files to link. This | |
f22eee08 | 447 | option may be used any number of times. @code{ld} will search its |
67afbcea | 448 | path-list for occurrences of @code{lib@var{archive}.a} for every |
4164f2d5 ILT |
449 | @var{archive} specified. |
450 | ||
451 | On systems which support shared libraries, @code{ld} may also search for | |
452 | libraries with extensions other than @code{.a}. Specifically, on ELF | |
453 | and SunOS systems, @code{ld} will search a directory for a library with | |
454 | an extension of @code{.so} before searching for one with an extension of | |
455 | @code{.a}. By convention, a @code{.so} extension indicates a shared | |
456 | library. | |
f22eee08 | 457 | |
b61364cc ILT |
458 | The linker will search an archive only once, at the location where it is |
459 | specified on the command line. If the archive defines a symbol which | |
460 | was undefined in some object which appeared before the archive on the | |
461 | command line, the linker will include the appropriate file(s) from the | |
462 | archive. However, an undefined symbol in an object appearing later on | |
463 | the command line will not cause the linker to search the archive again. | |
464 | ||
465 | See the @code{-(} option for a way to force the linker to search | |
466 | archives multiple times. | |
467 | ||
468 | You may list the same archive multiple times on the command line. | |
469 | ||
470 | @ifset GENERIC | |
471 | This type of archive searching is standard for Unix linkers. However, | |
472 | if you are using @code{ld} on AIX, note that it is different from the | |
473 | behaviour of the AIX linker. | |
474 | @end ifset | |
475 | ||
2c5c0674 RP |
476 | @cindex search directory, from cmd line |
477 | @kindex -L@var{dir} | |
67afbcea | 478 | @kindex --library-path=@var{dir} |
b4d4e8e3 | 479 | @item -L@var{searchdir} |
67afbcea | 480 | @itemx --library-path=@var{searchdir} |
ec40bbb8 | 481 | Add path @var{searchdir} to the list of paths that @code{ld} will search |
8ddef552 | 482 | for archive libraries and @code{ld} control scripts. You may use this |
0b0642d6 ILT |
483 | option any number of times. The directories are searched in the order |
484 | in which they are specified on the command line. Directories specified | |
485 | on the command line are searched before the default directories. All | |
486 | @code{-L} options apply to all @code{-l} options, regardless of the | |
487 | order in which the options appear. | |
f22eee08 | 488 | |
ec40bbb8 | 489 | @ifset UsesEnvVars |
2c5c0674 | 490 | The default set of paths searched (without being specified with |
ec40bbb8 DM |
491 | @samp{-L}) depends on which emulation mode @code{ld} is using, and in |
492 | some cases also on how it was configured. @xref{Environment}. | |
493 | @end ifset | |
494 | ||
495 | The paths can also be specified in a link script with the | |
0b0642d6 ILT |
496 | @code{SEARCH_DIR} command. Directories specified this way are searched |
497 | at the point in which the linker script appears in the command line. | |
f22eee08 | 498 | |
8ddef552 DM |
499 | @cindex emulation |
500 | @kindex -m @var{emulation} | |
501 | @item -m@var{emulation} | |
8ddef552 | 502 | Emulate the @var{emulation} linker. You can list the available |
f4175166 ILT |
503 | emulations with the @samp{--verbose} or @samp{-V} options. |
504 | ||
505 | If the @samp{-m} option is not used, the emulation is taken from the | |
506 | @code{LDEMULATION} environment variable, if that is defined. | |
507 | ||
508 | Otherwise, the default emulation depends upon how the linker was | |
509 | configured. | |
8ddef552 | 510 | |
67afbcea DE |
511 | @cindex link map |
512 | @kindex -M | |
513 | @kindex --print-map | |
514 | @item -M | |
515 | @itemx --print-map | |
af54556a ILT |
516 | Print a link map to the standard output. A link map provides |
517 | information about the link, including the following: | |
518 | ||
519 | @itemize @bullet | |
520 | @item | |
521 | Where object files and symbols are mapped into memory. | |
522 | @item | |
523 | How common symbols are allocated. | |
524 | @item | |
525 | All archive members included in the link, with a mention of the symbol | |
526 | which caused the archive member to be brought in. | |
527 | @end itemize | |
67afbcea DE |
528 | |
529 | @kindex -n | |
530 | @cindex read-only text | |
531 | @cindex NMAGIC | |
532 | @kindex --nmagic | |
533 | @item -n | |
534 | @itemx --nmagic | |
535 | Set the text segment to be read only, and mark the output as | |
536 | @code{NMAGIC} if possible. | |
537 | ||
2c5c0674 | 538 | @kindex -N |
67afbcea | 539 | @kindex --omagic |
2c5c0674 | 540 | @cindex read/write from cmd line |
67afbcea | 541 | @cindex OMAGIC |
f22eee08 | 542 | @item -N |
67afbcea | 543 | @itemx --omagic |
ec40bbb8 DM |
544 | Set the text and data sections to be readable and writable. Also, do |
545 | not page-align the data segment. If the output format supports Unix | |
546 | style magic numbers, mark the output as @code{OMAGIC}. | |
f22eee08 | 547 | |
67afbcea DE |
548 | @kindex -o @var{output} |
549 | @kindex --output=@var{output} | |
550 | @cindex naming the output file | |
551 | @item -o @var{output} | |
552 | @itemx --output=@var{output} | |
553 | Use @var{output} as the name for the program produced by @code{ld}; if this | |
554 | option is not specified, the name @file{a.out} is used by default. The | |
555 | script command @code{OUTPUT} can also specify the output file name. | |
f22eee08 | 556 | |
c470f2eb UD |
557 | @kindex -O @var{level} |
558 | @cindex generating optimized output | |
559 | @item -O @var{level} | |
560 | If @var{level} is a numeric values greater than zero @code{ld} optimizes | |
561 | the output. This might take significantly longer and therefore probably | |
562 | should only be enabled for the final binary. | |
563 | ||
67afbcea DE |
564 | @cindex partial link |
565 | @cindex relocatable output | |
566 | @kindex -r | |
567 | @kindex --relocateable | |
568 | @item -r | |
569 | @itemx --relocateable | |
570 | Generate relocatable output---i.e., generate an output file that can in | |
571 | turn serve as input to @code{ld}. This is often called @dfn{partial | |
572 | linking}. As a side effect, in environments that support standard Unix | |
573 | magic numbers, this option also sets the output file's magic number to | |
574 | @code{OMAGIC}. | |
575 | @c ; see @code{-N}. | |
576 | If this option is not specified, an absolute file is produced. When | |
577 | linking C++ programs, this option @emph{will not} resolve references to | |
578 | constructors; to do that, use @samp{-Ur}. | |
579 | ||
580 | This option does the same thing as @samp{-i}. | |
581 | ||
582 | @kindex -R @var{file} | |
583 | @kindex --just-symbols=@var{file} | |
584 | @cindex symbol-only input | |
585 | @item -R @var{filename} | |
586 | @itemx --just-symbols=@var{filename} | |
587 | Read symbol names and their addresses from @var{filename}, but do not | |
588 | relocate it or include it in the output. This allows your output file | |
589 | to refer symbolically to absolute locations of memory defined in other | |
590 | programs. You may use this option more than once. | |
591 | ||
592 | For compatibility with other ELF linkers, if the @code{-R} option is | |
593 | followed by a directory name, rather than a file name, it is treated as | |
594 | the @code{-rpath} option. | |
595 | ||
596 | @kindex -s | |
597 | @kindex --strip-all | |
598 | @cindex strip all symbols | |
599 | @item -s | |
600 | @itemx --strip-all | |
601 | Omit all symbol information from the output file. | |
602 | ||
603 | @kindex -S | |
604 | @kindex --strip-debug | |
605 | @cindex strip debugger symbols | |
606 | @item -S | |
607 | @itemx --strip-debug | |
608 | Omit debugger symbol information (but not all symbols) from the output file. | |
609 | ||
610 | @kindex -t | |
611 | @kindex --trace | |
612 | @cindex input files, displaying | |
613 | @item -t | |
614 | @itemx --trace | |
615 | Print the names of the input files as @code{ld} processes them. | |
616 | ||
617 | @kindex -T @var{script} | |
618 | @kindex --script=@var{script} | |
619 | @cindex script files | |
af54556a ILT |
620 | @item -T @var{scriptfile} |
621 | @itemx --script=@var{scriptfile} | |
622 | Use @var{scriptfile} as the linker script. This script replaces | |
623 | @code{ld}'s default linker script (rather than adding to it), so | |
cc28f8fb | 624 | @var{commandfile} must specify everything necessary to describe the |
af54556a | 625 | output file. You must use this option if you want to use a command |
cc28f8fb | 626 | which can only appear once in a linker script, such as the |
af54556a ILT |
627 | @code{SECTIONS} or @code{MEMORY} command. @xref{Scripts}. If |
628 | @var{scriptfile} does not exist in the current directory, @code{ld} | |
629 | looks for it in the directories specified by any preceding @samp{-L} | |
630 | options. Multiple @samp{-T} options accumulate. | |
67afbcea DE |
631 | |
632 | @kindex -u @var{symbol} | |
633 | @kindex --undefined=@var{symbol} | |
634 | @cindex undefined symbol | |
635 | @item -u @var{symbol} | |
636 | @itemx --undefined=@var{symbol} | |
b2529df1 ILT |
637 | Force @var{symbol} to be entered in the output file as an undefined |
638 | symbol. Doing this may, for example, trigger linking of additional | |
639 | modules from standard libraries. @samp{-u} may be repeated with | |
640 | different option arguments to enter additional undefined symbols. This | |
641 | option is equivalent to the @code{EXTERN} linker script command. | |
67afbcea DE |
642 | |
643 | @kindex -v | |
644 | @kindex -V | |
645 | @kindex --version | |
646 | @cindex version | |
647 | @item -v | |
648 | @itemx --version | |
649 | @itemx -V | |
650 | Display the version number for @code{ld}. The @code{-V} option also | |
651 | lists the supported emulations. | |
652 | ||
653 | @kindex -x | |
654 | @kindex --discard-all | |
655 | @cindex deleting local symbols | |
656 | @item -x | |
657 | @itemx --discard-all | |
658 | Delete all local symbols. | |
659 | ||
660 | @kindex -X | |
661 | @kindex --discard-locals | |
662 | @cindex local symbols, deleting | |
663 | @cindex L, deleting symbols beginning | |
664 | @item -X | |
665 | @itemx --discard-locals | |
666 | Delete all temporary local symbols. For most targets, this is all local | |
667 | symbols whose names begin with @samp{L}. | |
668 | ||
669 | @kindex -y @var{symbol} | |
670 | @kindex --trace-symbol=@var{symbol} | |
671 | @cindex symbol tracing | |
672 | @item -y @var{symbol} | |
673 | @itemx --trace-symbol=@var{symbol} | |
674 | Print the name of each linked file in which @var{symbol} appears. This | |
675 | option may be given any number of times. On many systems it is necessary | |
676 | to prepend an underscore. | |
677 | ||
678 | This option is useful when you have an undefined symbol in your link but | |
679 | don't know where the reference is coming from. | |
680 | ||
681 | @kindex -Y @var{path} | |
682 | @item -Y @var{path} | |
683 | Add @var{path} to the default library search path. This option exists | |
684 | for Solaris compatibility. | |
685 | ||
686 | @kindex -z @var{keyword} | |
687 | @item -z @var{keyword} | |
688 | This option is ignored for Solaris compatibility. | |
689 | ||
690 | @kindex -( | |
691 | @cindex groups of archives | |
692 | @item -( @var{archives} -) | |
693 | @itemx --start-group @var{archives} --end-group | |
694 | The @var{archives} should be a list of archive files. They may be | |
695 | either explicit file names, or @samp{-l} options. | |
696 | ||
697 | The specified archives are searched repeatedly until no new undefined | |
698 | references are created. Normally, an archive is searched only once in | |
699 | the order that it is specified on the command line. If a symbol in that | |
700 | archive is needed to resolve an undefined symbol referred to by an | |
701 | object in an archive that appears later on the command line, the linker | |
702 | would not be able to resolve that reference. By grouping the archives, | |
703 | they all be searched repeatedly until all possible references are | |
704 | resolved. | |
705 | ||
706 | Using this option has a significant performance cost. It is best to use | |
707 | it only when there are unavoidable circular references between two or | |
708 | more archives. | |
709 | ||
710 | @kindex -assert @var{keyword} | |
711 | @item -assert @var{keyword} | |
712 | This option is ignored for SunOS compatibility. | |
713 | ||
714 | @kindex -Bdynamic | |
715 | @kindex -dy | |
716 | @kindex -call_shared | |
717 | @item -Bdynamic | |
718 | @itemx -dy | |
719 | @itemx -call_shared | |
720 | Link against dynamic libraries. This is only meaningful on platforms | |
721 | for which shared libraries are supported. This option is normally the | |
722 | default on such platforms. The different variants of this option are | |
723 | for compatibility with various systems. You may use this option | |
724 | multiple times on the command line: it affects library searching for | |
725 | @code{-l} options which follow it. | |
726 | ||
727 | @kindex -Bstatic | |
728 | @kindex -dn | |
729 | @kindex -non_shared | |
730 | @kindex -static | |
731 | @item -Bstatic | |
732 | @itemx -dn | |
733 | @itemx -non_shared | |
734 | @itemx -static | |
735 | Do not link against shared libraries. This is only meaningful on | |
736 | platforms for which shared libraries are supported. The different | |
737 | variants of this option are for compatibility with various systems. You | |
738 | may use this option multiple times on the command line: it affects | |
739 | library searching for @code{-l} options which follow it. | |
740 | ||
741 | @kindex -Bsymbolic | |
742 | @item -Bsymbolic | |
743 | When creating a shared library, bind references to global symbols to the | |
744 | definition within the shared library, if any. Normally, it is possible | |
745 | for a program linked against a shared library to override the definition | |
746 | within the shared library. This option is only meaningful on ELF | |
747 | platforms which support shared libraries. | |
748 | ||
86bc0974 ILT |
749 | @cindex cross reference table |
750 | @kindex --cref | |
751 | @item --cref | |
752 | Output a cross reference table. If a linker map file is being | |
753 | generated, the cross reference table is printed to the map file. | |
754 | Otherwise, it is printed on the standard output. | |
755 | ||
756 | The format of the table is intentionally simple, so that it may be | |
757 | easily processed by a script if necessary. The symbols are printed out, | |
758 | sorted by name. For each symbol, a list of file names is given. If the | |
759 | symbol is defined, the first file listed is the location of the | |
760 | definition. The remaining files contain references to the symbol. | |
761 | ||
67afbcea DE |
762 | @cindex symbols, from command line |
763 | @kindex --defsym @var{symbol}=@var{exp} | |
764 | @item --defsym @var{symbol}=@var{expression} | |
765 | Create a global symbol in the output file, containing the absolute | |
766 | address given by @var{expression}. You may use this option as many | |
767 | times as necessary to define multiple symbols in the command line. A | |
768 | limited form of arithmetic is supported for the @var{expression} in this | |
769 | context: you may give a hexadecimal constant or the name of an existing | |
770 | symbol, or use @code{+} and @code{-} to add or subtract hexadecimal | |
771 | constants or symbols. If you need more elaborate expressions, consider | |
af54556a ILT |
772 | using the linker command language from a script (@pxref{Assignments,, |
773 | Assignment: Symbol Definitions}). @emph{Note:} there should be no white | |
774 | space between @var{symbol}, the equals sign (``@key{=}''), and | |
67afbcea DE |
775 | @var{expression}. |
776 | ||
777 | @cindex dynamic linker, from command line | |
778 | @kindex --dynamic-linker @var{file} | |
779 | @item --dynamic-linker @var{file} | |
780 | Set the name of the dynamic linker. This is only meaningful when | |
781 | generating dynamically linked ELF executables. The default dynamic | |
782 | linker is normally correct; don't use this unless you know what you are | |
783 | doing. | |
784 | ||
785 | @cindex big-endian objects | |
786 | @cindex endianness | |
787 | @kindex -EB | |
788 | @item -EB | |
789 | Link big-endian objects. This affects the default output format. | |
790 | ||
791 | @cindex little-endian objects | |
792 | @kindex -EL | |
793 | @item -EL | |
794 | Link little-endian objects. This affects the default output format. | |
795 | ||
796 | @cindex MIPS embedded PIC code | |
9fde46a4 ILT |
797 | @kindex --embedded-relocs |
798 | @item --embedded-relocs | |
67afbcea DE |
799 | This option is only meaningful when linking MIPS embedded PIC code, |
800 | generated by the -membedded-pic option to the @sc{gnu} compiler and | |
801 | assembler. It causes the linker to create a table which may be used at | |
802 | runtime to relocate any data which was statically initialized to pointer | |
803 | values. See the code in testsuite/ld-empic for details. | |
804 | ||
805 | @cindex help | |
806 | @cindex usage | |
807 | @kindex --help | |
808 | @item --help | |
809 | Print a summary of the command-line options on the standard output and exit. | |
810 | ||
67afbcea DE |
811 | @kindex -Map |
812 | @item -Map @var{mapfile} | |
af54556a ILT |
813 | Print a link map to the file @var{mapfile}. See the description of the |
814 | @samp{-M} option, above. | |
b4d4e8e3 | 815 | |
4551e108 | 816 | @cindex memory usage |
67afbcea DE |
817 | @kindex --no-keep-memory |
818 | @item --no-keep-memory | |
4551e108 ILT |
819 | @code{ld} normally optimizes for speed over memory usage by caching the |
820 | symbol tables of input files in memory. This option tells @code{ld} to | |
821 | instead optimize for memory usage, by rereading the symbol tables as | |
822 | necessary. This may be required if @code{ld} runs out of memory space | |
823 | while linking a large executable. | |
824 | ||
5a59e34d ILT |
825 | @kindex --no-warn-mismatch |
826 | @item --no-warn-mismatch | |
827 | Normally @code{ld} will give an error if you try to link together input | |
828 | files that are mismatched for some reason, perhaps because they have | |
829 | been compiled for different processors or for different endiannesses. | |
830 | This option tells @code{ld} that it should silently permit such possible | |
831 | errors. This option should only be used with care, in cases when you | |
832 | have taken some special action that ensures that the linker errors are | |
833 | inappropriate. | |
834 | ||
67afbcea DE |
835 | @kindex --no-whole-archive |
836 | @item --no-whole-archive | |
837 | Turn off the effect of the @code{--whole-archive} option for subsequent | |
838 | archive files. | |
839 | ||
840 | @cindex output file after errors | |
841 | @kindex --noinhibit-exec | |
842 | @item --noinhibit-exec | |
843 | Retain the executable output file whenever it is still usable. | |
844 | Normally, the linker will not produce an output file if it encounters | |
845 | errors during the link process; it exits without writing an output file | |
846 | when it issues any error whatsoever. | |
f22eee08 | 847 | |
7f9ae73e | 848 | @ifclear SingleFormat |
9fde46a4 ILT |
849 | @kindex --oformat |
850 | @item --oformat @var{output-format} | |
1fb57a5d RP |
851 | @code{ld} may be configured to support more than one kind of object |
852 | file. If your @code{ld} is configured this way, you can use the | |
9fde46a4 | 853 | @samp{--oformat} option to specify the binary format for the output |
1fb57a5d RP |
854 | object file. Even when @code{ld} is configured to support alternative |
855 | object formats, you don't usually need to specify this, as @code{ld} | |
856 | should be configured to produce as a default output format the most | |
857 | usual format on each machine. @var{output-format} is a text string, the | |
858 | name of a particular format supported by the BFD libraries. (You can | |
859 | list the available binary formats with @samp{objdump -i}.) The script | |
860 | command @code{OUTPUT_FORMAT} can also specify the output format, but | |
861 | this option overrides it. @xref{BFD}. | |
7f9ae73e | 862 | @end ifclear |
346535cc | 863 | |
67afbcea DE |
864 | @kindex -qmagic |
865 | @item -qmagic | |
866 | This option is ignored for Linux compatibility. | |
c653b370 | 867 | |
67afbcea DE |
868 | @kindex -Qy |
869 | @item -Qy | |
870 | This option is ignored for SVR4 compatibility. | |
c653b370 | 871 | |
67afbcea | 872 | @kindex --relax |
1c48127e RP |
873 | @cindex synthesizing linker |
874 | @cindex relaxing addressing modes | |
67afbcea | 875 | @item --relax |
1fb57a5d RP |
876 | An option with machine dependent effects. |
877 | @ifset GENERIC | |
67afbcea | 878 | This option is only supported on a few targets. |
1fb57a5d | 879 | @end ifset |
ec40bbb8 DM |
880 | @ifset H8300 |
881 | @xref{H8/300,,@code{ld} and the H8/300}. | |
882 | @end ifset | |
1fb57a5d RP |
883 | @ifset I960 |
884 | @xref{i960,, @code{ld} and the Intel 960 family}. | |
885 | @end ifset | |
1c48127e | 886 | |
e2ebb8ed | 887 | |
67afbcea DE |
888 | On some platforms, the @samp{--relax} option performs global |
889 | optimizations that become possible when the linker resolves addressing | |
890 | in the program, such as relaxing address modes and synthesizing new | |
891 | instructions in the output object file. | |
1c48127e | 892 | |
e2ebb8ed | 893 | On some platforms these link time global optimizations may make symbolic |
71b012a8 RH |
894 | debugging of the resulting executable impossible. |
895 | @ifset GENERIC | |
896 | This is known to be | |
e2ebb8ed | 897 | the case for the Matsushita MN10200 and MN10300 family of processors. |
71b012a8 | 898 | @end ifset |
e2ebb8ed | 899 | |
1fb57a5d | 900 | @ifset GENERIC |
9fde46a4 ILT |
901 | On platforms where this is not supported, @samp{--relax} is accepted, |
902 | but ignored. | |
1fb57a5d | 903 | @end ifset |
1c48127e | 904 | |
7c8fab26 RP |
905 | @cindex retaining specified symbols |
906 | @cindex stripping all but some symbols | |
907 | @cindex symbols, retaining selectively | |
67afbcea | 908 | @item --retain-symbols-file @var{filename} |
7c8fab26 RP |
909 | Retain @emph{only} the symbols listed in the file @var{filename}, |
910 | discarding all others. @var{filename} is simply a flat file, with one | |
911 | symbol name per line. This option is especially useful in environments | |
912 | @ifset GENERIC | |
913 | (such as VxWorks) | |
914 | @end ifset | |
915 | where a large global symbol table is accumulated gradually, to conserve | |
916 | run-time memory. | |
917 | ||
9fde46a4 | 918 | @samp{--retain-symbols-file} does @emph{not} discard undefined symbols, |
7c8fab26 RP |
919 | or symbols needed for relocations. |
920 | ||
9fde46a4 | 921 | You may only specify @samp{--retain-symbols-file} once in the command |
7c8fab26 RP |
922 | line. It overrides @samp{-s} and @samp{-S}. |
923 | ||
a1ad915d ILT |
924 | @ifset GENERIC |
925 | @item -rpath @var{dir} | |
926 | @cindex runtime library search path | |
927 | @kindex -rpath | |
0b0642d6 ILT |
928 | Add a directory to the runtime library search path. This is used when |
929 | linking an ELF executable with shared objects. All @code{-rpath} | |
930 | arguments are concatenated and passed to the runtime linker, which uses | |
e54bf1c1 ILT |
931 | them to locate shared objects at runtime. The @code{-rpath} option is |
932 | also used when locating shared objects which are needed by shared | |
933 | objects explicitly included in the link; see the description of the | |
c653b370 ILT |
934 | @code{-rpath-link} option. If @code{-rpath} is not used when linking an |
935 | ELF executable, the contents of the environment variable | |
936 | @code{LD_RUN_PATH} will be used if it is defined. | |
0b0642d6 ILT |
937 | |
938 | The @code{-rpath} option may also be used on SunOS. By default, on | |
939 | SunOS, the linker will form a runtime search patch out of all the | |
e54bf1c1 | 940 | @code{-L} options it is given. If a @code{-rpath} option is used, the |
0b0642d6 ILT |
941 | runtime search path will be formed exclusively using the @code{-rpath} |
942 | options, ignoring the @code{-L} options. This can be useful when using | |
943 | gcc, which adds many @code{-L} options which may be on NFS mounted | |
944 | filesystems. | |
c653b370 ILT |
945 | |
946 | For compatibility with other ELF linkers, if the @code{-R} option is | |
947 | followed by a directory name, rather than a file name, it is treated as | |
948 | the @code{-rpath} option. | |
a1ad915d ILT |
949 | @end ifset |
950 | ||
e54bf1c1 ILT |
951 | @ifset GENERIC |
952 | @cindex link-time runtime library search path | |
953 | @kindex -rpath-link | |
954 | @item -rpath-link @var{DIR} | |
955 | When using ELF or SunOS, one shared library may require another. This | |
956 | happens when an @code{ld -shared} link includes a shared library as one | |
957 | of the input files. | |
958 | ||
959 | When the linker encounters such a dependency when doing a non-shared, | |
af54556a | 960 | non-relocatable link, it will automatically try to locate the required |
e54bf1c1 ILT |
961 | shared library and include it in the link, if it is not included |
962 | explicitly. In such a case, the @code{-rpath-link} option | |
963 | specifies the first set of directories to search. The | |
964 | @code{-rpath-link} option may specify a sequence of directory names | |
965 | either by specifying a list of names separated by colons, or by | |
966 | appearing multiple times. | |
967 | ||
968 | The linker uses the following search paths to locate required shared | |
969 | libraries. | |
970 | @enumerate | |
971 | @item | |
972 | Any directories specified by @code{-rpath-link} options. | |
973 | @item | |
974 | Any directories specified by @code{-rpath} options. The difference | |
975 | between @code{-rpath} and @code{-rpath-link} is that directories | |
976 | specified by @code{-rpath} options are included in the executable and | |
977 | used at runtime, whereas the @code{-rpath-link} option is only effective | |
978 | at link time. | |
979 | @item | |
c653b370 ILT |
980 | On an ELF system, if the @code{-rpath} and @code{rpath-link} options |
981 | were not used, search the contents of the environment variable | |
982 | @code{LD_RUN_PATH}. | |
983 | @item | |
e54bf1c1 ILT |
984 | On SunOS, if the @code{-rpath} option was not used, search any |
985 | directories specified using @code{-L} options. | |
986 | @item | |
987 | For a native linker, the contents of the environment variable | |
988 | @code{LD_LIBRARY_PATH}. | |
989 | @item | |
990 | The default directories, normally @file{/lib} and @file{/usr/lib}. | |
6581c418 ILT |
991 | @item |
992 | For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} | |
993 | exists, the list of directories found in that file. | |
e54bf1c1 ILT |
994 | @end enumerate |
995 | ||
996 | If the required shared library is not found, the linker will issue a | |
997 | warning and continue with the link. | |
998 | @end ifset | |
999 | ||
67afbcea DE |
1000 | @kindex -shared |
1001 | @kindex -Bshareable | |
4551e108 | 1002 | @item -shared |
67afbcea | 1003 | @itemx -Bshareable |
4551e108 | 1004 | @cindex shared libraries |
67afbcea DE |
1005 | Create a shared library. This is currently only supported on ELF, XCOFF |
1006 | and SunOS platforms. On SunOS, the linker will automatically create a | |
e54bf1c1 ILT |
1007 | shared library if the @code{-e} option is not used and there are |
1008 | undefined symbols in the link. | |
4551e108 | 1009 | |
67afbcea DE |
1010 | @item --sort-common |
1011 | @kindex --sort-common | |
1012 | This option tells @code{ld} to sort the common symbols by size when it | |
1013 | places them in the appropriate output sections. First come all the one | |
1014 | byte symbols, then all the two bytes, then all the four bytes, and then | |
1015 | everything else. This is to prevent gaps between symbols due to | |
1016 | alignment constraints. | |
1017 | ||
1018 | @kindex --split-by-file | |
1019 | @item --split-by-file | |
1020 | Similar to @code{--split-by-reloc} but creates a new output section for | |
1021 | each input file. | |
1022 | ||
1023 | @kindex --split-by-reloc | |
1024 | @item --split-by-reloc @var{count} | |
1025 | Trys to creates extra sections in the output file so that no single | |
1026 | output section in the file contains more than @var{count} relocations. | |
1027 | This is useful when generating huge relocatable for downloading into | |
1028 | certain real time kernels with the COFF object file format; since COFF | |
1029 | cannot represent more than 65535 relocations in a single section. Note | |
1030 | that this will fail to work with object file formats which do not | |
1031 | support arbitrary sections. The linker will not split up individual | |
1032 | input sections for redistribution, so if a single input section contains | |
01bc8f35 ILT |
1033 | more than @var{count} relocations one output section will contain that |
1034 | many relocations. | |
1035 | ||
67afbcea DE |
1036 | @kindex --stats |
1037 | @item --stats | |
1038 | Compute and display statistics about the operation of the linker, such | |
1039 | as execution time and memory usage. | |
f22eee08 | 1040 | |
9fde46a4 | 1041 | @kindex --traditional-format |
c96386c4 | 1042 | @cindex traditional format |
9fde46a4 | 1043 | @item --traditional-format |
c96386c4 ILT |
1044 | For some targets, the output of @code{ld} is different in some ways from |
1045 | the output of some existing linker. This switch requests @code{ld} to | |
1046 | use the traditional format instead. | |
1047 | ||
1048 | @cindex dbx | |
1049 | For example, on SunOS, @code{ld} combines duplicate entries in the | |
1050 | symbol string table. This can reduce the size of an output file with | |
1051 | full debugging information by over 30 percent. Unfortunately, the SunOS | |
1052 | @code{dbx} program can not read the resulting program (@code{gdb} has no | |
9fde46a4 | 1053 | trouble). The @samp{--traditional-format} switch tells @code{ld} to not |
c96386c4 ILT |
1054 | combine duplicate entries. |
1055 | ||
67afbcea DE |
1056 | @kindex -Tbss @var{org} |
1057 | @kindex -Tdata @var{org} | |
1058 | @kindex -Ttext @var{org} | |
1059 | @cindex segment origins, cmd line | |
1060 | @item -Tbss @var{org} | |
1061 | @itemx -Tdata @var{org} | |
1062 | @itemx -Ttext @var{org} | |
1063 | Use @var{org} as the starting address for---respectively---the | |
1064 | @code{bss}, @code{data}, or the @code{text} segment of the output file. | |
1065 | @var{org} must be a single hexadecimal integer; | |
1066 | for compatibility with other linkers, you may omit the leading | |
1067 | @samp{0x} usually associated with hexadecimal values. | |
f22eee08 | 1068 | |
2c5c0674 | 1069 | @kindex -Ur |
b4d4e8e3 | 1070 | @cindex constructors |
d4e5e3c3 | 1071 | @item -Ur |
b4d4e8e3 | 1072 | For anything other than C++ programs, this option is equivalent to |
ec40bbb8 DM |
1073 | @samp{-r}: it generates relocatable output---i.e., an output file that can in |
1074 | turn serve as input to @code{ld}. When linking C++ programs, @samp{-Ur} | |
1fb57a5d | 1075 | @emph{does} resolve references to constructors, unlike @samp{-r}. |
3e27cc11 | 1076 | It does not work to use @samp{-Ur} on files that were themselves linked |
1fb57a5d | 1077 | with @samp{-Ur}; once the constructor table has been built, it cannot |
3e27cc11 DM |
1078 | be added to. Use @samp{-Ur} only for the last partial link, and |
1079 | @samp{-r} for the others. | |
b4d4e8e3 | 1080 | |
01bc8f35 | 1081 | @kindex --verbose |
cf2e4f5f | 1082 | @cindex verbose |
01bc8f35 | 1083 | @item --verbose |
1fb57a5d | 1084 | Display the version number for @code{ld} and list the linker emulations |
cf2e4f5f ILT |
1085 | supported. Display which input files can and cannot be opened. Display |
1086 | the linker script if using a default builtin script. | |
8ddef552 | 1087 | |
5a59e34d ILT |
1088 | @kindex --version-script=@var{version-scriptfile} |
1089 | @cindex version script, symbol versions | |
1090 | @itemx --version-script=@var{version-scriptfile} | |
1091 | Specify the name of a version script to the linker. This is typically | |
1092 | used when creating shared libraries to specify additional information | |
1093 | about the version heirarchy for the library being created. This option | |
1094 | is only meaningful on ELF platforms which support shared libraries. | |
af54556a | 1095 | @xref{VERSION}. |
5a59e34d | 1096 | |
458fc056 | 1097 | @kindex --warn-comon |
7f9ae73e RP |
1098 | @cindex warnings, on combining symbols |
1099 | @cindex combining symbols, warnings on | |
458fc056 | 1100 | @item --warn-common |
2a28d8b0 DM |
1101 | Warn when a common symbol is combined with another common symbol or with |
1102 | a symbol definition. Unix linkers allow this somewhat sloppy practice, | |
1103 | but linkers on some other operating systems do not. This option allows | |
1104 | you to find potential problems from combining global symbols. | |
1cd4cca9 DM |
1105 | Unfortunately, some C libraries use this practice, so you may get some |
1106 | warnings about symbols in the libraries as well as in your programs. | |
2a28d8b0 DM |
1107 | |
1108 | There are three kinds of global symbols, illustrated here by C examples: | |
1109 | ||
1110 | @table @samp | |
1111 | @item int i = 1; | |
1112 | A definition, which goes in the initialized data section of the output | |
1113 | file. | |
1114 | ||
1115 | @item extern int i; | |
1116 | An undefined reference, which does not allocate space. | |
1117 | There must be either a definition or a common symbol for the | |
1118 | variable somewhere. | |
1119 | ||
1120 | @item int i; | |
1121 | A common symbol. If there are only (one or more) common symbols for a | |
1122 | variable, it goes in the uninitialized data area of the output file. | |
1123 | The linker merges multiple common symbols for the same variable into a | |
1124 | single symbol. If they are of different sizes, it picks the largest | |
1125 | size. The linker turns a common symbol into a declaration, if there is | |
1126 | a definition of the same variable. | |
1127 | @end table | |
1128 | ||
458fc056 ILT |
1129 | The @samp{--warn-common} option can produce five kinds of warnings. |
1130 | Each warning consists of a pair of lines: the first describes the symbol | |
1131 | just encountered, and the second describes the previous symbol | |
1132 | encountered with the same name. One or both of the two symbols will be | |
1133 | a common symbol. | |
2a28d8b0 DM |
1134 | |
1135 | @enumerate | |
1136 | @item | |
1137 | Turning a common symbol into a reference, because there is already a | |
1138 | definition for the symbol. | |
1139 | @smallexample | |
8920addc RP |
1140 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
1141 | overridden by definition | |
2a28d8b0 DM |
1142 | @var{file}(@var{section}): warning: defined here |
1143 | @end smallexample | |
1144 | ||
1145 | @item | |
1146 | Turning a common symbol into a reference, because a later definition for | |
1147 | the symbol is encountered. This is the same as the previous case, | |
1148 | except that the symbols are encountered in a different order. | |
1149 | @smallexample | |
8920addc RP |
1150 | @var{file}(@var{section}): warning: definition of `@var{symbol}' |
1151 | overriding common | |
2a28d8b0 DM |
1152 | @var{file}(@var{section}): warning: common is here |
1153 | @end smallexample | |
1154 | ||
1155 | @item | |
1156 | Merging a common symbol with a previous same-sized common symbol. | |
1157 | @smallexample | |
8920addc RP |
1158 | @var{file}(@var{section}): warning: multiple common |
1159 | of `@var{symbol}' | |
2a28d8b0 DM |
1160 | @var{file}(@var{section}): warning: previous common is here |
1161 | @end smallexample | |
1162 | ||
1163 | @item | |
1164 | Merging a common symbol with a previous larger common symbol. | |
1165 | @smallexample | |
8920addc RP |
1166 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
1167 | overridden by larger common | |
2a28d8b0 DM |
1168 | @var{file}(@var{section}): warning: larger common is here |
1169 | @end smallexample | |
1170 | ||
1171 | @item | |
1172 | Merging a common symbol with a previous smaller common symbol. This is | |
1173 | the same as the previous case, except that the symbols are | |
1174 | encountered in a different order. | |
1175 | @smallexample | |
8920addc RP |
1176 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
1177 | overriding smaller common | |
2a28d8b0 DM |
1178 | @var{file}(@var{section}): warning: smaller common is here |
1179 | @end smallexample | |
1180 | @end enumerate | |
1181 | ||
458fc056 ILT |
1182 | @kindex --warn-constructors |
1183 | @item --warn-constructors | |
0b0642d6 ILT |
1184 | Warn if any global constructors are used. This is only useful for a few |
1185 | object file formats. For formats like COFF or ELF, the linker can not | |
1186 | detect the use of global constructors. | |
1187 | ||
458fc056 ILT |
1188 | @kindex --warn-multiple-gp |
1189 | @item --warn-multiple-gp | |
4b7d2399 ILT |
1190 | Warn if multiple global pointer values are required in the output file. |
1191 | This is only meaningful for certain processors, such as the Alpha. | |
1192 | Specifically, some processors put large-valued constants in a special | |
1193 | section. A special register (the global pointer) points into the middle | |
1194 | of this section, so that constants can be loaded efficiently via a | |
1195 | base-register relative addressing mode. Since the offset in | |
1196 | base-register relative mode is fixed and relatively small (e.g., 16 | |
1197 | bits), this limits the maximum size of the constant pool. Thus, in | |
1198 | large programs, it is often necessary to use multiple global pointer | |
1199 | values in order to be able to address all possible constants. This | |
1200 | option causes a warning to be issued whenever this case occurs. | |
1201 | ||
458fc056 | 1202 | @kindex --warn-once |
01bc8f35 ILT |
1203 | @cindex warnings, on undefined symbols |
1204 | @cindex undefined symbols, warnings on | |
458fc056 | 1205 | @item --warn-once |
01bc8f35 ILT |
1206 | Only warn once for each undefined symbol, rather than once per module |
1207 | which refers to it. | |
1208 | ||
458fc056 ILT |
1209 | @kindex --warn-section-align |
1210 | @cindex warnings, on section alignment | |
1211 | @cindex section alignment, warnings on | |
1212 | @item --warn-section-align | |
1213 | Warn if the address of an output section is changed because of | |
1214 | alignment. Typically, the alignment will be set by an input section. | |
1215 | The address will only be changed if it not explicitly specified; that | |
1216 | is, if the @code{SECTIONS} command does not specify a start address for | |
1217 | the section (@pxref{SECTIONS}). | |
1218 | ||
4551e108 ILT |
1219 | @kindex --whole-archive |
1220 | @cindex including an entire archive | |
4b7d2399 | 1221 | @item --whole-archive |
3c8deccc ILT |
1222 | For each archive mentioned on the command line after the |
1223 | @code{--whole-archive} option, include every object file in the archive | |
1224 | in the link, rather than searching the archive for the required object | |
1225 | files. This is normally used to turn an archive file into a shared | |
1226 | library, forcing every object to be included in the resulting shared | |
67afbcea | 1227 | library. This option may be used more than once. |
4551e108 | 1228 | |
e3d73386 ILT |
1229 | @kindex --wrap |
1230 | @item --wrap @var{symbol} | |
1231 | Use a wrapper function for @var{symbol}. Any undefined reference to | |
1232 | @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any | |
1233 | undefined reference to @code{__real_@var{symbol}} will be resolved to | |
1234 | @var{symbol}. | |
1235 | ||
1236 | This can be used to provide a wrapper for a system function. The | |
1237 | wrapper function should be called @code{__wrap_@var{symbol}}. If it | |
1238 | wishes to call the system function, it should call | |
1239 | @code{__real_@var{symbol}}. | |
1240 | ||
1241 | Here is a trivial example: | |
1242 | ||
1243 | @smallexample | |
1244 | void * | |
1245 | __wrap_malloc (int c) | |
1246 | @{ | |
1247 | printf ("malloc called with %ld\n", c); | |
1248 | return __real_malloc (c); | |
1249 | @} | |
1250 | @end smallexample | |
1251 | ||
1252 | If you link other code with this file using @code{--wrap malloc}, then | |
1253 | all calls to @code{malloc} will call the function @code{__wrap_malloc} | |
1254 | instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will | |
1255 | call the real @code{malloc} function. | |
1256 | ||
1257 | You may wish to provide a @code{__real_malloc} function as well, so that | |
1258 | links without the @code{--wrap} option will succeed. If you do this, | |
1259 | you should not put the definition of @code{__real_malloc} in the same | |
1260 | file as @code{__wrap_malloc}; if you do, the assembler may resolve the | |
1261 | call before the linker has a chance to wrap it to @code{malloc}. | |
1262 | ||
f22eee08 | 1263 | @end table |
b4d4e8e3 | 1264 | |
ec40bbb8 DM |
1265 | @ifset UsesEnvVars |
1266 | @node Environment | |
2c5c0674 RP |
1267 | @section Environment Variables |
1268 | ||
f4175166 ILT |
1269 | You can change the behavior of @code{ld} with the environment variables |
1270 | @code{GNUTARGET} and @code{LDEMULATION}. | |
2c5c0674 RP |
1271 | |
1272 | @kindex GNUTARGET | |
1273 | @cindex default input format | |
1274 | @code{GNUTARGET} determines the input-file object format if you don't | |
9fde46a4 | 1275 | use @samp{-b} (or its synonym @samp{--format}). Its value should be one |
2c5c0674 | 1276 | of the BFD names for an input format (@pxref{BFD}). If there is no |
246504a5 | 1277 | @code{GNUTARGET} in the environment, @code{ld} uses the natural format |
9fde46a4 ILT |
1278 | of the target. If @code{GNUTARGET} is set to @code{default} then BFD |
1279 | attempts to discover the input format by examining binary input files; | |
1280 | this method often succeeds, but there are potential ambiguities, since | |
1281 | there is no method of ensuring that the magic number used to specify | |
1282 | object-file formats is unique. However, the configuration procedure for | |
1283 | BFD on each system places the conventional format for that system first | |
1284 | in the search-list, so ambiguities are resolved in favor of convention. | |
f4175166 ILT |
1285 | |
1286 | @kindex LDEMULATION | |
1287 | @cindex default emulation | |
1288 | @cindex emulation, default | |
1289 | @code{LDEMULATION} determines the default emulation if you don't use the | |
1290 | @samp{-m} option. The emulation can affect various aspects of linker | |
1291 | behaviour, particularly the default linker script. You can list the | |
1292 | available emulations with the @samp{--verbose} or @samp{-V} options. If | |
1293 | the @samp{-m} option is not used, and the @code{LDEMULATION} environment | |
1294 | variable is not defined, the default emulation depends upon how the | |
1295 | linker was configured. | |
ec40bbb8 | 1296 | @end ifset |
2c5c0674 | 1297 | |
af54556a ILT |
1298 | @node Scripts |
1299 | @chapter Linker Scripts | |
f22eee08 | 1300 | |
af54556a ILT |
1301 | @cindex scripts |
1302 | @cindex linker scripts | |
2c5c0674 | 1303 | @cindex command files |
af54556a ILT |
1304 | Every link is controlled by a @dfn{linker script}. This script is |
1305 | written in the linker command language. | |
1306 | ||
1307 | The main purpose of the linker script is to describe how the sections in | |
1308 | the input files should be mapped into the output file, and to control | |
1309 | the memory layout of the output file. Most linker scripts do nothing | |
1310 | more than this. However, when necessary, the linker script can also | |
1311 | direct the linker to perform many other operations, using the commands | |
1312 | described below. | |
1313 | ||
1314 | The linker always uses a linker script. If you do not supply one | |
1315 | yourself, the linker will use a default script that is compiled into the | |
1316 | linker executable. You can use the @samp{--verbose} command line option | |
1317 | to display the default linker script. Certain command line options, | |
1318 | such as @samp{-r} or @samp{-N}, will affect the default linker script. | |
1319 | ||
1320 | You may supply your own linker script by using the @samp{-T} command | |
1321 | line option. When you do this, your linker script will replace the | |
1322 | default linker script. | |
1323 | ||
1324 | You may also use linker scripts implicitly by naming them as input files | |
f8cf2baa ILT |
1325 | to the linker, as though they were files to be linked. @xref{Implicit |
1326 | Linker Scripts}. | |
f22eee08 | 1327 | |
af54556a ILT |
1328 | @menu |
1329 | * Basic Script Concepts:: Basic Linker Script Concepts | |
1330 | * Script Format:: Linker Script Format | |
1331 | * Simple Example:: Simple Linker Script Example | |
1332 | * Simple Commands:: Simple Linker Script Commands | |
1333 | * Assignments:: Assigning Values to Symbols | |
1334 | * SECTIONS:: SECTIONS Command | |
1335 | * MEMORY:: MEMORY Command | |
1336 | * PHDRS:: PHDRS Command | |
1337 | * VERSION:: VERSION Command | |
1338 | * Expressions:: Expressions in Linker Scripts | |
f8cf2baa | 1339 | * Implicit Linker Scripts:: Implicit Linker Scripts |
af54556a ILT |
1340 | @end menu |
1341 | ||
1342 | @node Basic Script Concepts | |
1343 | @section Basic Linker Script Concepts | |
1344 | @cindex linker script concepts | |
1345 | We need to define some basic concepts and vocabulary in order to | |
1346 | describe the linker script language. | |
1347 | ||
1348 | The linker combines input files into a single output file. The output | |
1349 | file and each input file are in a special data format known as an | |
1350 | @dfn{object file format}. Each file is called an @dfn{object file}. | |
1351 | The output file is often called an @dfn{executable}, but for our | |
1352 | purposes we will also call it an object file. Each object file has, | |
1353 | among other things, a list of @dfn{sections}. We sometimes refer to a | |
1354 | section in an input file as an @dfn{input section}; similarly, a section | |
1355 | in the output file is an @dfn{output section}. | |
1356 | ||
1357 | Each section in an object file has a name and a size. Most sections | |
1358 | also have an associated block of data, known as the @dfn{section | |
1359 | contents}. A section may be marked as @dfn{loadable}, which mean that | |
1360 | the contents should be loaded into memory when the output file is run. | |
1361 | A section with no contents may be @dfn{allocatable}, which means that an | |
1362 | area in memory should be set aside, but nothing in particular should be | |
1363 | loaded there (in some cases this memory must be zeroed out). A section | |
1364 | which is neither loadable nor allocatable typically contains some sort | |
1365 | of debugging information. | |
1366 | ||
1367 | Every loadable or allocatable output section has two addresses. The | |
1368 | first is the @dfn{VMA}, or virtual memory address. This is the address | |
36a8f215 | 1369 | the section will have when the output file is run. The second is the |
af54556a ILT |
1370 | @dfn{LMA}, or load memory address. This is the address at which the |
1371 | section will be loaded. In most cases the two addresses will be the | |
1372 | same. An example of when they might be different is when a data section | |
1373 | is loaded into ROM, and then copied into RAM when the program starts up | |
1374 | (this technique is often used to initialize global variables in a ROM | |
1375 | based system). In this case the ROM address would be the LMA, and the | |
1376 | RAM address would be the VMA. | |
1377 | ||
1378 | You can see the sections in an object file by using the @code{objdump} | |
1379 | program with the @samp{-h} option. | |
1380 | ||
1381 | Every object file also has a list of @dfn{symbols}, known as the | |
1382 | @dfn{symbol table}. A symbol may be defined or undefined. Each symbol | |
1383 | has a name, and each defined symbol has an address, among other | |
1384 | information. If you compile a C or C++ program into an object file, you | |
1385 | will get a defined symbol for every defined function and global or | |
1386 | static variable. Every undefined function or global variable which is | |
1387 | referenced in the input file will become an undefined symbol. | |
1388 | ||
1389 | You can see the symbols in an object file by using the @code{nm} | |
1390 | program, or by using the @code{objdump} program with the @samp{-t} | |
1391 | option. | |
1392 | ||
1393 | @node Script Format | |
1394 | @section Linker Script Format | |
1395 | @cindex linker script format | |
1396 | Linker scripts are text files. | |
1397 | ||
1398 | You write a linker script as a series of commands. Each command is | |
1399 | either a keyword, possibly followed by arguments, or an assignment to a | |
1400 | symbol. You may separate commands using semicolons. Whitespace is | |
1401 | generally ignored. | |
1402 | ||
1403 | Strings such as file or format names can normally be entered directly. | |
1404 | If the file name contains a character such as a comma which would | |
1405 | otherwise serve to separate file names, you may put the file name in | |
1406 | double quotes. There is no way to use a double quote character in a | |
1407 | file name. | |
1408 | ||
1409 | You may include comments in linker scripts just as in C, delimited by | |
1410 | @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent | |
1411 | to whitespace. | |
1412 | ||
1413 | @node Simple Example | |
1414 | @section Simple Linker Script Example | |
1415 | @cindex linker script example | |
1416 | @cindex example of linker script | |
1417 | Many linker scripts are fairly simple. | |
1418 | ||
1419 | The simplest possible linker script has just one command: | |
1420 | @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the | |
1421 | memory layout of the output file. | |
1422 | ||
1423 | The @samp{SECTIONS} command is a powerful command. Here we will | |
1424 | describe a simple use of it. Let's assume your program consists only of | |
1425 | code, initialized data, and uninitialized data. These will be in the | |
1426 | @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively. | |
1427 | Let's assume further that these are the only sections which appear in | |
1428 | your input files. | |
1429 | ||
1430 | For this example, let's say that the code should be loaded at address | |
1431 | 0x10000, and that the data should start at address 0x8000000. Here is a | |
1432 | linker script which will do that: | |
1433 | @smallexample | |
1434 | SECTIONS | |
1435 | @{ | |
1436 | . = 0x10000; | |
1437 | .text : @{ *(.text) @} | |
1438 | . = 0x8000000; | |
1439 | .data : @{ *(.data) @} | |
1440 | .bss : @{ *(.bss) @} | |
1441 | @} | |
1442 | @end smallexample | |
f9c5c231 | 1443 | |
af54556a ILT |
1444 | You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, |
1445 | followed by a series of symbol assignments and output section | |
1446 | descriptions enclosed in curly braces. | |
1447 | ||
1448 | The first line in the above example sets the special symbol @samp{.}, | |
1449 | which is the location counter. If you do not specify the address of an | |
1450 | output section in some other way (other ways are described later), the | |
1451 | address is set from the current value of the location counter. The | |
1452 | location counter is then incremented by the size of the output section. | |
1453 | ||
36a8f215 ILT |
1454 | The first line inside the @samp{SECTIONS} command of the above example |
1455 | sets the value of the special symbol @samp{.}, which is the location | |
1456 | counter. If you do not specify the address of an output section in some | |
1457 | other way (other ways are described later), the address is set from the | |
1458 | current value of the location counter. The location counter is then | |
1459 | incremented by the size of the output section. At the start of the | |
1460 | @samp{SECTIONS} command, the location counter has the value @samp{0}. | |
1461 | ||
af54556a ILT |
1462 | The second line defines an output section, @samp{.text}. The colon is |
1463 | required syntax which may be ignored for now. Within the curly braces | |
1464 | after the output section name, you list the names of the input sections | |
1465 | which should be placed into this output section. The @samp{*} is a | |
1466 | wildcard which matches any file name. The expression @samp{*(.text)} | |
1467 | means all @samp{.text} input sections in all input files. | |
1468 | ||
1469 | Since the location counter is @samp{0x10000} when the output section | |
1470 | @samp{.text} is defined, the linker will set the address of the | |
1471 | @samp{.text} section in the output file to be @samp{0x10000}. | |
1472 | ||
1473 | The remaining lines define the @samp{.data} and @samp{.bss} sections in | |
36a8f215 ILT |
1474 | the output file. The linker will place the @samp{.data} output section |
1475 | at address @samp{0x8000000}. After the linker places the @samp{.data} | |
1476 | output section, the value of the location counter will be | |
1477 | @samp{0x8000000} plus the size of the @samp{.data} output section. The | |
1478 | effect is that the linker will place the @samp{.bss} output section | |
1479 | immediately after the @samp{.data} output section in memory | |
1480 | ||
1481 | The linker will ensure that each output section has the required | |
1482 | alignment, by increasing the location counter if necessary. In this | |
1483 | example, the specified addresses for the @samp{.text} and @samp{.data} | |
1484 | sections will probably satisfy any alignment constraints, but the linker | |
1485 | may have to create a small gap between the @samp{.data} and @samp{.bss} | |
1486 | sections. | |
af54556a ILT |
1487 | |
1488 | That's it! That's a simple and complete linker script. | |
1489 | ||
1490 | @node Simple Commands | |
1491 | @section Simple Linker Script Commands | |
1492 | @cindex linker script simple commands | |
1493 | In this section we describe the simple linker script commands. | |
2c5c0674 | 1494 | |
2c5c0674 | 1495 | @menu |
af54556a ILT |
1496 | * Entry Point:: Setting the entry point |
1497 | * File Commands:: Commands dealing with files | |
1498 | @ifclear SingleFormat | |
1499 | * Format Commands:: Commands dealing with object file formats | |
1500 | @end ifclear | |
1501 | ||
1502 | * Miscellaneous Commands:: Other linker script commands | |
2c5c0674 RP |
1503 | @end menu |
1504 | ||
af54556a ILT |
1505 | @node Entry Point |
1506 | @subsection Setting the entry point | |
1507 | @kindex ENTRY(@var{symbol}) | |
1508 | @cindex start of execution | |
1509 | @cindex first instruction | |
1510 | @cindex entry point | |
1511 | The first instruction to execute in a program is called the @dfn{entry | |
1512 | point}. You can use the @code{ENTRY} linker script command to set the | |
1513 | entry point. The argument is a symbol name: | |
1514 | @smallexample | |
1515 | ENTRY(@var{symbol}) | |
1516 | @end smallexample | |
2c5c0674 | 1517 | |
af54556a ILT |
1518 | There are several ways to set the entry point. The linker will set the |
1519 | entry point by trying each of the following methods in order, and | |
1520 | stopping when one of them succeeds: | |
b4d4e8e3 | 1521 | @itemize @bullet |
2c5c0674 | 1522 | @item |
af54556a | 1523 | the @samp{-e} @var{entry} command-line option; |
2c5c0674 | 1524 | @item |
af54556a | 1525 | the @code{ENTRY(@var{symbol})} command in a linker script; |
2c5c0674 | 1526 | @item |
af54556a | 1527 | the value of the symbol @code{start}, if defined; |
2c5c0674 | 1528 | @item |
af54556a | 1529 | the address of the first byte of the @samp{.text} section, if present; |
2c5c0674 | 1530 | @item |
af54556a | 1531 | The address @code{0}. |
b4d4e8e3 | 1532 | @end itemize |
f22eee08 | 1533 | |
af54556a ILT |
1534 | @node File Commands |
1535 | @subsection Commands dealing with files | |
1536 | @cindex linker script file commands | |
1537 | Several linker script commands deal with files. | |
f22eee08 | 1538 | |
af54556a ILT |
1539 | @table @code |
1540 | @item INCLUDE @var{filename} | |
1541 | @kindex INCLUDE @var{filename} | |
1542 | @cindex including a linker script | |
1543 | Include the linker script @var{filename} at this point. The file will | |
1544 | be searched for in the current directory, and in any directory specified | |
1545 | with the @code{-L} option. You can nest calls to @code{INCLUDE} up to | |
1546 | 10 levels deep. | |
f22eee08 | 1547 | |
af54556a ILT |
1548 | @item INPUT(@var{file}, @var{file}, @dots{}) |
1549 | @itemx INPUT(@var{file} @var{file} @dots{}) | |
1550 | @kindex INPUT(@var{files}) | |
1551 | @cindex input files in linker scripts | |
1552 | @cindex input object files in linker scripts | |
1553 | @cindex linker script input object files | |
1554 | The @code{INPUT} command directs the linker to include the named files | |
1555 | in the link, as though they were named on the command line. | |
1556 | ||
1557 | For example, if you always want to include @file{subr.o} any time you do | |
1558 | a link, but you can't be bothered to put it on every link command line, | |
1559 | then you can put @samp{INPUT (subr.o)} in your linker script. | |
1560 | ||
1561 | In fact, if you like, you can list all of your input files in the linker | |
1562 | script, and then invoke the linker with nothing but a @samp{-T} option. | |
1563 | ||
1564 | The linker will first try to open the file in the current directory. If | |
1565 | it is not found, the linker will search through the archive library | |
1566 | search path. See the description of @samp{-L} in @ref{Options,,Command | |
1567 | Line Options}. | |
1568 | ||
1569 | If you use @samp{INPUT (-l@var{file})}, @code{ld} will transform the | |
1570 | name to @code{lib@var{file}.a}, as with the command line argument | |
1571 | @samp{-l}. | |
1572 | ||
1573 | When you use the @code{INPUT} command in an implicit linker script, the | |
1574 | files will be included in the link at the point at which the linker | |
1575 | script file is included. This can affect archive searching. | |
1576 | ||
1577 | @item GROUP(@var{file}, @var{file}, @dots{}) | |
1578 | @itemx GROUP(@var{file} @var{file} @dots{}) | |
1579 | @kindex GROUP(@var{files}) | |
1580 | @cindex grouping input files | |
1581 | The @code{GROUP} command is like @code{INPUT}, except that the named | |
1582 | files should all be archives, and they are searched repeatedly until no | |
1583 | new undefined references are created. See the description of @samp{-(} | |
1584 | in @ref{Options,,Command Line Options}. | |
1585 | ||
1586 | @item OUTPUT(@var{filename}) | |
1587 | @kindex OUTPUT(@var{filename}) | |
1588 | @cindex output file name in linker scripot | |
1589 | The @code{OUTPUT} command names the output file. Using | |
1590 | @code{OUTPUT(@var{filename})} in the linker script is exactly like using | |
1591 | @samp{-o @var{filename}} on the command line (@pxref{Options,,Command | |
1592 | Line Options}). If both are used, the command line option takes | |
1593 | precedence. | |
1594 | ||
1595 | You can use the @code{OUTPUT} command to define a default name for the | |
1596 | output file other than the usual default of @file{a.out}. | |
1597 | ||
1598 | @item SEARCH_DIR(@var{path}) | |
1599 | @kindex SEARCH_DIR(@var{path}) | |
1600 | @cindex library search path in linker script | |
1601 | @cindex archive search path in linker script | |
1602 | @cindex search path in linker script | |
1603 | The @code{SEARCH_DIR} command adds @var{path} to the list of paths where | |
1604 | @code{ld} looks for archive libraries. Using | |
1605 | @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} | |
1606 | on the command line (@pxref{Options,,Command Line Options}). If both | |
1607 | are used, then the linker will search both paths. Paths specified using | |
1608 | the command line option are searched first. | |
1609 | ||
1610 | @item STARTUP(@var{filename}) | |
1611 | @kindex STARTUP(@var{filename}) | |
1612 | @cindex first input file | |
1613 | The @code{STARTUP} command is just like the @code{INPUT} command, except | |
1614 | that @var{filename} will become the first input file to be linked, as | |
1615 | though it were specified first on the command line. This may be useful | |
1616 | when using a system in which the entry point is always the start of the | |
1617 | first file. | |
1618 | @end table | |
f22eee08 | 1619 | |
af54556a ILT |
1620 | @ifclear SingleFormat |
1621 | @node Format Commands | |
1622 | @subsection Commands dealing with object file formats | |
1623 | A couple of linker script commands deal with object file formats. | |
f22eee08 | 1624 | |
af54556a ILT |
1625 | @table @code |
1626 | @item OUTPUT_FORMAT(@var{bfdname}) | |
1627 | @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) | |
1628 | @kindex OUTPUT_FORMAT(@var{bfdname}) | |
1629 | @cindex output file format in linker script | |
1630 | The @code{OUTPUT_FORMAT} command names the BFD format to use for the | |
1631 | output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is | |
1632 | exactly like using @samp{-oformat @var{bfdname}} on the command line | |
1633 | (@pxref{Options,,Command Line Options}). If both are used, the command | |
1634 | line option takes precedence. | |
1635 | ||
1636 | You can use @code{OUTPUT_FORMAT} with three arguments to use different | |
1637 | formats based on the @samp{-EB} and @samp{-EL} command line options. | |
1638 | This permits the linker script to set the output format based on the | |
1639 | desired endianness. | |
1640 | ||
1641 | If neither @samp{-EB} nor @samp{-EL} are used, then the output format | |
1642 | will be the first argument, @var{default}. If @samp{-EB} is used, the | |
1643 | output format will be the second argument, @var{big}. If @samp{-EL} is | |
1644 | used, the output format will be the third argument, @var{little}. | |
1645 | ||
1646 | For example, the default linker script for the MIPS ELF target uses this | |
1647 | command: | |
c653b370 | 1648 | @smallexample |
af54556a | 1649 | OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) |
c653b370 | 1650 | @end smallexample |
af54556a ILT |
1651 | This says that the default format for the output file is |
1652 | @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line | |
1653 | option, the output file will be created in the @samp{elf32-littlemips} | |
1654 | format. | |
1655 | ||
1656 | @item TARGET(@var{bfdname}) | |
1657 | @kindex TARGET(@var{bfdname}) | |
1658 | @cindex input file format in linker script | |
1659 | The @code{TARGET} command names the BFD format to use when reading input | |
1660 | files. It affects subsequent @code{INPUT} and @code{GROUP} commands. | |
1661 | This command is like using @samp{-b @var{bfdname}} on the command line | |
1662 | (@pxref{Options,,Command Line Options}). If the @code{TARGET} command | |
1663 | is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} | |
1664 | command is also used to set the format for the output file. @xref{BFD}. | |
1665 | @end table | |
1666 | @end ifclear | |
b4d4e8e3 | 1667 | |
af54556a ILT |
1668 | @node Miscellaneous Commands |
1669 | @subsection Other linker script commands | |
1670 | There are a few other linker scripts commands. | |
b4d4e8e3 | 1671 | |
af54556a | 1672 | @table @code |
c470f2eb UD |
1673 | @item ASSERT(@var{exp}, @var{message}) |
1674 | @kindex ASSERT | |
1675 | @cindex assertion in linker script | |
1676 | Ensure that @var{exp} is non-zero. If it is zero, then exit the linker | |
1677 | with an error code, and print @var{message}. | |
1678 | ||
b2529df1 ILT |
1679 | @item EXTERN(@var{symbol} @var{symbol} @dots{}) |
1680 | @kindex EXTERN | |
1681 | @cindex undefined symbol in linker script | |
1682 | Force @var{symbol} to be entered in the output file as an undefined | |
1683 | symbol. Doing this may, for example, trigger linking of additional | |
1684 | modules from standard libraries. You may list several @var{symbol}s for | |
1685 | each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This | |
1686 | command has the same effect as the @samp{-u} command-line option. | |
1687 | ||
af54556a ILT |
1688 | @item FORCE_COMMON_ALLOCATION |
1689 | @kindex FORCE_COMMON_ALLOCATION | |
ed1cc83d | 1690 | @cindex common allocation in linker script |
af54556a ILT |
1691 | This command has the same effect as the @samp{-d} command-line option: |
1692 | to make @code{ld} assign space to common symbols even if a relocatable | |
1693 | output file is specified (@samp{-r}). | |
1fb57a5d | 1694 | |
af54556a ILT |
1695 | @item NOCROSSREFS(@var{section} @var{section} @dots{}) |
1696 | @kindex NOCROSSREFS(@var{sections}) | |
1697 | @cindex cross references | |
1698 | This command may be used to tell @code{ld} to issue an error about any | |
1699 | references among certain output sections. | |
b4d4e8e3 | 1700 | |
af54556a ILT |
1701 | In certain types of programs, particularly on embedded systems when |
1702 | using overlays, when one section is loaded into memory, another section | |
1703 | will not be. Any direct references between the two sections would be | |
1704 | errors. For example, it would be an error if code in one section called | |
1705 | a function defined in the other section. | |
d4e5e3c3 | 1706 | |
af54556a ILT |
1707 | The @code{NOCROSSREFS} command takes a list of output section names. If |
1708 | @code{ld} detects any cross references between the sections, it reports | |
1709 | an error and returns a non-zero exit status. Note that the | |
1710 | @code{NOCROSSREFS} command uses output section names, not input section | |
1711 | names. | |
f22eee08 | 1712 | |
af54556a ILT |
1713 | @ifclear SingleFormat |
1714 | @item OUTPUT_ARCH(@var{bfdarch}) | |
1715 | @kindex OUTPUT_ARCH(@var{bfdarch}) | |
1716 | @cindex machine architecture | |
1717 | @cindex architecture | |
1718 | Specify a particular output machine architecture. The argument is one | |
1719 | of the names used by the BFD library (@pxref{BFD}). You can see the | |
1720 | architecture of an object file by using the @code{objdump} program with | |
1721 | the @samp{-f} option. | |
1722 | @end ifclear | |
1723 | @end table | |
b4d4e8e3 | 1724 | |
af54556a ILT |
1725 | @node Assignments |
1726 | @section Assigning Values to Symbols | |
2c5c0674 RP |
1727 | @cindex assignment in scripts |
1728 | @cindex symbol definition, scripts | |
1729 | @cindex variables, defining | |
af54556a ILT |
1730 | You may assign a value to a symbol in a linker script. This will define |
1731 | the symbol as a global symbol. | |
1732 | ||
1733 | @menu | |
1734 | * Simple Assignments:: Simple Assignments | |
1735 | * PROVIDE:: PROVIDE | |
1736 | @end menu | |
1737 | ||
1738 | @node Simple Assignments | |
1739 | @subsection Simple Assignments | |
1740 | ||
1741 | You may assign to a symbol using any of the C assignment operators: | |
b4d4e8e3 RP |
1742 | |
1743 | @table @code | |
af54556a | 1744 | @item @var{symbol} = @var{expression} ; |
b4d4e8e3 RP |
1745 | @itemx @var{symbol} += @var{expression} ; |
1746 | @itemx @var{symbol} -= @var{expression} ; | |
1747 | @itemx @var{symbol} *= @var{expression} ; | |
1748 | @itemx @var{symbol} /= @var{expression} ; | |
af54556a ILT |
1749 | @itemx @var{symbol} <<= @var{expression} ; |
1750 | @itemx @var{symbol} >>= @var{expression} ; | |
1751 | @itemx @var{symbol} &= @var{expression} ; | |
1752 | @itemx @var{symbol} |= @var{expression} ; | |
b4d4e8e3 RP |
1753 | @end table |
1754 | ||
af54556a ILT |
1755 | The first case will define @var{symbol} to the value of |
1756 | @var{expression}. In the other cases, @var{symbol} must already be | |
1757 | defined, and the value will be adjusted accordingly. | |
2c5c0674 | 1758 | |
af54556a ILT |
1759 | The special symbol name @samp{.} indicates the location counter. You |
1760 | may only use this within a @code{SECTIONS} command. | |
b4d4e8e3 | 1761 | |
af54556a | 1762 | The semicolon after @var{expression} is required. |
b4d4e8e3 | 1763 | |
af54556a | 1764 | Expressions are defined below; see @ref{Expressions}. |
b4d4e8e3 | 1765 | |
af54556a ILT |
1766 | You may write symbol assignments as commands in their own right, or as |
1767 | statements within a @code{SECTIONS} command, or as part of an output | |
1768 | section description in a @code{SECTIONS} command. | |
1769 | ||
1770 | The section of the symbol will be set from the section of the | |
1771 | expression; for more information, see @ref{Expression Section}. | |
1772 | ||
1773 | Here is an example showing the three different places that symbol | |
1774 | assignments may be used: | |
b4d4e8e3 | 1775 | |
c653b370 | 1776 | @smallexample |
af54556a ILT |
1777 | floating_point = 0; |
1778 | SECTIONS | |
1779 | @{ | |
1780 | .text : | |
1781 | @{ | |
1782 | *(.text) | |
1783 | _etext = .; | |
d4e5e3c3 | 1784 | @} |
af54556a ILT |
1785 | _bdata = (. + 3) & ~ 4; |
1786 | .data : @{ *(.data) @} | |
1787 | @} | |
c653b370 | 1788 | @end smallexample |
2c5c0674 | 1789 | @noindent |
af54556a ILT |
1790 | In this example, the symbol @samp{floating_point} will be defined as |
1791 | zero. The symbol @samp{_etext} will be defined as the address following | |
1792 | the last @samp{.text} input section. The symbol @samp{_bdata} will be | |
1793 | defined as the address following the @samp{.text} output section aligned | |
1794 | upward to a 4 byte boundary. | |
1795 | ||
1796 | @node PROVIDE | |
1797 | @subsection PROVIDE | |
1798 | @cindex PROVIDE | |
0b3499f6 | 1799 | In some cases, it is desirable for a linker script to define a symbol |
af54556a ILT |
1800 | only if it is referenced and is not defined by any object included in |
1801 | the link. For example, traditional linkers defined the symbol | |
1802 | @samp{etext}. However, ANSI C requires that the user be able to use | |
1803 | @samp{etext} as a function name without encountering an error. The | |
1804 | @code{PROVIDE} keyword may be used to define a symbol, such as | |
0b3499f6 ILT |
1805 | @samp{etext}, only if it is referenced but not defined. The syntax is |
1806 | @code{PROVIDE(@var{symbol} = @var{expression})}. | |
b4d4e8e3 | 1807 | |
af54556a | 1808 | Here is an example of using @code{PROVIDE} to define @samp{etext}: |
c653b370 | 1809 | @smallexample |
af54556a ILT |
1810 | SECTIONS |
1811 | @{ | |
1812 | .text : | |
d4e5e3c3 | 1813 | @{ |
af54556a ILT |
1814 | *(.text) |
1815 | _etext = .; | |
1816 | PROVIDE(etext = .); | |
d4e5e3c3 | 1817 | @} |
af54556a | 1818 | @} |
c653b370 | 1819 | @end smallexample |
d4e5e3c3 | 1820 | |
36a8f215 ILT |
1821 | In this example, if the program defines @samp{_etext} (with a leading |
1822 | underscore), the linker will give a multiple definition error. If, on | |
1823 | the other hand, the program defines @samp{etext} (with no leading | |
1824 | underscore), the linker will silently use the definition in the program. | |
1825 | If the program references @samp{etext} but does not define it, the | |
1826 | linker will use the definition in the linker script. | |
f22eee08 | 1827 | |
af54556a ILT |
1828 | @node SECTIONS |
1829 | @section SECTIONS command | |
1830 | @kindex SECTIONS | |
1831 | The @code{SECTIONS} command tells the linker how to map input sections | |
1832 | into output sections, and how to place the output sections in memory. | |
2c5c0674 | 1833 | |
af54556a | 1834 | The format of the @code{SECTIONS} command is: |
c653b370 | 1835 | @smallexample |
af54556a ILT |
1836 | SECTIONS |
1837 | @{ | |
1838 | @var{sections-command} | |
1839 | @var{sections-command} | |
1840 | @dots{} | |
1841 | @} | |
c653b370 | 1842 | @end smallexample |
f22eee08 | 1843 | |
af54556a | 1844 | Each @var{sections-command} may of be one of the following: |
b61364cc | 1845 | |
af54556a ILT |
1846 | @itemize @bullet |
1847 | @item | |
1848 | an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) | |
1849 | @item | |
1850 | a symbol assignment (@pxref{Assignments}) | |
1851 | @item | |
1852 | an output section description | |
1853 | @item | |
1854 | an overlay description | |
1855 | @end itemize | |
2c5c0674 | 1856 | |
af54556a ILT |
1857 | The @code{ENTRY} command and symbol assignments are permitted inside the |
1858 | @code{SECTIONS} command for convenience in using the location counter in | |
1859 | those commands. This can also make the linker script easier to | |
1860 | understand because you can use those commands at meaningful points in | |
1861 | the layout of the output file. | |
67afbcea | 1862 | |
af54556a ILT |
1863 | Output section descriptions and overlay descriptions are described |
1864 | below. | |
67afbcea | 1865 | |
af54556a ILT |
1866 | If you do not use a @code{SECTIONS} command in your linker script, the |
1867 | linker will place each input section into an identically named output | |
1868 | section in the order that the sections are first encountered in the | |
1869 | input files. If all input sections are present in the first file, for | |
1870 | example, the order of sections in the output file will match the order | |
1871 | in the first input file. The first section will be at address zero. | |
67afbcea | 1872 | |
af54556a ILT |
1873 | @menu |
1874 | * Output Section Description:: Output section description | |
1875 | * Output Section Name:: Output section name | |
1876 | * Output Section Address:: Output section address | |
1877 | * Input Section:: Input section description | |
1878 | * Output Section Data:: Output section data | |
1879 | * Output Section Keywords:: Output section keywords | |
1880 | * Output Section Discarding:: Output section discarding | |
1881 | * Output Section Attributes:: Output section attributes | |
1882 | * Overlay Description:: Overlay description | |
1883 | @end menu | |
67afbcea | 1884 | |
af54556a ILT |
1885 | @node Output Section Description |
1886 | @subsection Output section description | |
1887 | The full description of an output section looks like this: | |
c653b370 | 1888 | @smallexample |
af54556a ILT |
1889 | @group |
1890 | @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] | |
d4e5e3c3 | 1891 | @{ |
af54556a ILT |
1892 | @var{output-section-command} |
1893 | @var{output-section-command} | |
d4e5e3c3 | 1894 | @dots{} |
af54556a | 1895 | @} [>@var{region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
c653b370 ILT |
1896 | @end group |
1897 | @end smallexample | |
5a59e34d | 1898 | |
af54556a | 1899 | Most output sections do not use most of the optional section attributes. |
5a59e34d | 1900 | |
af54556a ILT |
1901 | The whitespace around @var{section} is required, so that the section |
1902 | name is unambiguous. The colon and the curly braces are also required. | |
1903 | The line breaks and other white space are optional. | |
d4e5e3c3 | 1904 | |
af54556a | 1905 | Each @var{output-section-command} may be one of the following: |
d4e5e3c3 | 1906 | |
af54556a ILT |
1907 | @itemize @bullet |
1908 | @item | |
1909 | a symbol assignment (@pxref{Assignments}) | |
1910 | @item | |
1911 | an input section description (@pxref{Input Section}) | |
1912 | @item | |
1913 | data values to include directly (@pxref{Output Section Data}) | |
1914 | @item | |
1915 | a special output section keyword (@pxref{Output Section Keywords}) | |
1916 | @end itemize | |
f22eee08 | 1917 | |
af54556a ILT |
1918 | @node Output Section Name |
1919 | @subsection Output section name | |
1920 | @cindex name, section | |
1921 | @cindex section name | |
1922 | The name of the output section is @var{section}. @var{section} must | |
1923 | meet the constraints of your output format. In formats which only | |
1924 | support a limited number of sections, such as @code{a.out}, the name | |
1925 | must be one of the names supported by the format (@code{a.out}, for | |
1926 | example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the | |
1927 | output format supports any number of sections, but with numbers and not | |
1928 | names (as is the case for Oasys), the name should be supplied as a | |
1929 | quoted numeric string. A section name may consist of any sequence of | |
1930 | characters, but a name which contains any unusual characters such as | |
1931 | commas must be quoted. | |
1932 | ||
1933 | The output section name @samp{/DISCARD/} is special; @ref{Output Section | |
1934 | Discarding}. | |
1935 | ||
1936 | @node Output Section Address | |
1937 | @subsection Output section address | |
1938 | @cindex address, section | |
1939 | @cindex section address | |
1940 | The @var{address} is an expression for the VMA (the virtual memory | |
1941 | address) of the output section. If you do not provide @var{address}, | |
1942 | the linker will set it based on @var{region} if present, or otherwise | |
1943 | based on the current value of the location counter. | |
1944 | ||
1945 | If you provide @var{address}, the address of the output section will be | |
1946 | set to precisely that. If you provide neither @var{address} nor | |
1947 | @var{region}, then the address of the output section will be set to the | |
1948 | current value of the location counter aligned to the alignment | |
1949 | requirements of the output section. The alignment requirement of the | |
1950 | output section is the strictest alignment of any input section contained | |
1951 | within the output section. | |
1952 | ||
1953 | For example, | |
c653b370 | 1954 | @smallexample |
af54556a | 1955 | .text . : @{ *(.text) @} |
c653b370 | 1956 | @end smallexample |
af54556a ILT |
1957 | @noindent |
1958 | and | |
1959 | @smallexample | |
1960 | .text : @{ *(.text) @} | |
1961 | @end smallexample | |
1962 | @noindent | |
1963 | are subtly different. The first will set the address of the | |
1964 | @samp{.text} output section to the current value of the location | |
1965 | counter. The second will set it to the current value of the location | |
1966 | counter aligned to the strictest alignment of a @samp{.text} input | |
1967 | section. | |
1968 | ||
1969 | The @var{address} may be an arbitrary expression; @ref{Expressions}. | |
1970 | For example, if you want to align the section on a 0x10 byte boundary, | |
1971 | so that the lowest four bits of the section address are zero, you could | |
1972 | do something like this: | |
1973 | @smallexample | |
1974 | .text ALIGN(0x10) : @{ *(.text) @} | |
1975 | @end smallexample | |
1976 | @noindent | |
1977 | This works because @code{ALIGN} returns the current location counter | |
1978 | aligned upward to the specified value. | |
f22eee08 | 1979 | |
af54556a ILT |
1980 | Specifying @var{address} for a section will change the value of the |
1981 | location counter. | |
67c4333b | 1982 | |
af54556a ILT |
1983 | @node Input Section |
1984 | @subsection Input section description | |
1985 | @cindex input sections | |
1986 | @cindex mapping input sections to output sections | |
1987 | The most common output section command is an input section description. | |
67c4333b | 1988 | |
af54556a ILT |
1989 | The input section description is the most basic linker script operation. |
1990 | You use output sections to tell the linker how to lay out your program | |
1991 | in memory. You use input section descriptions to tell the linker how to | |
1992 | map the input files into your memory layout. | |
67c4333b | 1993 | |
af54556a ILT |
1994 | @menu |
1995 | * Input Section Basics:: Input section basics | |
1996 | * Input Section Wildcards:: Input section wildcard patterns | |
1997 | * Input Section Common:: Input section for common symbols | |
71b012a8 | 1998 | * Input Section Keep:: Input section and garbage collection |
af54556a ILT |
1999 | * Input Section Example:: Input section example |
2000 | @end menu | |
b4d4e8e3 | 2001 | |
af54556a ILT |
2002 | @node Input Section Basics |
2003 | @subsubsection Input section basics | |
2004 | @cindex input section basics | |
2005 | An input section description consists of a file name optionally followed | |
2006 | by a list of section names in parentheses. | |
f22eee08 | 2007 | |
af54556a ILT |
2008 | The file name and the section name may be wildcard patterns, which we |
2009 | describe further below (@pxref{Input Section Wildcards}). | |
b4d4e8e3 | 2010 | |
af54556a ILT |
2011 | The most common input section description is to include all input |
2012 | sections with a particular name in the output section. For example, to | |
2013 | include all input @samp{.text} sections, you would write: | |
2014 | @smallexample | |
2015 | *(.text) | |
2016 | @end smallexample | |
2017 | @noindent | |
2018 | Here the @samp{*} is a wildcard which matches any file name. | |
2c5c0674 | 2019 | |
af54556a | 2020 | There are two ways to include more than one section: |
c653b370 | 2021 | @smallexample |
af54556a ILT |
2022 | *(.text .rdata) |
2023 | *(.text) *(.rdata) | |
c653b370 | 2024 | @end smallexample |
b4d4e8e3 | 2025 | @noindent |
af54556a ILT |
2026 | The difference between these is the order in which the @samp{.text} and |
2027 | @samp{.rdata} input sections will appear in the output section. In the | |
2028 | first example, they will be intermingled. In the second example, all | |
2029 | @samp{.text} input sections will appear first, followed by all | |
2030 | @samp{.rdata} input sections. | |
2031 | ||
2032 | You can specify a file name to include sections from a particular file. | |
2033 | You would do this if one or more of your files contain special data that | |
2034 | needs to be at a particular location in memory. For example: | |
c653b370 | 2035 | @smallexample |
af54556a | 2036 | data.o(.data) |
c653b370 | 2037 | @end smallexample |
f9d3d71a | 2038 | |
af54556a ILT |
2039 | If you use a file name without a list of sections, then all sections in |
2040 | the input file will be included in the output section. This is not | |
2041 | commonly done, but it may by useful on occasion. For example: | |
2042 | @smallexample | |
2043 | data.o | |
2044 | @end smallexample | |
67c4333b | 2045 | |
af54556a ILT |
2046 | When you use a file name which does not contain any wild card |
2047 | characters, the linker will first see if you also specified the file | |
2048 | name on the linker command line or in an @code{INPUT} command. If you | |
2049 | did not, the linker will attempt to open the file as an input file, as | |
2050 | though it appeared on the command line. Note that this differs from an | |
2051 | @code{INPUT} command, because the linker will not search for the file in | |
2052 | the archive search path. | |
b4d4e8e3 | 2053 | |
af54556a ILT |
2054 | @node Input Section Wildcards |
2055 | @subsubsection Input section wildcard patterns | |
2056 | @cindex input section wildcards | |
2057 | @cindex wildcard file name patterns | |
2058 | @cindex file name wildcard patterns | |
2059 | @cindex section name wildcard patterns | |
2060 | In an input section description, either the file name or the section | |
2061 | name or both may be wildcard patterns. | |
f22eee08 | 2062 | |
af54556a ILT |
2063 | The file name of @samp{*} seen in many examples is a simple wildcard |
2064 | pattern for the file name. | |
f22eee08 | 2065 | |
af54556a | 2066 | The wildcard patterns are like those used by the Unix shell. |
b4d4e8e3 | 2067 | |
af54556a ILT |
2068 | @table @samp |
2069 | @item * | |
2070 | matches any number of characters | |
2071 | @item ? | |
2072 | matches any single character | |
2073 | @item [@var{chars}] | |
2074 | matches a single instance of any of the @var{chars}; the @samp{-} | |
2075 | character may be used to specify a range of characters, as in | |
2076 | @samp{[a-z]} to match any lower case letter | |
2077 | @item \ | |
2078 | quotes the following character | |
2079 | @end table | |
86bc0974 | 2080 | |
c2ba3684 ILT |
2081 | When a file name is matched with a wildcard, the wildcard characters |
2082 | will not match a @samp{/} character (used to separate directory names on | |
86bc0974 | 2083 | Unix). A pattern consisting of a single @samp{*} character is an |
af54556a ILT |
2084 | exception; it will always match any file name, whether it contains a |
2085 | @samp{/} or not. In a section name, the wildcard characters will match | |
2086 | a @samp{/} character. | |
2087 | ||
2088 | File name wildcard patterns only match files which are explicitly | |
2089 | specified on the command line or in an @code{INPUT} command. The linker | |
2090 | does not search directories to expand wildcards. | |
2091 | ||
2092 | If a file name matches more than one wildcard pattern, or if a file name | |
2093 | appears explicitly and is also matched by a wildcard pattern, the linker | |
2094 | will use the first match in the linker script. For example, this | |
2095 | sequence of input section descriptions is probably in error, because the | |
2096 | @file{data.o} rule will not be used: | |
2097 | @smallexample | |
2098 | .data : @{ *(.data) @} | |
2099 | .data1 : @{ data.o(.data) @} | |
2100 | @end smallexample | |
86bc0974 | 2101 | |
da713b8f ILT |
2102 | @cindex SORT |
2103 | Normally, the linker will place files and sections matched by wildcards | |
2104 | in the order in which they are seen during the link. You can change | |
2105 | this by using the @code{SORT} keyword, which appears before a wildcard | |
2106 | pattern in parentheses (e.g., @code{SORT(.text*)}). When the | |
2107 | @code{SORT} keyword is used, the linker will sort the files or sections | |
2108 | into ascending order by name before placing them in the output file. | |
2109 | ||
af54556a ILT |
2110 | If you ever get confused about where input sections are going, use the |
2111 | @samp{-M} linker option to generate a map file. The map file shows | |
2112 | precisely how input sections are mapped to output sections. | |
d4e5e3c3 | 2113 | |
af54556a ILT |
2114 | This example shows how wildcard patterns might be used to partition |
2115 | files. This linker script directs the linker to place all @samp{.text} | |
2116 | sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. | |
2117 | The linker will place the @samp{.data} section from all files beginning | |
2118 | with an upper case character in @samp{.DATA}; for all other files, the | |
2119 | linker will place the @samp{.data} section in @samp{.data}. | |
c653b370 ILT |
2120 | @smallexample |
2121 | @group | |
af54556a | 2122 | SECTIONS @{ |
d76ae847 | 2123 | .text : @{ *(.text) @} |
af54556a ILT |
2124 | .DATA : @{ [A-Z]*(.data) @} |
2125 | .data : @{ *(.data) @} | |
2126 | .bss : @{ *(.bss) @} | |
2127 | @} | |
c653b370 ILT |
2128 | @end group |
2129 | @end smallexample | |
b4d4e8e3 | 2130 | |
af54556a ILT |
2131 | @node Input Section Common |
2132 | @subsubsection Input section for common symbols | |
2133 | @cindex common symbol placement | |
2134 | @cindex uninitialized data placement | |
2135 | A special notation is needed for common symbols, because in many object | |
2136 | file formats common symbols do not have a particular input section. The | |
2137 | linker treats common symbols as though they are in an input section | |
2138 | named @samp{COMMON}. | |
2139 | ||
2140 | You may use file names with the @samp{COMMON} section just as with any | |
2141 | other input sections. You can use this to place common symbols from a | |
2142 | particular input file in one section while common symbols from other | |
2143 | input files are placed in another section. | |
2144 | ||
2145 | In most cases, common symbols in input files will be placed in the | |
2146 | @samp{.bss} section in the output file. For example: | |
2147 | @smallexample | |
2148 | .bss @{ *(.bss) *(COMMON) @} | |
2149 | @end smallexample | |
2150 | ||
2151 | @cindex scommon section | |
2152 | @cindex small common symbols | |
2153 | Some object file formats have more than one type of common symbol. For | |
2154 | example, the MIPS ELF object file format distinguishes standard common | |
2155 | symbols and small common symbols. In this case, the linker will use a | |
2156 | different special section name for other types of common symbols. In | |
2157 | the case of MIPS ELF, the linker uses @samp{COMMON} for standard common | |
2158 | symbols and @samp{.scommon} for small common symbols. This permits you | |
2159 | to map the different types of common symbols into memory at different | |
2160 | locations. | |
2161 | ||
2162 | @cindex [COMMON] | |
2163 | You will sometimes see @samp{[COMMON]} in old linker scripts. This | |
2164 | notation is now considered obsolete. It is equivalent to | |
2165 | @samp{*(COMMON)}. | |
2166 | ||
71b012a8 RH |
2167 | @node Input Section Keep |
2168 | @subsubsection Input section and garbage collection | |
2169 | @cindex KEEP | |
2170 | @cindex garbage collection | |
2171 | When link-time garbage collection is in use (@samp{--gc-sections}), | |
2172 | it is often useful to mark sections that should not be eliminated. | |
2173 | This is accomplished by surrounding an input section's wildcard entry | |
2174 | with @code{KEEP()}, as in @code{KEEP(*(.init))} or | |
2175 | @code{KEEP(SORT(*)(.ctors))}. | |
2176 | ||
af54556a ILT |
2177 | @node Input Section Example |
2178 | @subsubsection Input section example | |
2179 | The following example is a complete linker script. It tells the linker | |
2180 | to read all of the sections from file @file{all.o} and place them at the | |
2181 | start of output section @samp{outputa} which starts at location | |
2182 | @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} | |
2183 | follows immediately, in the same output section. All of section | |
2184 | @samp{.input2} from @file{foo.o} goes into output section | |
2185 | @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. | |
2186 | All of the remaining @samp{.input1} and @samp{.input2} sections from any | |
2187 | files are written to output section @samp{outputc}. | |
b4d4e8e3 | 2188 | |
c653b370 ILT |
2189 | @smallexample |
2190 | @group | |
2c5c0674 | 2191 | SECTIONS @{ |
d4e5e3c3 DM |
2192 | outputa 0x10000 : |
2193 | @{ | |
2194 | all.o | |
2195 | foo.o (.input1) | |
2196 | @} | |
2197 | outputb : | |
2198 | @{ | |
2199 | foo.o (.input2) | |
2200 | foo1.o (.input1) | |
2201 | @} | |
2202 | outputc : | |
2203 | @{ | |
2204 | *(.input1) | |
2205 | *(.input2) | |
2206 | @} | |
2c5c0674 | 2207 | @} |
c653b370 ILT |
2208 | @end group |
2209 | @end smallexample | |
b4d4e8e3 | 2210 | |
af54556a ILT |
2211 | @node Output Section Data |
2212 | @subsection Output section data | |
2213 | @cindex data | |
2214 | @cindex section data | |
2215 | @cindex output section data | |
2216 | @kindex BYTE(@var{expression}) | |
2217 | @kindex SHORT(@var{expression}) | |
2218 | @kindex LONG(@var{expression}) | |
2219 | @kindex QUAD(@var{expression}) | |
2220 | @kindex SQUAD(@var{expression}) | |
2221 | You can include explicit bytes of data in an output section by using | |
2222 | @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as | |
2223 | an output section command. Each keyword is followed by an expression in | |
2224 | parentheses providing the value to store (@pxref{Expressions}). The | |
2225 | value of the expression is stored at the current value of the location | |
2226 | counter. | |
2227 | ||
2228 | The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands | |
2229 | store one, two, four, and eight bytes (respectively). After storing the | |
2230 | bytes, the location counter is incremented by the number of bytes | |
2231 | stored. | |
2232 | ||
2233 | For example, this will store the byte 1 followed by the four byte value | |
2234 | of the symbol @samp{addr}: | |
2235 | @smallexample | |
2236 | BYTE(1) | |
2237 | LONG(addr) | |
2238 | @end smallexample | |
2239 | ||
2240 | When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the | |
2241 | same; they both store an 8 byte, or 64 bit, value. When both host and | |
2242 | target are 32 bits, an expression is computed as 32 bits. In this case | |
2243 | @code{QUAD} stores a 32 bit value zero extended to 64 bits, and | |
2244 | @code{SQUAD} stores a 32 bit value sign extended to 64 bits. | |
86bc0974 | 2245 | |
af54556a ILT |
2246 | If the object file format of the output file has an explicit endianness, |
2247 | which is the normal case, the value will be stored in that endianness. | |
2248 | When the object file format does not have an explicit endianness, as is | |
2249 | true of, for example, S-records, the value will be stored in the | |
2250 | endianness of the first input object file. | |
2251 | ||
2252 | @kindex FILL(@var{expression}) | |
2253 | @cindex holes, filling | |
2254 | @cindex unspecified memory | |
2255 | You may use the @code{FILL} command to set the fill pattern for the | |
2256 | current section. It is followed by an expression in parentheses. Any | |
2257 | otherwise unspecified regions of memory within the section (for example, | |
2258 | gaps left due to the required alignment of input sections) are filled | |
2259 | with the two least significant bytes of the expression, repeated as | |
2260 | necessary. A @code{FILL} statement covers memory locations after the | |
2261 | point at which it occurs in the section definition; by including more | |
2262 | than one @code{FILL} statement, you can have different fill patterns in | |
2263 | different parts of an output section. | |
2264 | ||
2265 | This example shows how to fill unspecified regions of memory with the | |
2266 | value @samp{0x9090}: | |
86bc0974 | 2267 | @smallexample |
af54556a | 2268 | FILL(0x9090) |
86bc0974 ILT |
2269 | @end smallexample |
2270 | ||
af54556a ILT |
2271 | The @code{FILL} command is similar to the @samp{=@var{fillexp}} output |
2272 | section attribute (@pxref{Output Section Fill}), but it only affects the | |
2273 | part of the section following the @code{FILL} command, rather than the | |
2274 | entire section. If both are used, the @code{FILL} command takes | |
2275 | precedence. | |
67c4333b | 2276 | |
af54556a ILT |
2277 | @node Output Section Keywords |
2278 | @subsection Output section keywords | |
2279 | There are a couple of keywords which can appear as output section | |
2280 | commands. | |
f22eee08 | 2281 | |
b4d4e8e3 | 2282 | @table @code |
af54556a | 2283 | @kindex CREATE_OBJECT_SYMBOLS |
2c5c0674 RP |
2284 | @cindex input filename symbols |
2285 | @cindex filename symbols | |
d4e5e3c3 | 2286 | @item CREATE_OBJECT_SYMBOLS |
af54556a ILT |
2287 | The command tells the linker to create a symbol for each input file. |
2288 | The name of each symbol will be the name of the corresponding input | |
2289 | file. The section of each symbol will be the output section in which | |
2290 | the @code{CREATE_OBJECT_SYMBOLS} command appears. | |
b4d4e8e3 | 2291 | |
af54556a ILT |
2292 | This is conventional for the a.out object file format. It is not |
2293 | normally used for any other object file format. | |
f22eee08 | 2294 | |
af54556a ILT |
2295 | @kindex CONSTRUCTORS |
2296 | @cindex C++ constructors, arranging in link | |
2297 | @cindex constructors, arranging in link | |
2298 | @item CONSTRUCTORS | |
2299 | When linking using the a.out object file format, the linker uses an | |
2300 | unusual set construct to support C++ global constructors and | |
2301 | destructors. When linking object file formats which do not support | |
2302 | arbitrary sections, such as ECOFF and XCOFF, the linker will | |
2303 | automatically recognize C++ global constructors and destructors by name. | |
2304 | For these object file formats, the @code{CONSTRUCTORS} command tells the | |
2305 | linker to place constructor information in the output section where the | |
2306 | @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is | |
2307 | ignored for other object file formats. | |
f22eee08 | 2308 | |
af54556a ILT |
2309 | The symbol @w{@code{__CTOR_LIST__}} marks the start of the global |
2310 | constructors, and the symbol @w{@code{__DTOR_LIST}} marks the end. The | |
2311 | first word in the list is the number of entries, followed by the address | |
2312 | of each constructor or destructor, followed by a zero word. The | |
2313 | compiler must arrange to actually run the code. For these object file | |
2314 | formats @sc{gnu} C++ normally calls constructors from a subroutine | |
2315 | @code{__main}; a call to @code{__main} is automatically inserted into | |
2316 | the startup code for @code{main}. @sc{gnu} C++ normally runs | |
2317 | destructors either by using @code{atexit}, or directly from the function | |
2318 | @code{exit}. | |
2c5c0674 | 2319 | |
af54556a ILT |
2320 | For object file formats such as @code{COFF} or @code{ELF} which support |
2321 | arbitrary section names, @sc{gnu} C++ will normally arrange to put the | |
2322 | addresses of global constructors and destructors into the @code{.ctors} | |
2323 | and @code{.dtors} sections. Placing the following sequence into your | |
2324 | linker script will build the sort of table which the @sc{gnu} C++ | |
2325 | runtime code expects to see. | |
d4e5e3c3 | 2326 | |
c653b370 | 2327 | @smallexample |
af54556a ILT |
2328 | __CTOR_LIST__ = .; |
2329 | LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) | |
2330 | *(.ctors) | |
2331 | LONG(0) | |
2332 | __CTOR_END__ = .; | |
2333 | __DTOR_LIST__ = .; | |
2334 | LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) | |
2335 | *(.dtors) | |
2336 | LONG(0) | |
2337 | __DTOR_END__ = .; | |
c653b370 | 2338 | @end smallexample |
d4e5e3c3 | 2339 | |
c470f2eb UD |
2340 | If you are using the @sc{gnu} C++ support for initialization priority, |
2341 | which provides some control over the order in which global constructors | |
2342 | are run, you must sort the constructors at link time to ensure that they | |
2343 | are executed in the correct order. When using the @code{CONSTRUCTORS} | |
2344 | command, use @samp{SORT(CONSTRUCTORS)} instead. When using the | |
2345 | @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT(.ctors))} and | |
2346 | @samp{*(SORT(.dtors))} instead of just @samp{*(.ctors)} and | |
2347 | @samp{*(.dtors)}. | |
2348 | ||
af54556a ILT |
2349 | Normally the compiler and linker will handle these issues automatically, |
2350 | and you will not need to concern yourself with them. However, you may | |
2351 | need to consider this if you are using C++ and writing your own linker | |
2352 | scripts. | |
c470f2eb | 2353 | |
b4d4e8e3 RP |
2354 | @end table |
2355 | ||
af54556a ILT |
2356 | @node Output Section Discarding |
2357 | @subsection Output section discarding | |
2358 | @cindex discarding sections | |
2359 | @cindex sections, discarding | |
2360 | @cindex removing sections | |
2361 | The linker will not create output section which do not have any | |
2362 | contents. This is for convenience when referring to input sections that | |
2363 | may or may not be present in any of the input files. For example: | |
d76ae847 | 2364 | @smallexample |
af54556a | 2365 | .foo @{ *(.foo) @} |
d76ae847 | 2366 | @end smallexample |
af54556a ILT |
2367 | @noindent |
2368 | will only create a @samp{.foo} section in the output file if there is a | |
2369 | @samp{.foo} section in at least one input file. | |
b4d4e8e3 | 2370 | |
af54556a ILT |
2371 | If you use anything other than an input section description as an output |
2372 | section command, such as a symbol assignment, then the output section | |
2373 | will always be created, even if there are no matching input sections. | |
f22eee08 | 2374 | |
71b012a8 | 2375 | @cindex /DISCARD/ |
af54556a ILT |
2376 | The special output section name @samp{/DISCARD/} may be used to discard |
2377 | input sections. Any input sections which are assigned to an output | |
2378 | section named @samp{/DISCARD/} are not included in the output file. | |
d4e5e3c3 | 2379 | |
af54556a ILT |
2380 | @node Output Section Attributes |
2381 | @subsection Output section attributes | |
2382 | @cindex output section attributes | |
2383 | We showed above that the full description of an output section looked | |
2384 | like this: | |
c653b370 | 2385 | @smallexample |
af54556a ILT |
2386 | @group |
2387 | @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] | |
2388 | @{ | |
2389 | @var{output-section-command} | |
2390 | @var{output-section-command} | |
d4e5e3c3 | 2391 | @dots{} |
af54556a | 2392 | @} [>@var{region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
c653b370 ILT |
2393 | @end group |
2394 | @end smallexample | |
af54556a ILT |
2395 | We've already described @var{section}, @var{address}, and |
2396 | @var{output-section-command}. In this section we will describe the | |
2397 | remaining section attributes. | |
2398 | ||
2399 | @menu | |
2400 | * Output Section Type:: Output section type | |
2401 | * Output Section LMA:: Output section LMA | |
2402 | * Output Section Region:: Output section region | |
2403 | * Output Section Phdr:: Output section phdr | |
2404 | * Output Section Fill:: Output section fill | |
2405 | @end menu | |
f22eee08 | 2406 | |
af54556a ILT |
2407 | @node Output Section Type |
2408 | @subsubsection Output section type | |
2409 | Each output section may have a type. The type is a keyword in | |
2410 | parentheses. The following types are defined: | |
2411 | ||
2412 | @table @code | |
2413 | @item NOLOAD | |
2414 | The section should be marked as not loadable, so that it will not be | |
2415 | loaded into memory when the program is run. | |
2416 | @item DSECT | |
2417 | @itemx COPY | |
2418 | @itemx INFO | |
2419 | @itemx OVERLAY | |
2420 | These type names are supported for backward compatibility, and are | |
2421 | rarely used. They all have the same effect: the section should be | |
2422 | marked as not allocatable, so that no memory is allocated for the | |
2423 | section when the program is run. | |
2424 | @end table | |
f22eee08 | 2425 | |
d76ae847 RP |
2426 | @kindex NOLOAD |
2427 | @cindex prevent unnecessary loading | |
67c4333b | 2428 | @cindex loading, preventing |
af54556a ILT |
2429 | The linker normally sets the attributes of an output section based on |
2430 | the input sections which map into it. You can override this by using | |
2431 | the section type. For example, in the script sample below, the | |
2432 | @samp{ROM} section is addressed at memory location @samp{0} and does not | |
2433 | need to be loaded when the program is run. The contents of the | |
2434 | @samp{ROM} section will appear in the linker output file as usual. | |
c653b370 ILT |
2435 | @smallexample |
2436 | @group | |
d76ae847 | 2437 | SECTIONS @{ |
af54556a | 2438 | ROM 0 (NOLOAD) : @{ @dots{} @} |
d4e5e3c3 | 2439 | @dots{} |
d76ae847 | 2440 | @} |
c653b370 ILT |
2441 | @end group |
2442 | @end smallexample | |
d76ae847 | 2443 | |
af54556a ILT |
2444 | @node Output Section LMA |
2445 | @subsubsection Output section LMA | |
2446 | @kindex AT(@var{lma}) | |
2447 | @cindex load address | |
2448 | @cindex section load address | |
2449 | Every section has a virtual address (VMA) and a load address (LMA); see | |
2450 | @ref{Basic Script Concepts}. The address expression which may appear in | |
2451 | an output section description sets the VMA (@pxref{Output Section | |
2452 | Address}). | |
2453 | ||
2454 | The linker will normally set the LMA equal to the VMA. You can change | |
2455 | that by using the @code{AT} keyword. The expression @var{lma} that | |
2456 | follows the @code{AT} keyword specifies the load address of the section. | |
2457 | ||
2458 | @cindex ROM initialized data | |
2459 | @cindex initialized data in ROM | |
2460 | This feature is designed to make it easy to build a ROM image. For | |
2461 | example, the following linker script creates three output sections: one | |
2462 | called @samp{.text}, which starts at @code{0x1000}, one called | |
2463 | @samp{.mdata}, which is loaded at the end of the @samp{.text} section | |
2464 | even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold | |
2465 | uninitialized data at address @code{0x3000}. The symbol @code{_data} is | |
2466 | defined with the value @code{0x2000}, which shows that the location | |
2467 | counter holds the VMA value, not the LMA value. | |
67c4333b RP |
2468 | |
2469 | @smallexample | |
c653b370 | 2470 | @group |
67c4333b | 2471 | SECTIONS |
139c8857 RP |
2472 | @{ |
2473 | .text 0x1000 : @{ *(.text) _etext = . ; @} | |
2474 | .mdata 0x2000 : | |
af54556a | 2475 | AT ( ADDR (.text) + SIZEOF (.text) ) |
139c8857 RP |
2476 | @{ _data = . ; *(.data); _edata = . ; @} |
2477 | .bss 0x3000 : | |
2478 | @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} | |
67c4333b | 2479 | @} |
c653b370 | 2480 | @end group |
67c4333b RP |
2481 | @end smallexample |
2482 | ||
af54556a ILT |
2483 | The run-time initialization code for use with a program generated with |
2484 | this linker script would include something like the following, to copy | |
2485 | the initialized data from the ROM image to its runtime address. Notice | |
2486 | how this code takes advantage of the symbols defined by the linker | |
2487 | script. | |
67c4333b | 2488 | |
139c8857 | 2489 | @smallexample |
c653b370 | 2490 | @group |
af54556a ILT |
2491 | extern char _etext, _data, _edata, _bstart, _bend; |
2492 | char *src = &_etext; | |
2493 | char *dst = &_data; | |
67c4333b | 2494 | |
139c8857 | 2495 | /* ROM has data at end of text; copy it. */ |
af54556a | 2496 | while (dst < &_edata) @{ |
139c8857 | 2497 | *dst++ = *src++; |
67c4333b RP |
2498 | @} |
2499 | ||
2500 | /* Zero bss */ | |
af54556a | 2501 | for (dst = &_bstart; dst< &_bend; dst++) |
139c8857 | 2502 | *dst = 0; |
c653b370 | 2503 | @end group |
139c8857 | 2504 | @end smallexample |
67c4333b | 2505 | |
af54556a ILT |
2506 | @node Output Section Region |
2507 | @subsubsection Output section region | |
f9d3d71a ILT |
2508 | @kindex >@var{region} |
2509 | @cindex section, assigning to memory region | |
2510 | @cindex memory regions and sections | |
af54556a ILT |
2511 | You can assign a section to a previously defined region of memory by |
2512 | using @samp{>@var{region}}. @xref{MEMORY}. | |
2513 | ||
2514 | Here is a simple example: | |
2515 | @smallexample | |
2516 | @group | |
2517 | MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} | |
2518 | SECTIONS @{ ROM : @{ *(.text) @} >rom @} | |
2519 | @end group | |
2520 | @end smallexample | |
f9d3d71a | 2521 | |
af54556a ILT |
2522 | @node Output Section Phdr |
2523 | @subsubsection Output section phdr | |
c653b370 ILT |
2524 | @kindex :@var{phdr} |
2525 | @cindex section, assigning to program header | |
2526 | @cindex program headers and sections | |
af54556a ILT |
2527 | You can assign a section to a previously defined program segment by |
2528 | using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to | |
2529 | one or more segments, then all subsequent allocated sections will be | |
2530 | assigned to those segments as well, unless they use an explicitly | |
36a8f215 ILT |
2531 | @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the |
2532 | linker to not put the section in any segment at all. | |
af54556a ILT |
2533 | |
2534 | Here is a simple example: | |
2535 | @smallexample | |
2536 | @group | |
2537 | PHDRS @{ text PT_LOAD ; @} | |
2538 | SECTIONS @{ .text : @{ *(.text) @} :text @} | |
2539 | @end group | |
2540 | @end smallexample | |
2541 | ||
2542 | @node Output Section Fill | |
2543 | @subsubsection Output section fill | |
2544 | @kindex =@var{fillexp} | |
2c5c0674 RP |
2545 | @cindex section fill pattern |
2546 | @cindex fill pattern, entire section | |
af54556a ILT |
2547 | You can set the fill pattern for an entire section by using |
2548 | @samp{=@var{fillexp}}. @var{fillexp} is an expression | |
2549 | (@pxref{Expressions}). Any otherwise unspecified regions of memory | |
2550 | within the output section (for example, gaps left due to the required | |
2551 | alignment of input sections) will be filled with the two least | |
2552 | significant bytes of the value, repeated as necessary. | |
f22eee08 | 2553 | |
af54556a ILT |
2554 | You can also change the fill value with a @code{FILL} command in the |
2555 | output section commands; see @ref{Output Section Data}. | |
b4d4e8e3 | 2556 | |
af54556a ILT |
2557 | Here is a simple example: |
2558 | @smallexample | |
2559 | @group | |
2560 | SECTIONS @{ .text : @{ *(.text) @} =0x9090 @} | |
2561 | @end group | |
2562 | @end smallexample | |
2563 | ||
2564 | @node Overlay Description | |
2565 | @subsection Overlay description | |
b61364cc ILT |
2566 | @kindex OVERLAY |
2567 | @cindex overlays | |
af54556a ILT |
2568 | An overlay description provides an easy way to describe sections which |
2569 | are to be loaded as part of a single memory image but are to be run at | |
2570 | the same memory address. At run time, some sort of overlay manager will | |
2571 | copy the overlaid sections in and out of the runtime memory address as | |
2572 | required, perhaps by simply manipulating addressing bits. This approach | |
2573 | can be useful, for example, when a certain region of memory is faster | |
2574 | than another. | |
2575 | ||
2576 | Overlays are described using the @code{OVERLAY} command. The | |
2577 | @code{OVERLAY} command is used within a @code{SECTIONS} command, like an | |
2578 | output section description. The full syntax of the @code{OVERLAY} | |
2579 | command is as follows: | |
b61364cc ILT |
2580 | @smallexample |
2581 | @group | |
af54556a ILT |
2582 | OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] |
2583 | @{ | |
2584 | @var{secname1} | |
2585 | @{ | |
2586 | @var{output-section-command} | |
2587 | @var{output-section-command} | |
2588 | @dots{} | |
2589 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
2590 | @var{secname2} | |
2591 | @{ | |
2592 | @var{output-section-command} | |
2593 | @var{output-section-command} | |
2594 | @dots{} | |
2595 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
2596 | @dots{} | |
2597 | @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] | |
b61364cc ILT |
2598 | @end group |
2599 | @end smallexample | |
2600 | ||
2601 | Everything is optional except @code{OVERLAY} (a keyword), and each | |
2602 | section must have a name (@var{secname1} and @var{secname2} above). The | |
2603 | section definitions within the @code{OVERLAY} construct are identical to | |
2604 | those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), | |
2605 | except that no addresses and no memory regions may be defined for | |
2606 | sections within an @code{OVERLAY}. | |
2607 | ||
2608 | The sections are all defined with the same starting address. The load | |
2609 | addresses of the sections are arranged such that they are consecutive in | |
2610 | memory starting at the load address used for the @code{OVERLAY} as a | |
2611 | whole (as with normal section definitions, the load address is optional, | |
2612 | and defaults to the start address; the start address is also optional, | |
af54556a | 2613 | and defaults to the current value of the location counter). |
b61364cc ILT |
2614 | |
2615 | If the @code{NOCROSSREFS} keyword is used, and there any references | |
2616 | among the sections, the linker will report an error. Since the sections | |
2617 | all run at the same address, it normally does not make sense for one | |
af54556a | 2618 | section to refer directly to another. @xref{Miscellaneous Commands, |
b61364cc ILT |
2619 | NOCROSSREFS}. |
2620 | ||
2621 | For each section within the @code{OVERLAY}, the linker automatically | |
2622 | defines two symbols. The symbol @code{__load_start_@var{secname}} is | |
2623 | defined as the starting load address of the section. The symbol | |
2624 | @code{__load_stop_@var{secname}} is defined as the final load address of | |
2625 | the section. Any characters within @var{secname} which are not legal | |
2626 | within C identifiers are removed. C (or assembler) code may use these | |
2627 | symbols to move the overlaid sections around as necessary. | |
2628 | ||
af54556a ILT |
2629 | At the end of the overlay, the value of the location counter is set to |
2630 | the start address of the overlay plus the size of the largest section. | |
b61364cc ILT |
2631 | |
2632 | Here is an example. Remember that this would appear inside a | |
2633 | @code{SECTIONS} construct. | |
b61364cc ILT |
2634 | @smallexample |
2635 | @group | |
2636 | OVERLAY 0x1000 : AT (0x4000) | |
2637 | @{ | |
2638 | .text0 @{ o1/*.o(.text) @} | |
2639 | .text1 @{ o2/*.o(.text) @} | |
2640 | @} | |
2641 | @end group | |
2642 | @end smallexample | |
af54556a ILT |
2643 | @noindent |
2644 | This will define both @samp{.text0} and @samp{.text1} to start at | |
2645 | address 0x1000. @samp{.text0} will be loaded at address 0x4000, and | |
2646 | @samp{.text1} will be loaded immediately after @samp{.text0}. The | |
b61364cc ILT |
2647 | following symbols will be defined: @code{__load_start_text0}, |
2648 | @code{__load_stop_text0}, @code{__load_start_text1}, | |
2649 | @code{__load_stop_text1}. | |
2650 | ||
2651 | C code to copy overlay @code{.text1} into the overlay area might look | |
2652 | like the following. | |
2653 | ||
2654 | @smallexample | |
2655 | @group | |
2656 | extern char __load_start_text1, __load_stop_text1; | |
2657 | memcpy ((char *) 0x1000, &__load_start_text1, | |
2658 | &__load_stop_text1 - &__load_start_text1); | |
2659 | @end group | |
2660 | @end smallexample | |
2661 | ||
2662 | Note that the @code{OVERLAY} command is just syntactic sugar, since | |
2663 | everything it does can be done using the more basic commands. The above | |
2664 | example could have been written identically as follows. | |
2665 | ||
2666 | @smallexample | |
2667 | @group | |
2668 | .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} | |
2669 | __load_start_text0 = LOADADDR (.text0); | |
2670 | __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0); | |
2671 | .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} | |
2672 | __load_start_text1 = LOADADDR (.text1); | |
2673 | __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1); | |
2674 | . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); | |
2675 | @end group | |
2676 | @end smallexample | |
2677 | ||
af54556a ILT |
2678 | @node MEMORY |
2679 | @section MEMORY command | |
2680 | @kindex MEMORY | |
2681 | @cindex memory regions | |
2682 | @cindex regions of memory | |
2683 | @cindex allocating memory | |
2684 | @cindex discontinuous memory | |
2685 | The linker's default configuration permits allocation of all available | |
2686 | memory. You can override this by using the @code{MEMORY} command. | |
2687 | ||
2688 | The @code{MEMORY} command describes the location and size of blocks of | |
2689 | memory in the target. You can use it to describe which memory regions | |
2690 | may be used by the linker, and which memory regions it must avoid. You | |
2691 | can then assign sections to particular memory regions. The linker will | |
2692 | set section addresses based on the memory regions, and will warn about | |
2693 | regions that become too full. The linker will not shuffle sections | |
2694 | around to fit into the available regions. | |
2695 | ||
2696 | A linker script may contain at most one use of the @code{MEMORY} | |
2697 | command. However, you can define as many blocks of memory within it as | |
2698 | you wish. The syntax is: | |
2699 | @smallexample | |
2700 | @group | |
2701 | MEMORY | |
2702 | @{ | |
2703 | @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} | |
2704 | @dots{} | |
2705 | @} | |
2706 | @end group | |
2707 | @end smallexample | |
2708 | ||
2709 | The @var{name} is a name used in the linker script to refer to the | |
2710 | region. The region name has no meaning outside of the linker script. | |
2711 | Region names are stored in a separate name space, and will not conflict | |
2712 | with symbol names, file names, or section names. Each memory region | |
2713 | must have a distinct name. | |
2714 | ||
2715 | @cindex memory region attributes | |
2716 | The @var{attr} string is an optional list of attributes that specify | |
2717 | whether to use a particular memory region for an input section which is | |
2718 | not explicitly mapped in the linker script. As described in | |
2719 | @ref{SECTIONS}, if you do not specify an output section for some input | |
2720 | section, the linker will create an output section with the same name as | |
2721 | the input section. If you define region attributes, the linker will use | |
2722 | them to select the memory region for the output section that it creates. | |
2723 | ||
2724 | The @var{attr} string must consist only of the following characters: | |
2725 | @table @samp | |
2726 | @item R | |
2727 | Read-only section | |
2728 | @item W | |
2729 | Read/write section | |
2730 | @item X | |
2731 | Executable section | |
2732 | @item A | |
2733 | Allocatable section | |
2734 | @item I | |
2735 | Initialized section | |
2736 | @item L | |
2737 | Same as @samp{I} | |
2738 | @item ! | |
2739 | Invert the sense of any of the preceding attributes | |
2740 | @end table | |
2741 | ||
2742 | If a unmapped section matches any of the listed attributes other than | |
2743 | @samp{!}, it will be placed in the memory region. The @samp{!} | |
2744 | attribute reverses this test, so that an unmapped section will be placed | |
2745 | in the memory region only if it does not match any of the listed | |
2746 | attributes. | |
2747 | ||
2748 | @kindex ORIGIN = | |
2749 | @kindex o = | |
2750 | @kindex org = | |
2751 | The @var{origin} is an expression for the start address of the memory | |
2752 | region. The expression must evaluate to a constant before memory | |
2753 | allocation is performed, which means that you may not use any section | |
2754 | relative symbols. The keyword @code{ORIGIN} may be abbreviated to | |
2755 | @code{org} or @code{o} (but not, for example, @code{ORG}). | |
2756 | ||
2757 | @kindex LENGTH = | |
2758 | @kindex len = | |
2759 | @kindex l = | |
2760 | The @var{len} is an expression for the size in bytes of the memory | |
2761 | region. As with the @var{origin} expression, the expression must | |
2762 | evaluate to a constant before memory allocation is performed. The | |
2763 | keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. | |
2764 | ||
2765 | In the following example, we specify that there are two memory regions | |
2766 | available for allocation: one starting at @samp{0} for 256 kilobytes, | |
2767 | and the other starting at @samp{0x40000000} for four megabytes. The | |
2768 | linker will place into the @samp{rom} memory region every section which | |
2769 | is not explicitly mapped into a memory region, and is either read-only | |
2770 | or executable. The linker will place other sections which are not | |
2771 | explicitly mapped into a memory region into the @samp{ram} memory | |
2772 | region. | |
2773 | ||
2774 | @smallexample | |
2775 | @group | |
2776 | MEMORY | |
2777 | @{ | |
2778 | rom (rx) : ORIGIN = 0, LENGTH = 256K | |
2779 | ram (!rx) : org = 0x40000000, l = 4M | |
2780 | @} | |
2781 | @end group | |
2782 | @end smallexample | |
2783 | ||
f8a86f8f ILT |
2784 | Once you define a memory region, you can direct the linker to place |
2785 | specific output sections into that memory region by using the | |
2786 | @samp{>@var{region}} output section attribute. For example, if you have | |
2787 | a memory region named @samp{mem}, you would use @samp{>mem} in the | |
2788 | output section definition. @xref{Output Section Region}. If no address | |
2789 | was specified for the output section, the linker will set the address to | |
2790 | the next available address within the memory region. If the combined | |
2791 | output sections directed to a memory region are too large for the | |
2792 | region, the linker will issue an error message. | |
af54556a | 2793 | |
c653b370 | 2794 | @node PHDRS |
af54556a | 2795 | @section PHDRS Command |
c653b370 | 2796 | @kindex PHDRS |
b61364cc ILT |
2797 | @cindex program headers |
2798 | @cindex ELF program headers | |
af54556a ILT |
2799 | @cindex program segments |
2800 | @cindex segments, ELF | |
2801 | The ELF object file format uses @dfn{program headers}, also knows as | |
2802 | @dfn{segments}. The program headers describe how the program should be | |
2803 | loaded into memory. You can print them out by using the @code{objdump} | |
2804 | program with the @samp{-p} option. | |
2805 | ||
2806 | When you run an ELF program on a native ELF system, the system loader | |
2807 | reads the program headers in order to figure out how to load the | |
2808 | program. This will only work if the program headers are set correctly. | |
2809 | This manual does not describe the details of how the system loader | |
2810 | interprets program headers; for more information, see the ELF ABI. | |
2811 | ||
2812 | The linker will create reasonable program headers by default. However, | |
2813 | in some cases, you may need to specify the program headers more | |
2814 | precisely. You may use the @code{PHDRS} command for this purpose. When | |
2815 | the linker sees the @code{PHDRS} command in the linker script, it will | |
2816 | not create any program headers other than the ones specified. | |
2817 | ||
2818 | The linker only pays attention to the @code{PHDRS} command when | |
2819 | generating an ELF output file. In other cases, the linker will simply | |
2820 | ignore @code{PHDRS}. | |
c653b370 ILT |
2821 | |
2822 | This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, | |
2823 | @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. | |
2824 | ||
2825 | @smallexample | |
2826 | @group | |
2827 | PHDRS | |
2828 | @{ | |
2829 | @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] | |
2830 | [ FLAGS ( @var{flags} ) ] ; | |
2831 | @} | |
2832 | @end group | |
2833 | @end smallexample | |
2834 | ||
2835 | The @var{name} is used only for reference in the @code{SECTIONS} command | |
af54556a ILT |
2836 | of the linker script. It is not put into the output file. Program |
2837 | header names are stored in a separate name space, and will not conflict | |
2838 | with symbol names, file names, or section names. Each program header | |
2839 | must have a distinct name. | |
2840 | ||
2841 | Certain program header types describe segments of memory which the | |
2842 | system loader will load from the file. In the linker script, you | |
2843 | specify the contents of these segments by placing allocatable output | |
2844 | sections in the segments. You use the @samp{:@var{phdr}} output section | |
2845 | attribute to place a section in a particular segment. @xref{Output | |
2846 | Section Phdr}. | |
2847 | ||
2848 | It is normal to put certain sections in more than one segment. This | |
2849 | merely implies that one segment of memory contains another. You may | |
2850 | repeat @samp{:@var{phdr}}, using it once for each segment which should | |
2851 | contain the section. | |
2852 | ||
2853 | If you place a section in one or more segments using @samp{:@var{phdr}}, | |
2854 | then the linker will place all subsequent allocatable sections which do | |
2855 | not specify @samp{:@var{phdr}} in the same segments. This is for | |
c653b370 | 2856 | convenience, since generally a whole set of contiguous sections will be |
36a8f215 ILT |
2857 | placed in a single segment. You can use @code{:NONE} to override the |
2858 | default segment and tell the linker to not put the section in any | |
2859 | segment at all. | |
c653b370 | 2860 | |
af54556a ILT |
2861 | @kindex FILEHDR |
2862 | @kindex PHDRS | |
2863 | You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after | |
2864 | the program header type to further describe the contents of the segment. | |
c653b370 ILT |
2865 | The @code{FILEHDR} keyword means that the segment should include the ELF |
2866 | file header. The @code{PHDRS} keyword means that the segment should | |
2867 | include the ELF program headers themselves. | |
2868 | ||
2869 | The @var{type} may be one of the following. The numbers indicate the | |
2870 | value of the keyword. | |
2871 | ||
2872 | @table @asis | |
2873 | @item @code{PT_NULL} (0) | |
2874 | Indicates an unused program header. | |
2875 | ||
2876 | @item @code{PT_LOAD} (1) | |
2877 | Indicates that this program header describes a segment to be loaded from | |
2878 | the file. | |
2879 | ||
2880 | @item @code{PT_DYNAMIC} (2) | |
2881 | Indicates a segment where dynamic linking information can be found. | |
2882 | ||
2883 | @item @code{PT_INTERP} (3) | |
2884 | Indicates a segment where the name of the program interpreter may be | |
2885 | found. | |
2886 | ||
2887 | @item @code{PT_NOTE} (4) | |
2888 | Indicates a segment holding note information. | |
2889 | ||
2890 | @item @code{PT_SHLIB} (5) | |
2891 | A reserved program header type, defined but not specified by the ELF | |
2892 | ABI. | |
2893 | ||
2894 | @item @code{PT_PHDR} (6) | |
2895 | Indicates a segment where the program headers may be found. | |
2896 | ||
2897 | @item @var{expression} | |
2898 | An expression giving the numeric type of the program header. This may | |
2899 | be used for types not defined above. | |
2900 | @end table | |
2901 | ||
af54556a ILT |
2902 | You can specify that a segment should be loaded at a particular address |
2903 | in memory by using an @code{AT} expression. This is identical to the | |
2904 | @code{AT} command used as an output section attribute (@pxref{Output | |
2905 | Section LMA}). The @code{AT} command for a program header overrides the | |
2906 | output section attribute. | |
c653b370 | 2907 | |
af54556a ILT |
2908 | The linker will normally set the segment flags based on the sections |
2909 | which comprise the segment. You may use the @code{FLAGS} keyword to | |
2910 | explicitly specify the segment flags. The value of @var{flags} must be | |
2911 | an integer. It is used to set the @code{p_flags} field of the program | |
2912 | header. | |
c653b370 | 2913 | |
af54556a ILT |
2914 | Here is an example of @code{PHDRS}. This shows a typical set of program |
2915 | headers used on a native ELF system. | |
c653b370 ILT |
2916 | |
2917 | @example | |
2918 | @group | |
2919 | PHDRS | |
2920 | @{ | |
2921 | headers PT_PHDR PHDRS ; | |
2922 | interp PT_INTERP ; | |
2923 | text PT_LOAD FILEHDR PHDRS ; | |
2924 | data PT_LOAD ; | |
2925 | dynamic PT_DYNAMIC ; | |
2926 | @} | |
2927 | ||
2928 | SECTIONS | |
2929 | @{ | |
2930 | . = SIZEOF_HEADERS; | |
2931 | .interp : @{ *(.interp) @} :text :interp | |
2932 | .text : @{ *(.text) @} :text | |
2933 | .rodata : @{ *(.rodata) @} /* defaults to :text */ | |
2934 | @dots{} | |
2935 | . = . + 0x1000; /* move to a new page in memory */ | |
2936 | .data : @{ *(.data) @} :data | |
2937 | .dynamic : @{ *(.dynamic) @} :data :dynamic | |
2938 | @dots{} | |
2939 | @} | |
2940 | @end group | |
2941 | @end example | |
2942 | ||
af54556a ILT |
2943 | @node VERSION |
2944 | @section VERSION Command | |
5a59e34d ILT |
2945 | @kindex VERSION @{script text@} |
2946 | @cindex symbol versions | |
2947 | @cindex version script | |
2948 | @cindex versions of symbols | |
af54556a ILT |
2949 | The linker supports symbol versions when using ELF. Symbol versions are |
2950 | only useful when using shared libraries. The dynamic linker can use | |
2951 | symbol versions to select a specific version of a function when it runs | |
2952 | a program that may have been linked against an earlier version of the | |
2953 | shared library. | |
2954 | ||
2955 | You can include a version script directly in the main linker script, or | |
2956 | you can supply the version script as an implicit linker script. You can | |
2957 | also use the @samp{--version-script} linker option. | |
2958 | ||
2959 | The syntax of the @code{VERSION} command is simply | |
5a59e34d | 2960 | @smallexample |
af54556a | 2961 | VERSION @{ version-script-commands @} |
5a59e34d | 2962 | @end smallexample |
af54556a ILT |
2963 | |
2964 | The format of the version script commands is identical to that used by | |
2965 | Sun's linker in Solaris 2.5. The version script defines a tree of | |
2966 | version nodes. You specify the node names and interdependencies in the | |
2967 | version script. You can specify which symbols are bound to which | |
2968 | version nodes, and you can reduce a specified set of symbols to local | |
2969 | scope so that they are not globally visible outside of the shared | |
5a59e34d ILT |
2970 | library. |
2971 | ||
2972 | The easiest way to demonstrate the version script language is with a few | |
2973 | examples. | |
2974 | ||
2975 | @smallexample | |
2976 | VERS_1.1 @{ | |
2977 | global: | |
2978 | foo1; | |
2979 | local: | |
2980 | old*; | |
2981 | original*; | |
2982 | new*; | |
2983 | @}; | |
2984 | ||
2985 | VERS_1.2 @{ | |
2986 | foo2; | |
2987 | @} VERS_1.1; | |
2988 | ||
2989 | VERS_2.0 @{ | |
2990 | bar1; bar2; | |
2991 | @} VERS_1.2; | |
2992 | @end smallexample | |
2993 | ||
af54556a ILT |
2994 | This example version script defines three version nodes. The first |
2995 | version node defined is @samp{VERS_1.1}; it has no other dependencies. | |
2996 | The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces | |
2997 | a number of symbols to local scope so that they are not visible outside | |
2998 | of the shared library. | |
5a59e34d | 2999 | |
af54556a ILT |
3000 | Next, the version script defines node @samp{VERS_1.2}. This node |
3001 | depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} | |
3002 | to the version node @samp{VERS_1.2}. | |
5a59e34d | 3003 | |
af54556a ILT |
3004 | Finally, the version script defines node @samp{VERS_2.0}. This node |
3005 | depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} | |
3006 | and @samp{bar2} are bound to the version node @samp{VERS_2.0}. | |
5a59e34d | 3007 | |
af54556a ILT |
3008 | When the linker finds a symbol defined in a library which is not |
3009 | specifically bound to a version node, it will effectively bind it to an | |
3010 | unspecified base version of the library. You can bind all otherwise | |
3011 | unspecified symbols to a given version node by using @samp{global: *} | |
3012 | somewhere in the version script. | |
5a59e34d | 3013 | |
af54556a ILT |
3014 | The names of the version nodes have no specific meaning other than what |
3015 | they might suggest to the person reading them. The @samp{2.0} version | |
3016 | could just as well have appeared in between @samp{1.1} and @samp{1.2}. | |
3017 | However, this would be a confusing way to write a version script. | |
5a59e34d ILT |
3018 | |
3019 | When you link an application against a shared library that has versioned | |
af54556a ILT |
3020 | symbols, the application itself knows which version of each symbol it |
3021 | requires, and it also knows which version nodes it needs from each | |
3022 | shared library it is linked against. Thus at runtime, the dynamic | |
3023 | loader can make a quick check to make sure that the libraries you have | |
3024 | linked against do in fact supply all of the version nodes that the | |
3025 | application will need to resolve all of the dynamic symbols. In this | |
3026 | way it is possible for the dynamic linker to know with certainty that | |
3027 | all external symbols that it needs will be resolvable without having to | |
3028 | search for each symbol reference. | |
5a59e34d ILT |
3029 | |
3030 | The symbol versioning is in effect a much more sophisticated way of | |
3031 | doing minor version checking that SunOS does. The fundamental problem | |
3032 | that is being addressed here is that typically references to external | |
3033 | functions are bound on an as-needed basis, and are not all bound when | |
3034 | the application starts up. If a shared library is out of date, a | |
3035 | required interface may be missing; when the application tries to use | |
3036 | that interface, it may suddenly and unexpectedly fail. With symbol | |
3037 | versioning, the user will get a warning when they start their program if | |
3038 | the libraries being used with the application are too old. | |
3039 | ||
3040 | There are several GNU extensions to Sun's versioning approach. The | |
3041 | first of these is the ability to bind a symbol to a version node in the | |
3042 | source file where the symbol is defined instead of in the versioning | |
3043 | script. This was done mainly to reduce the burden on the library | |
af54556a | 3044 | maintainer. You can do this by putting something like: |
5a59e34d ILT |
3045 | @smallexample |
3046 | __asm__(".symver original_foo,foo@@VERS_1.1"); | |
3047 | @end smallexample | |
af54556a ILT |
3048 | @noindent |
3049 | in the C source file. This renames the function @samp{original_foo} to | |
5a59e34d ILT |
3050 | be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. |
3051 | The @samp{local:} directive can be used to prevent the symbol | |
3052 | @samp{original_foo} from being exported. | |
3053 | ||
af54556a ILT |
3054 | The second GNU extension is to allow multiple versions of the same |
3055 | function to appear in a given shared library. In this way you can make | |
3056 | an incompatible change to an interface without increasing the major | |
3057 | version number of the shared library, while still allowing applications | |
3058 | linked against the old interface to continue to function. | |
5a59e34d | 3059 | |
af54556a ILT |
3060 | To do this, you must use multiple @samp{.symver} directives in the |
3061 | source file. Here is an example: | |
5a59e34d ILT |
3062 | |
3063 | @smallexample | |
3064 | __asm__(".symver original_foo,foo@@"); | |
3065 | __asm__(".symver old_foo,foo@@VERS_1.1"); | |
3066 | __asm__(".symver old_foo1,foo@@VERS_1.2"); | |
3067 | __asm__(".symver new_foo,foo@@@@VERS_2.0"); | |
3068 | @end smallexample | |
3069 | ||
3070 | In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the | |
3071 | unspecified base version of the symbol. The source file that contains this | |
3072 | example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, | |
3073 | @samp{old_foo1}, and @samp{new_foo}. | |
3074 | ||
3075 | When you have multiple definitions of a given symbol, there needs to be | |
3076 | some way to specify a default version to which external references to | |
af54556a ILT |
3077 | this symbol will be bound. You can do this with the |
3078 | @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only | |
3079 | declare one version of a symbol as the default in this manner; otherwise | |
3080 | you would effectively have multiple definitions of the same symbol. | |
5a59e34d ILT |
3081 | |
3082 | If you wish to bind a reference to a specific version of the symbol | |
3083 | within the shared library, you can use the aliases of convenience | |
3084 | (i.e. @samp{old_foo}), or you can use the @samp{.symver} directive to | |
3085 | specifically bind to an external version of the function in question. | |
3086 | ||
af54556a ILT |
3087 | @node Expressions |
3088 | @section Expressions in Linker Scripts | |
3089 | @cindex expressions | |
3090 | @cindex arithmetic | |
3091 | The syntax for expressions in the linker script language is identical to | |
3092 | that of C expressions. All expressions are evaluated as integers. All | |
3093 | expressions are evaluated in the same size, which is 32 bits if both the | |
3094 | host and target are 32 bits, and is otherwise 64 bits. | |
b4d4e8e3 | 3095 | |
af54556a | 3096 | You can use and set symbol values in expressions. |
a1d393cf | 3097 | |
af54556a ILT |
3098 | The linker defines several special purpose builtin functions for use in |
3099 | expressions. | |
a1d393cf | 3100 | |
af54556a ILT |
3101 | @menu |
3102 | * Constants:: Constants | |
3103 | * Symbols:: Symbol Names | |
3104 | * Location Counter:: The Location Counter | |
3105 | * Operators:: Operators | |
3106 | * Evaluation:: Evaluation | |
3107 | * Expression Section:: The Section of an Expression | |
3108 | * Builtin Functions:: Builtin Functions | |
3109 | @end menu | |
a1d393cf | 3110 | |
af54556a ILT |
3111 | @node Constants |
3112 | @subsection Constants | |
3113 | @cindex integer notation | |
3114 | @cindex constants in linker scripts | |
3115 | All constants are integers. | |
3116 | ||
3117 | As in C, the linker considers an integer beginning with @samp{0} to be | |
3118 | octal, and an integer beginning with @samp{0x} or @samp{0X} to be | |
3119 | hexadecimal. The linker considers other integers to be decimal. | |
3120 | ||
3121 | @cindex scaled integers | |
3122 | @cindex K and M integer suffixes | |
3123 | @cindex M and K integer suffixes | |
3124 | @cindex suffixes for integers | |
3125 | @cindex integer suffixes | |
3126 | In addition, you can use the suffixes @code{K} and @code{M} to scale a | |
3127 | constant by | |
3128 | @c TEXI2ROFF-KILL | |
3129 | @ifinfo | |
3130 | @c END TEXI2ROFF-KILL | |
3131 | @code{1024} or @code{1024*1024} | |
3132 | @c TEXI2ROFF-KILL | |
3133 | @end ifinfo | |
3134 | @tex | |
3135 | ${\rm 1024}$ or ${\rm 1024}^2$ | |
3136 | @end tex | |
3137 | @c END TEXI2ROFF-KILL | |
3138 | respectively. For example, the following all refer to the same quantity: | |
a1d393cf | 3139 | @smallexample |
af54556a ILT |
3140 | _fourk_1 = 4K; |
3141 | _fourk_2 = 4096; | |
3142 | _fourk_3 = 0x1000; | |
a1d393cf ILT |
3143 | @end smallexample |
3144 | ||
af54556a ILT |
3145 | @node Symbols |
3146 | @subsection Symbol Names | |
3147 | @cindex symbol names | |
3148 | @cindex names | |
3149 | @cindex quoted symbol names | |
3150 | @kindex " | |
3151 | Unless quoted, symbol names start with a letter, underscore, or period | |
3152 | and may include letters, digits, underscores, periods, and hyphens. | |
3153 | Unquoted symbol names must not conflict with any keywords. You can | |
3154 | specify a symbol which contains odd characters or has the same name as a | |
3155 | keyword by surrounding the symbol name in double quotes: | |
3156 | @smallexample | |
3157 | "SECTION" = 9; | |
3158 | "with a space" = "also with a space" + 10; | |
3159 | @end smallexample | |
1fb57a5d | 3160 | |
af54556a ILT |
3161 | Since symbols can contain many non-alphabetic characters, it is safest |
3162 | to delimit symbols with spaces. For example, @samp{A-B} is one symbol, | |
3163 | whereas @samp{A - B} is an expression involving subtraction. | |
2c5c0674 | 3164 | |
af54556a ILT |
3165 | @node Location Counter |
3166 | @subsection The Location Counter | |
3167 | @kindex . | |
3168 | @cindex dot | |
3169 | @cindex location counter | |
3170 | @cindex current output location | |
3171 | The special linker variable @dfn{dot} @samp{.} always contains the | |
3172 | current output location counter. Since the @code{.} always refers to a | |
3173 | location in an output section, it may only appear in an expression | |
3174 | within a @code{SECTIONS} command. The @code{.} symbol may appear | |
3175 | anywhere that an ordinary symbol is allowed in an expression. | |
b4d4e8e3 | 3176 | |
af54556a ILT |
3177 | @cindex holes |
3178 | Assigning a value to @code{.} will cause the location counter to be | |
3179 | moved. This may be used to create holes in the output section. The | |
3180 | location counter may never be moved backwards. | |
3181 | ||
3182 | @smallexample | |
3183 | SECTIONS | |
3184 | @{ | |
3185 | output : | |
3186 | @{ | |
3187 | file1(.text) | |
3188 | . = . + 1000; | |
3189 | file2(.text) | |
3190 | . += 1000; | |
3191 | file3(.text) | |
3192 | @} = 0x1234; | |
3193 | @} | |
3194 | @end smallexample | |
3195 | @noindent | |
3196 | In the previous example, the @samp{.text} section from @file{file1} is | |
3197 | located at the beginning of the output section @samp{output}. It is | |
3198 | followed by a 1000 byte gap. Then the @samp{.text} section from | |
3199 | @file{file2} appears, also with a 1000 byte gap following before the | |
3200 | @samp{.text} section from @file{file3}. The notation @samp{= 0x1234} | |
3201 | specifies what data to write in the gaps (@pxref{Output Section Fill}). | |
5a59e34d | 3202 | |
af54556a ILT |
3203 | @need 2000 |
3204 | @node Operators | |
3205 | @subsection Operators | |
3206 | @cindex operators for arithmetic | |
3207 | @cindex arithmetic operators | |
3208 | @cindex precedence in expressions | |
3209 | The linker recognizes the standard C set of arithmetic operators, with | |
3210 | the standard bindings and precedence levels: | |
3211 | @c TEXI2ROFF-KILL | |
3212 | @ifinfo | |
3213 | @c END TEXI2ROFF-KILL | |
3214 | @smallexample | |
3215 | precedence associativity Operators Notes | |
3216 | (highest) | |
3217 | 1 left ! - ~ (1) | |
3218 | 2 left * / % | |
3219 | 3 left + - | |
3220 | 4 left >> << | |
3221 | 5 left == != > < <= >= | |
3222 | 6 left & | |
3223 | 7 left | | |
3224 | 8 left && | |
3225 | 9 left || | |
3226 | 10 right ? : | |
3227 | 11 right &= += -= *= /= (2) | |
3228 | (lowest) | |
3229 | @end smallexample | |
3230 | Notes: | |
3231 | (1) Prefix operators | |
3232 | (2) @xref{Assignments}. | |
3233 | @c TEXI2ROFF-KILL | |
3234 | @end ifinfo | |
3235 | @tex | |
3236 | \vskip \baselineskip | |
3237 | %"lispnarrowing" is the extra indent used generally for smallexample | |
3238 | \hskip\lispnarrowing\vbox{\offinterlineskip | |
3239 | \hrule | |
3240 | \halign | |
3241 | {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr | |
3242 | height2pt&\omit&&\omit&&\omit&\cr | |
3243 | &Precedence&& Associativity &&{\rm Operators}&\cr | |
3244 | height2pt&\omit&&\omit&&\omit&\cr | |
3245 | \noalign{\hrule} | |
3246 | height2pt&\omit&&\omit&&\omit&\cr | |
3247 | &highest&&&&&\cr | |
3248 | % '176 is tilde, '~' in tt font | |
3249 | &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr | |
3250 | &2&&left&&* / \%&\cr | |
3251 | &3&&left&&+ -&\cr | |
3252 | &4&&left&&>> <<&\cr | |
3253 | &5&&left&&== != > < <= >=&\cr | |
3254 | &6&&left&&\&&\cr | |
3255 | &7&&left&&|&\cr | |
3256 | &8&&left&&{\&\&}&\cr | |
3257 | &9&&left&&||&\cr | |
3258 | &10&&right&&? :&\cr | |
3259 | &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr | |
3260 | &lowest&&&&&\cr | |
3261 | height2pt&\omit&&\omit&&\omit&\cr} | |
3262 | \hrule} | |
3263 | @end tex | |
3264 | @iftex | |
3265 | { | |
3266 | @obeylines@parskip=0pt@parindent=0pt | |
3267 | @dag@quad Prefix operators. | |
3268 | @ddag@quad @xref{Assignments}. | |
3269 | } | |
3270 | @end iftex | |
3271 | @c END TEXI2ROFF-KILL | |
7f9ae73e | 3272 | |
af54556a ILT |
3273 | @node Evaluation |
3274 | @subsection Evaluation | |
3275 | @cindex lazy evaluation | |
3276 | @cindex expression evaluation order | |
3277 | The linker evaluates expressions lazily. It only computes the value of | |
3278 | an expression when absolutely necessary. | |
b4d4e8e3 | 3279 | |
af54556a ILT |
3280 | The linker needs some information, such as the value of the start |
3281 | address of the first section, and the origins and lengths of memory | |
3282 | regions, in order to do any linking at all. These values are computed | |
3283 | as soon as possible when the linker reads in the linker script. | |
01bc8f35 | 3284 | |
af54556a ILT |
3285 | However, other values (such as symbol values) are not known or needed |
3286 | until after storage allocation. Such values are evaluated later, when | |
3287 | other information (such as the sizes of output sections) is available | |
3288 | for use in the symbol assignment expression. | |
0b3499f6 | 3289 | |
af54556a ILT |
3290 | The sizes of sections cannot be known until after allocation, so |
3291 | assignments dependent upon these are not performed until after | |
3292 | allocation. | |
b4d4e8e3 | 3293 | |
af54556a ILT |
3294 | Some expressions, such as those depending upon the location counter |
3295 | @samp{.}, must be evaluated during section allocation. | |
2c5c0674 | 3296 | |
af54556a ILT |
3297 | If the result of an expression is required, but the value is not |
3298 | available, then an error results. For example, a script like the | |
3299 | following | |
3300 | @smallexample | |
3301 | @group | |
3302 | SECTIONS | |
3303 | @{ | |
3304 | .text 9+this_isnt_constant : | |
3305 | @{ *(.text) @} | |
3306 | @} | |
3307 | @end group | |
3308 | @end smallexample | |
3309 | @noindent | |
3310 | will cause the error message @samp{non constant expression for initial | |
3311 | address}. | |
3312 | ||
3313 | @node Expression Section | |
3314 | @subsection The Section of an Expression | |
3315 | @cindex expression sections | |
3316 | @cindex absolute expressions | |
3317 | @cindex relative expressions | |
3318 | @cindex absolute and relocatable symbols | |
3319 | @cindex relocatable and absolute symbols | |
3320 | @cindex symbols, relocatable and absolute | |
3321 | When the linker evaluates an expression, the result is either absolute | |
3322 | or relative to some section. A relative expression is expressed as a | |
3323 | fixed offset from the base of a section. | |
3324 | ||
3325 | The position of the expression within the linker script determines | |
3326 | whether it is absolute or relative. An expression which appears within | |
3327 | an output section definition is relative to the base of the output | |
3328 | section. An expression which appears elsewhere will be absolute. | |
3329 | ||
3330 | A symbol set to a relative expression will be relocatable if you request | |
3331 | relocatable output using the @samp{-r} option. That means that a | |
3332 | further link operation may change the value of the symbol. The symbol's | |
3333 | section will be the section of the relative expression. | |
3334 | ||
3335 | A symbol set to an absolute expression will retain the same value | |
3336 | through any further link operation. The symbol will be absolute, and | |
3337 | will not have any particular associated section. | |
3338 | ||
3339 | You can use the builtin function @code{ABSOLUTE} to force an expression | |
3340 | to be absolute when it would otherwise be relative. For example, to | |
3341 | create an absolute symbol set to the address of the end of the output | |
3342 | section @samp{.data}: | |
3343 | @smallexample | |
3344 | SECTIONS | |
3345 | @{ | |
3346 | .data : @{ *(.data) _edata = ABSOLUTE(.); @} | |
3347 | @} | |
3348 | @end smallexample | |
3349 | @noindent | |
3350 | If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the | |
3351 | @samp{.data} section. | |
2c5c0674 | 3352 | |
af54556a ILT |
3353 | @node Builtin Functions |
3354 | @subsection Builtin Functions | |
3355 | @cindex functions in expressions | |
3356 | The linker script language includes a number of builtin functions for | |
3357 | use in linker script expressions. | |
2c5c0674 | 3358 | |
af54556a ILT |
3359 | @table @code |
3360 | @item ABSOLUTE(@var{exp}) | |
3361 | @kindex ABSOLUTE(@var{exp}) | |
3362 | @cindex expression, absolute | |
3363 | Return the absolute (non-relocatable, as opposed to non-negative) value | |
3364 | of the expression @var{exp}. Primarily useful to assign an absolute | |
3365 | value to a symbol within a section definition, where symbol values are | |
3366 | normally section relative. @xref{Expression Section}. | |
b4d4e8e3 | 3367 | |
af54556a ILT |
3368 | @item ADDR(@var{section}) |
3369 | @kindex ADDR(@var{section}) | |
3370 | @cindex section address in expression | |
3371 | Return the absolute address (the VMA) of the named @var{section}. Your | |
3372 | script must previously have defined the location of that section. In | |
3373 | the following example, @code{symbol_1} and @code{symbol_2} are assigned | |
3374 | identical values: | |
3375 | @smallexample | |
3376 | @group | |
3377 | SECTIONS @{ @dots{} | |
3378 | .output1 : | |
3379 | @{ | |
3380 | start_of_output_1 = ABSOLUTE(.); | |
3381 | @dots{} | |
3382 | @} | |
3383 | .output : | |
3384 | @{ | |
3385 | symbol_1 = ADDR(.output1); | |
3386 | symbol_2 = start_of_output_1; | |
3387 | @} | |
3388 | @dots{} @} | |
3389 | @end group | |
3390 | @end smallexample | |
2c5c0674 | 3391 | |
af54556a ILT |
3392 | @item ALIGN(@var{exp}) |
3393 | @kindex ALIGN(@var{exp}) | |
3394 | @cindex round up location counter | |
3395 | @cindex align location counter | |
3396 | Return the location counter (@code{.}) aligned to the next @var{exp} | |
3397 | boundary. @var{exp} must be an expression whose value is a power of | |
3398 | two. This is equivalent to | |
3399 | @smallexample | |
3400 | (. + @var{exp} - 1) & ~(@var{exp} - 1) | |
3401 | @end smallexample | |
582dd77f | 3402 | |
af54556a ILT |
3403 | @code{ALIGN} doesn't change the value of the location counter---it just |
3404 | does arithmetic on it. Here is an example which aligns the output | |
3405 | @code{.data} section to the next @code{0x2000} byte boundary after the | |
3406 | preceding section and sets a variable within the section to the next | |
3407 | @code{0x8000} boundary after the input sections: | |
3408 | @smallexample | |
3409 | @group | |
3410 | SECTIONS @{ @dots{} | |
3411 | .data ALIGN(0x2000): @{ | |
3412 | *(.data) | |
3413 | variable = ALIGN(0x8000); | |
3414 | @} | |
3415 | @dots{} @} | |
3416 | @end group | |
3417 | @end smallexample | |
3418 | @noindent | |
3419 | The first use of @code{ALIGN} in this example specifies the location of | |
3420 | a section because it is used as the optional @var{address} attribute of | |
3421 | a section definition (@pxref{Output Section Address}). The second use | |
3422 | of @code{ALIGN} is used to defines the value of a symbol. | |
582dd77f | 3423 | |
af54556a | 3424 | The builtin function @code{NEXT} is closely related to @code{ALIGN}. |
582dd77f | 3425 | |
af54556a ILT |
3426 | @item BLOCK(@var{exp}) |
3427 | @kindex BLOCK(@var{exp}) | |
3428 | This is a synonym for @code{ALIGN}, for compatibility with older linker | |
3429 | scripts. It is most often seen when setting the address of an output | |
3430 | section. | |
3431 | ||
3432 | @item DEFINED(@var{symbol}) | |
3433 | @kindex DEFINED(@var{symbol}) | |
3434 | @cindex symbol defaults | |
3435 | Return 1 if @var{symbol} is in the linker global symbol table and is | |
3436 | defined, otherwise return 0. You can use this function to provide | |
3437 | default values for symbols. For example, the following script fragment | |
3438 | shows how to set a global symbol @samp{begin} to the first location in | |
3439 | the @samp{.text} section---but if a symbol called @samp{begin} already | |
3440 | existed, its value is preserved: | |
3441 | ||
3442 | @smallexample | |
3443 | @group | |
da713b8f | 3444 | SECTIONS @{ @dots{} |
af54556a ILT |
3445 | .text : @{ |
3446 | begin = DEFINED(begin) ? begin : . ; | |
3447 | @dots{} | |
3448 | @} | |
da713b8f ILT |
3449 | @dots{} |
3450 | @} | |
af54556a ILT |
3451 | @end group |
3452 | @end smallexample | |
3453 | ||
3454 | @item LOADADDR(@var{section}) | |
3455 | @kindex LOADADDR(@var{section}) | |
3456 | @cindex section load address in expression | |
3457 | Return the absolute LMA of the named @var{section}. This is normally | |
3458 | the same as @code{ADDR}, but it may be different if the @code{AT} | |
3459 | attribute is used in the output section definition (@pxref{Output | |
3460 | Section LMA}). | |
3461 | ||
3462 | @kindex MAX | |
3463 | @item MAX(@var{exp1}, @var{exp2}) | |
3464 | Returns the maximum of @var{exp1} and @var{exp2}. | |
3465 | ||
3466 | @kindex MIN | |
3467 | @item MIN(@var{exp1}, @var{exp2}) | |
3468 | Returns the minimum of @var{exp1} and @var{exp2}. | |
3469 | ||
3470 | @item NEXT(@var{exp}) | |
3471 | @kindex NEXT(@var{exp}) | |
3472 | @cindex unallocated address, next | |
3473 | Return the next unallocated address that is a multiple of @var{exp}. | |
3474 | This function is closely related to @code{ALIGN(@var{exp})}; unless you | |
3475 | use the @code{MEMORY} command to define discontinuous memory for the | |
3476 | output file, the two functions are equivalent. | |
3477 | ||
3478 | @item SIZEOF(@var{section}) | |
3479 | @kindex SIZEOF(@var{section}) | |
3480 | @cindex section size | |
3481 | Return the size in bytes of the named @var{section}, if that section has | |
3482 | been allocated. If the section has not been allocated when this is | |
3483 | evaluated, the linker will report an error. In the following example, | |
3484 | @code{symbol_1} and @code{symbol_2} are assigned identical values: | |
3485 | @smallexample | |
3486 | @group | |
3487 | SECTIONS@{ @dots{} | |
3488 | .output @{ | |
3489 | .start = . ; | |
3490 | @dots{} | |
3491 | .end = . ; | |
3492 | @} | |
3493 | symbol_1 = .end - .start ; | |
3494 | symbol_2 = SIZEOF(.output); | |
3495 | @dots{} @} | |
3496 | @end group | |
3497 | @end smallexample | |
3498 | ||
3499 | @item SIZEOF_HEADERS | |
3500 | @itemx sizeof_headers | |
3501 | @kindex SIZEOF_HEADERS | |
3502 | @cindex header size | |
3503 | Return the size in bytes of the output file's headers. This is | |
3504 | information which appears at the start of the output file. You can use | |
3505 | this number when setting the start address of the first section, if you | |
3506 | choose, to facilitate paging. | |
3507 | ||
3508 | @cindex not enough room for program headers | |
3509 | @cindex program headers, not enough room | |
3510 | When producing an ELF output file, if the linker script uses the | |
3511 | @code{SIZEOF_HEADERS} builtin function, the linker must compute the | |
3512 | number of program headers before it has determined all the section | |
3513 | addresses and sizes. If the linker later discovers that it needs | |
3514 | additional program headers, it will report an error @samp{not enough | |
3515 | room for program headers}. To avoid this error, you must avoid using | |
3516 | the @code{SIZEOF_HEADERS} function, or you must rework your linker | |
3517 | script to avoid forcing the linker to use additional program headers, or | |
3518 | you must define the program headers yourself using the @code{PHDRS} | |
3519 | command (@pxref{PHDRS}). | |
b4d4e8e3 RP |
3520 | @end table |
3521 | ||
f8cf2baa ILT |
3522 | @node Implicit Linker Scripts |
3523 | @section Implicit Linker Scripts | |
3524 | @cindex implicit linker scripts | |
3525 | If you specify a linker input file which the linker can not recognize as | |
3526 | an object file or an archive file, it will try to read the file as a | |
3527 | linker script. If the file can not be parsed as a linker script, the | |
3528 | linker will report an error. | |
3529 | ||
3530 | An implicit linker script will not replace the default linker script. | |
3531 | ||
3532 | Typically an implicit linker script would contain only symbol | |
3533 | assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION} | |
3534 | commands. | |
3535 | ||
3536 | Any input files read because of an implicit linker script will be read | |
3537 | at the position in the command line where the implicit linker script was | |
3538 | read. This can affect archive searching. | |
3539 | ||
ec40bbb8 DM |
3540 | @ifset GENERIC |
3541 | @node Machine Dependent | |
1c48127e RP |
3542 | @chapter Machine Dependent Features |
3543 | ||
3544 | @cindex machine dependencies | |
246504a5 RP |
3545 | @code{ld} has additional features on some platforms; the following |
3546 | sections describe them. Machines where @code{ld} has no additional | |
1c48127e RP |
3547 | functionality are not listed. |
3548 | ||
3549 | @menu | |
246504a5 RP |
3550 | * H8/300:: @code{ld} and the H8/300 |
3551 | * i960:: @code{ld} and the Intel 960 family | |
da713b8f | 3552 | * ARM:: @code{ld} and the ARM family |
1c48127e | 3553 | @end menu |
ec40bbb8 DM |
3554 | @end ifset |
3555 | ||
7f9ae73e | 3556 | @c FIXME! This could use @raisesections/@lowersections, but there seems to be a conflict |
ec40bbb8 DM |
3557 | @c between those and node-defaulting. |
3558 | @ifset H8300 | |
3559 | @ifclear GENERIC | |
7f9ae73e | 3560 | @raisesections |
ec40bbb8 | 3561 | @end ifclear |
da713b8f | 3562 | |
ec40bbb8 | 3563 | @node H8/300 |
246504a5 | 3564 | @section @code{ld} and the H8/300 |
1c48127e RP |
3565 | |
3566 | @cindex H8/300 support | |
246504a5 | 3567 | For the H8/300, @code{ld} can perform these global optimizations when |
9fde46a4 | 3568 | you specify the @samp{--relax} command-line option. |
1c48127e RP |
3569 | |
3570 | @table @emph | |
d76ae847 | 3571 | @cindex relaxing on H8/300 |
c653b370 | 3572 | @item relaxing address modes |
246504a5 | 3573 | @code{ld} finds all @code{jsr} and @code{jmp} instructions whose |
1c48127e RP |
3574 | targets are within eight bits, and turns them into eight-bit |
3575 | program-counter relative @code{bsr} and @code{bra} instructions, | |
3576 | respectively. | |
3577 | ||
d76ae847 | 3578 | @cindex synthesizing on H8/300 |
c653b370 | 3579 | @item synthesizing instructions |
1c48127e | 3580 | @c FIXME: specifically mov.b, or any mov instructions really? |
246504a5 | 3581 | @code{ld} finds all @code{mov.b} instructions which use the |
1c48127e RP |
3582 | sixteen-bit absolute address form, but refer to the top |
3583 | page of memory, and changes them to use the eight-bit address form. | |
3584 | (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into | |
3585 | @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the | |
3586 | top page of memory). | |
3587 | @end table | |
da713b8f | 3588 | |
ec40bbb8 | 3589 | @ifclear GENERIC |
7f9ae73e | 3590 | @lowersections |
ec40bbb8 DM |
3591 | @end ifclear |
3592 | @end ifset | |
3593 | ||
f9d3d71a ILT |
3594 | @ifclear GENERIC |
3595 | @ifset Hitachi | |
3596 | @c This stuff is pointless to say unless you're especially concerned | |
3597 | @c with Hitachi chips; don't enable it for generic case, please. | |
3598 | @node Hitachi | |
3599 | @chapter @code{ld} and other Hitachi chips | |
3600 | ||
3601 | @code{ld} also supports the H8/300H, the H8/500, and the Hitachi SH. No | |
3602 | special features, commands, or command-line options are required for | |
3603 | these chips. | |
3604 | @end ifset | |
3605 | @end ifclear | |
3606 | ||
ec40bbb8 DM |
3607 | @ifset I960 |
3608 | @ifclear GENERIC | |
7f9ae73e | 3609 | @raisesections |
ec40bbb8 | 3610 | @end ifclear |
da713b8f | 3611 | |
ec40bbb8 | 3612 | @node i960 |
246504a5 | 3613 | @section @code{ld} and the Intel 960 family |
1c48127e RP |
3614 | |
3615 | @cindex i960 support | |
d76ae847 | 3616 | |
1c48127e RP |
3617 | You can use the @samp{-A@var{architecture}} command line option to |
3618 | specify one of the two-letter names identifying members of the 960 | |
3619 | family; the option specifies the desired output target, and warns of any | |
3620 | incompatible instructions in the input files. It also modifies the | |
3621 | linker's search strategy for archive libraries, to support the use of | |
3622 | libraries specific to each particular architecture, by including in the | |
3623 | search loop names suffixed with the string identifying the architecture. | |
3624 | ||
246504a5 | 3625 | For example, if your @code{ld} command line included @w{@samp{-ACA}} as |
1c48127e | 3626 | well as @w{@samp{-ltry}}, the linker would look (in its built-in search |
ec40bbb8 | 3627 | paths, and in any paths you specify with @samp{-L}) for a library with |
1c48127e | 3628 | the names |
ec40bbb8 | 3629 | |
c653b370 ILT |
3630 | @smallexample |
3631 | @group | |
1c48127e RP |
3632 | try |
3633 | libtry.a | |
3634 | tryca | |
3635 | libtryca.a | |
c653b370 ILT |
3636 | @end group |
3637 | @end smallexample | |
ec40bbb8 | 3638 | |
1c48127e RP |
3639 | @noindent |
3640 | The first two possibilities would be considered in any event; the last | |
3641 | two are due to the use of @w{@samp{-ACA}}. | |
3642 | ||
ec40bbb8 | 3643 | You can meaningfully use @samp{-A} more than once on a command line, since |
1c48127e | 3644 | the 960 architecture family allows combination of target architectures; each |
ec40bbb8 | 3645 | use will add another pair of name variants to search for when @w{@samp{-l}} |
1c48127e | 3646 | specifies a library. |
1fb57a5d | 3647 | |
9fde46a4 | 3648 | @cindex @code{--relax} on i960 |
1fb57a5d | 3649 | @cindex relaxing on i960 |
9fde46a4 ILT |
3650 | @code{ld} supports the @samp{--relax} option for the i960 family. If |
3651 | you specify @samp{--relax}, @code{ld} finds all @code{balx} and | |
3652 | @code{calx} instructions whose targets are within 24 bits, and turns | |
3653 | them into 24-bit program-counter relative @code{bal} and @code{cal} | |
1fb57a5d RP |
3654 | instructions, respectively. @code{ld} also turns @code{cal} |
3655 | instructions into @code{bal} instructions when it determines that the | |
3656 | target subroutine is a leaf routine (that is, the target subroutine does | |
3657 | not itself call any subroutines). | |
3658 | ||
ec40bbb8 | 3659 | @ifclear GENERIC |
7f9ae73e | 3660 | @lowersections |
ec40bbb8 DM |
3661 | @end ifclear |
3662 | @end ifset | |
1c48127e | 3663 | |
da713b8f ILT |
3664 | @ifclear GENERIC |
3665 | @raisesections | |
3666 | @end ifclear | |
3667 | ||
3668 | @node ARM | |
3669 | @section @code{ld}'s support for interworking between ARM and Thumb code | |
3670 | ||
3671 | @cindex ARM interworking support | |
3672 | @cindex --support-old-code | |
3673 | For the ARM, @code{ld} will generate code stubs to allow functions calls | |
3674 | betweem ARM and Thumb code. These stubs only work with code that has | |
3675 | been compiled and assembled with the @samp{-mthumb-interwork} command | |
3676 | line option. If it is necessary to link with old ARM object files or | |
3677 | libraries, which have not been compiled with the -mthumb-interwork | |
3678 | option then the @samp{--support-old-code} command line switch should be | |
3679 | given to the linker. This will make it generate larger stub functions | |
3680 | which will work with non-interworking aware ARM code. Note, however, | |
3681 | the linker does not support generating stubs for function calls to | |
3682 | non-interworking aware Thumb code. | |
3683 | ||
3684 | @ifclear GENERIC | |
3685 | @lowersections | |
3686 | @end ifclear | |
3687 | ||
ec40bbb8 DM |
3688 | @ifclear SingleFormat |
3689 | @node BFD | |
f22eee08 RP |
3690 | @chapter BFD |
3691 | ||
2c5c0674 RP |
3692 | @cindex back end |
3693 | @cindex object file management | |
d4e5e3c3 DM |
3694 | @cindex object formats available |
3695 | @kindex objdump -i | |
2c5c0674 RP |
3696 | The linker accesses object and archive files using the BFD libraries. |
3697 | These libraries allow the linker to use the same routines to operate on | |
3698 | object files whatever the object file format. A different object file | |
3699 | format can be supported simply by creating a new BFD back end and adding | |
d4e5e3c3 DM |
3700 | it to the library. To conserve runtime memory, however, the linker and |
3701 | associated tools are usually configured to support only a subset of the | |
3702 | object file formats available. You can use @code{objdump -i} | |
1c48127e | 3703 | (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to |
d4e5e3c3 | 3704 | list all the formats available for your configuration. |
f22eee08 | 3705 | |
2c5c0674 RP |
3706 | @cindex BFD requirements |
3707 | @cindex requirements for BFD | |
3708 | As with most implementations, BFD is a compromise between | |
f22eee08 | 3709 | several conflicting requirements. The major factor influencing |
2c5c0674 RP |
3710 | BFD design was efficiency: any time used converting between |
3711 | formats is time which would not have been spent had BFD not | |
f22eee08 | 3712 | been involved. This is partly offset by abstraction payback; since |
2c5c0674 | 3713 | BFD simplifies applications and back ends, more time and care |
f22eee08 RP |
3714 | may be spent optimizing algorithms for a greater speed. |
3715 | ||
2c5c0674 RP |
3716 | One minor artifact of the BFD solution which you should bear in |
3717 | mind is the potential for information loss. There are two places where | |
ec40bbb8 | 3718 | useful information can be lost using the BFD mechanism: during |
2c5c0674 RP |
3719 | conversion and during output. @xref{BFD information loss}. |
3720 | ||
3721 | @menu | |
2d59b2c3 | 3722 | * BFD outline:: How it works: an outline of BFD |
2c5c0674 | 3723 | @end menu |
f22eee08 | 3724 | |
ec40bbb8 | 3725 | @node BFD outline |
b4d4e8e3 | 3726 | @section How it works: an outline of BFD |
2c5c0674 | 3727 | @cindex opening object files |
3e27cc11 | 3728 | @include bfdsumm.texi |
ec40bbb8 | 3729 | @end ifclear |
f22eee08 | 3730 | |
9fde46a4 ILT |
3731 | @node Reporting Bugs |
3732 | @chapter Reporting Bugs | |
3733 | @cindex bugs in @code{ld} | |
3734 | @cindex reporting bugs in @code{ld} | |
3735 | ||
3736 | Your bug reports play an essential role in making @code{ld} reliable. | |
3737 | ||
3738 | Reporting a bug may help you by bringing a solution to your problem, or | |
3739 | it may not. But in any case the principal function of a bug report is | |
3740 | to help the entire community by making the next version of @code{ld} | |
3741 | work better. Bug reports are your contribution to the maintenance of | |
3742 | @code{ld}. | |
3743 | ||
3744 | In order for a bug report to serve its purpose, you must include the | |
3745 | information that enables us to fix the bug. | |
3746 | ||
3747 | @menu | |
3748 | * Bug Criteria:: Have you found a bug? | |
3749 | * Bug Reporting:: How to report bugs | |
3750 | @end menu | |
3751 | ||
3752 | @node Bug Criteria | |
3753 | @section Have you found a bug? | |
3754 | @cindex bug criteria | |
3755 | ||
3756 | If you are not sure whether you have found a bug, here are some guidelines: | |
3757 | ||
3758 | @itemize @bullet | |
3759 | @cindex fatal signal | |
3760 | @cindex linker crash | |
3761 | @cindex crash of linker | |
3762 | @item | |
3763 | If the linker gets a fatal signal, for any input whatever, that is a | |
3764 | @code{ld} bug. Reliable linkers never crash. | |
3765 | ||
3766 | @cindex error on valid input | |
3767 | @item | |
3768 | If @code{ld} produces an error message for valid input, that is a bug. | |
3769 | ||
3770 | @cindex invalid input | |
3771 | @item | |
3772 | If @code{ld} does not produce an error message for invalid input, that | |
3773 | may be a bug. In the general case, the linker can not verify that | |
3774 | object files are correct. | |
3775 | ||
3776 | @item | |
3777 | If you are an experienced user of linkers, your suggestions for | |
3778 | improvement of @code{ld} are welcome in any case. | |
3779 | @end itemize | |
3780 | ||
3781 | @node Bug Reporting | |
3782 | @section How to report bugs | |
3783 | @cindex bug reports | |
3784 | @cindex @code{ld} bugs, reporting | |
3785 | ||
3786 | A number of companies and individuals offer support for @sc{gnu} | |
3787 | products. If you obtained @code{ld} from a support organization, we | |
3788 | recommend you contact that organization first. | |
3789 | ||
3790 | You can find contact information for many support companies and | |
3791 | individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs | |
3792 | distribution. | |
3793 | ||
af54556a ILT |
3794 | Otherwise, send bug reports for @code{ld} to |
3795 | @samp{bug-gnu-utils@@gnu.org}. | |
9fde46a4 ILT |
3796 | |
3797 | The fundamental principle of reporting bugs usefully is this: | |
3798 | @strong{report all the facts}. If you are not sure whether to state a | |
3799 | fact or leave it out, state it! | |
3800 | ||
3801 | Often people omit facts because they think they know what causes the | |
3802 | problem and assume that some details do not matter. Thus, you might | |
3803 | assume that the name of a symbol you use in an example does not matter. | |
3804 | Well, probably it does not, but one cannot be sure. Perhaps the bug is | |
3805 | a stray memory reference which happens to fetch from the location where | |
3806 | that name is stored in memory; perhaps, if the name were different, the | |
3807 | contents of that location would fool the linker into doing the right | |
3808 | thing despite the bug. Play it safe and give a specific, complete | |
3809 | example. That is the easiest thing for you to do, and the most helpful. | |
3810 | ||
3811 | Keep in mind that the purpose of a bug report is to enable us to fix the bug if | |
3812 | it is new to us. Therefore, always write your bug reports on the assumption | |
3813 | that the bug has not been reported previously. | |
3814 | ||
3815 | Sometimes people give a few sketchy facts and ask, ``Does this ring a | |
3816 | bell?'' Those bug reports are useless, and we urge everyone to | |
3817 | @emph{refuse to respond to them} except to chide the sender to report | |
3818 | bugs properly. | |
3819 | ||
3820 | To enable us to fix the bug, you should include all these things: | |
3821 | ||
3822 | @itemize @bullet | |
3823 | @item | |
3824 | The version of @code{ld}. @code{ld} announces it if you start it with | |
3825 | the @samp{--version} argument. | |
3826 | ||
3827 | Without this, we will not know whether there is any point in looking for | |
3828 | the bug in the current version of @code{ld}. | |
3829 | ||
3830 | @item | |
3831 | Any patches you may have applied to the @code{ld} source, including any | |
3832 | patches made to the @code{BFD} library. | |
3833 | ||
3834 | @item | |
3835 | The type of machine you are using, and the operating system name and | |
3836 | version number. | |
3837 | ||
3838 | @item | |
3839 | What compiler (and its version) was used to compile @code{ld}---e.g. | |
3840 | ``@code{gcc-2.7}''. | |
3841 | ||
3842 | @item | |
3843 | The command arguments you gave the linker to link your example and | |
3844 | observe the bug. To guarantee you will not omit something important, | |
3845 | list them all. A copy of the Makefile (or the output from make) is | |
3846 | sufficient. | |
3847 | ||
3848 | If we were to try to guess the arguments, we would probably guess wrong | |
3849 | and then we might not encounter the bug. | |
3850 | ||
3851 | @item | |
3852 | A complete input file, or set of input files, that will reproduce the | |
3853 | bug. It is generally most helpful to send the actual object files, | |
3854 | uuencoded if necessary to get them through the mail system. Making them | |
3855 | available for anonymous FTP is not as good, but may be the only | |
3856 | reasonable choice for large object files. | |
3857 | ||
3858 | If the source files were assembled using @code{gas} or compiled using | |
3859 | @code{gcc}, then it may be OK to send the source files rather than the | |
3860 | object files. In this case, be sure to say exactly what version of | |
3861 | @code{gas} or @code{gcc} was used to produce the object files. Also say | |
3862 | how @code{gas} or @code{gcc} were configured. | |
3863 | ||
3864 | @item | |
3865 | A description of what behavior you observe that you believe is | |
3866 | incorrect. For example, ``It gets a fatal signal.'' | |
3867 | ||
3868 | Of course, if the bug is that @code{ld} gets a fatal signal, then we | |
3869 | will certainly notice it. But if the bug is incorrect output, we might | |
3870 | not notice unless it is glaringly wrong. You might as well not give us | |
3871 | a chance to make a mistake. | |
3872 | ||
3873 | Even if the problem you experience is a fatal signal, you should still | |
3874 | say so explicitly. Suppose something strange is going on, such as, your | |
3875 | copy of @code{ld} is out of synch, or you have encountered a bug in the | |
3876 | C library on your system. (This has happened!) Your copy might crash | |
3877 | and ours would not. If you told us to expect a crash, then when ours | |
3878 | fails to crash, we would know that the bug was not happening for us. If | |
3879 | you had not told us to expect a crash, then we would not be able to draw | |
3880 | any conclusion from our observations. | |
3881 | ||
3882 | @item | |
3883 | If you wish to suggest changes to the @code{ld} source, send us context | |
3884 | diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or | |
3885 | @samp{-p} option. Always send diffs from the old file to the new file. | |
3886 | If you even discuss something in the @code{ld} source, refer to it by | |
3887 | context, not by line number. | |
3888 | ||
3889 | The line numbers in our development sources will not match those in your | |
3890 | sources. Your line numbers would convey no useful information to us. | |
3891 | @end itemize | |
3892 | ||
3893 | Here are some things that are not necessary: | |
3894 | ||
3895 | @itemize @bullet | |
3896 | @item | |
3897 | A description of the envelope of the bug. | |
3898 | ||
3899 | Often people who encounter a bug spend a lot of time investigating | |
3900 | which changes to the input file will make the bug go away and which | |
3901 | changes will not affect it. | |
3902 | ||
3903 | This is often time consuming and not very useful, because the way we | |
3904 | will find the bug is by running a single example under the debugger | |
3905 | with breakpoints, not by pure deduction from a series of examples. | |
3906 | We recommend that you save your time for something else. | |
3907 | ||
3908 | Of course, if you can find a simpler example to report @emph{instead} | |
3909 | of the original one, that is a convenience for us. Errors in the | |
3910 | output will be easier to spot, running under the debugger will take | |
3911 | less time, and so on. | |
3912 | ||
3913 | However, simplification is not vital; if you do not want to do this, | |
3914 | report the bug anyway and send us the entire test case you used. | |
3915 | ||
3916 | @item | |
3917 | A patch for the bug. | |
3918 | ||
3919 | A patch for the bug does help us if it is a good one. But do not omit | |
3920 | the necessary information, such as the test case, on the assumption that | |
3921 | a patch is all we need. We might see problems with your patch and decide | |
3922 | to fix the problem another way, or we might not understand it at all. | |
3923 | ||
de220cbd ILT |
3924 | Sometimes with a program as complicated as @code{ld} it is very hard to |
3925 | construct an example that will make the program follow a certain path | |
3926 | through the code. If you do not send us the example, we will not be | |
3927 | able to construct one, so we will not be able to verify that the bug is | |
3928 | fixed. | |
9fde46a4 ILT |
3929 | |
3930 | And if we cannot understand what bug you are trying to fix, or why your | |
3931 | patch should be an improvement, we will not install it. A test case will | |
3932 | help us to understand. | |
3933 | ||
3934 | @item | |
3935 | A guess about what the bug is or what it depends on. | |
3936 | ||
3937 | Such guesses are usually wrong. Even we cannot guess right about such | |
3938 | things without first using the debugger to find the facts. | |
3939 | @end itemize | |
3940 | ||
ec40bbb8 | 3941 | @node MRI |
2d59b2c3 RP |
3942 | @appendix MRI Compatible Script Files |
3943 | @cindex MRI compatibility | |
3944 | To aid users making the transition to @sc{gnu} @code{ld} from the MRI | |
3945 | linker, @code{ld} can use MRI compatible linker scripts as an | |
3946 | alternative to the more general-purpose linker scripting language | |
af54556a ILT |
3947 | described in @ref{Scripts}. MRI compatible linker scripts have a much |
3948 | simpler command set than the scripting language otherwise used with | |
3949 | @code{ld}. @sc{gnu} @code{ld} supports the most commonly used MRI | |
3950 | linker commands; these commands are described here. | |
2d59b2c3 | 3951 | |
867a1b8a DM |
3952 | In general, MRI scripts aren't of much use with the @code{a.out} object |
3953 | file format, since it only has three sections and MRI scripts lack some | |
3954 | features to make use of them. | |
3955 | ||
2d59b2c3 RP |
3956 | You can specify a file containing an MRI-compatible script using the |
3957 | @samp{-c} command-line option. | |
3958 | ||
3959 | Each command in an MRI-compatible script occupies its own line; each | |
3960 | command line starts with the keyword that identifies the command (though | |
3961 | blank lines are also allowed for punctuation). If a line of an | |
3962 | MRI-compatible script begins with an unrecognized keyword, @code{ld} | |
3963 | issues a warning message, but continues processing the script. | |
3964 | ||
3965 | Lines beginning with @samp{*} are comments. | |
3966 | ||
3967 | You can write these commands using all upper-case letters, or all | |
3968 | lower case; for example, @samp{chip} is the same as @samp{CHIP}. | |
3969 | The following list shows only the upper-case form of each command. | |
3970 | ||
3971 | @table @code | |
d4e5e3c3 | 3972 | @cindex @code{ABSOLUTE} (MRI) |
2d59b2c3 | 3973 | @item ABSOLUTE @var{secname} |
c653b370 | 3974 | @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname} |
2d59b2c3 RP |
3975 | Normally, @code{ld} includes in the output file all sections from all |
3976 | the input files. However, in an MRI-compatible script, you can use the | |
3977 | @code{ABSOLUTE} command to restrict the sections that will be present in | |
3978 | your output program. If the @code{ABSOLUTE} command is used at all in a | |
3979 | script, then only the sections named explicitly in @code{ABSOLUTE} | |
3980 | commands will appear in the linker output. You can still use other | |
3981 | input sections (whatever you select on the command line, or using | |
7b015547 | 3982 | @code{LOAD}) to resolve addresses in the output file. |
2d59b2c3 | 3983 | |
2d59b2c3 | 3984 | @cindex @code{ALIAS} (MRI) |
d4e5e3c3 | 3985 | @item ALIAS @var{out-secname}, @var{in-secname} |
2d59b2c3 RP |
3986 | Use this command to place the data from input section @var{in-secname} |
3987 | in a section called @var{out-secname} in the linker output file. | |
3988 | ||
3989 | @var{in-secname} may be an integer. | |
3990 | ||
e54bf1c1 ILT |
3991 | @cindex @code{ALIGN} (MRI) |
3992 | @item ALIGN @var{secname} = @var{expression} | |
3993 | Align the section called @var{secname} to @var{expression}. The | |
3994 | @var{expression} should be a power of two. | |
3995 | ||
2d59b2c3 | 3996 | @cindex @code{BASE} (MRI) |
d4e5e3c3 | 3997 | @item BASE @var{expression} |
2d59b2c3 RP |
3998 | Use the value of @var{expression} as the lowest address (other than |
3999 | absolute addresses) in the output file. | |
4000 | ||
d4e5e3c3 | 4001 | @cindex @code{CHIP} (MRI) |
2d59b2c3 RP |
4002 | @item CHIP @var{expression} |
4003 | @itemx CHIP @var{expression}, @var{expression} | |
ec40bbb8 | 4004 | This command does nothing; it is accepted only for compatibility. |
2d59b2c3 | 4005 | |
2d59b2c3 | 4006 | @cindex @code{END} (MRI) |
d4e5e3c3 | 4007 | @item END |
2d59b2c3 RP |
4008 | This command does nothing whatever; it's only accepted for compatibility. |
4009 | ||
2d59b2c3 | 4010 | @cindex @code{FORMAT} (MRI) |
d4e5e3c3 | 4011 | @item FORMAT @var{output-format} |
2d59b2c3 RP |
4012 | Similar to the @code{OUTPUT_FORMAT} command in the more general linker |
4013 | language, but restricted to one of these output formats: | |
d4e5e3c3 | 4014 | |
2d59b2c3 RP |
4015 | @enumerate |
4016 | @item | |
4017 | S-records, if @var{output-format} is @samp{S} | |
4018 | ||
4019 | @item | |
4020 | IEEE, if @var{output-format} is @samp{IEEE} | |
4021 | ||
4022 | @item | |
4023 | COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is | |
4024 | @samp{COFF} | |
4025 | @end enumerate | |
4026 | ||
2d59b2c3 | 4027 | @cindex @code{LIST} (MRI) |
d4e5e3c3 | 4028 | @item LIST @var{anything}@dots{} |
2d59b2c3 RP |
4029 | Print (to the standard output file) a link map, as produced by the |
4030 | @code{ld} command-line option @samp{-M}. | |
4031 | ||
ec40bbb8 DM |
4032 | The keyword @code{LIST} may be followed by anything on the |
4033 | same line, with no change in its effect. | |
2d59b2c3 | 4034 | |
d4e5e3c3 | 4035 | @cindex @code{LOAD} (MRI) |
2d59b2c3 | 4036 | @item LOAD @var{filename} |
c653b370 | 4037 | @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename} |
2d59b2c3 RP |
4038 | Include one or more object file @var{filename} in the link; this has the |
4039 | same effect as specifying @var{filename} directly on the @code{ld} | |
4040 | command line. | |
4041 | ||
2d59b2c3 | 4042 | @cindex @code{NAME} (MRI) |
d4e5e3c3 | 4043 | @item NAME @var{output-name} |
2d59b2c3 RP |
4044 | @var{output-name} is the name for the program produced by @code{ld}; the |
4045 | MRI-compatible command @code{NAME} is equivalent to the command-line | |
4046 | option @samp{-o} or the general script language command @code{OUTPUT}. | |
4047 | ||
d4e5e3c3 | 4048 | @cindex @code{ORDER} (MRI) |
2d59b2c3 RP |
4049 | @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname} |
4050 | @itemx ORDER @var{secname} @var{secname} @var{secname} | |
ec40bbb8 DM |
4051 | Normally, @code{ld} orders the sections in its output file in the |
4052 | order in which they first appear in the input files. In an MRI-compatible | |
4053 | script, you can override this ordering with the @code{ORDER} command. The | |
2d59b2c3 RP |
4054 | sections you list with @code{ORDER} will appear first in your output |
4055 | file, in the order specified. | |
4056 | ||
d4e5e3c3 | 4057 | @cindex @code{PUBLIC} (MRI) |
2d59b2c3 RP |
4058 | @item PUBLIC @var{name}=@var{expression} |
4059 | @itemx PUBLIC @var{name},@var{expression} | |
4060 | @itemx PUBLIC @var{name} @var{expression} | |
ec40bbb8 | 4061 | Supply a value (@var{expression}) for external symbol |
2d59b2c3 RP |
4062 | @var{name} used in the linker input files. |
4063 | ||
d4e5e3c3 | 4064 | @cindex @code{SECT} (MRI) |
2d59b2c3 RP |
4065 | @item SECT @var{secname}, @var{expression} |
4066 | @itemx SECT @var{secname}=@var{expression} | |
4067 | @itemx SECT @var{secname} @var{expression} | |
2d59b2c3 RP |
4068 | You can use any of these three forms of the @code{SECT} command to |
4069 | specify the start address (@var{expression}) for section @var{secname}. | |
4070 | If you have more than one @code{SECT} statement for the same | |
4071 | @var{secname}, only the @emph{first} sets the start address. | |
4072 | @end table | |
4073 | ||
ec40bbb8 | 4074 | @node Index |
2c5c0674 RP |
4075 | @unnumbered Index |
4076 | ||
4077 | @printindex cp | |
4078 | ||
4079 | @tex | |
4080 | % I think something like @colophon should be in texinfo. In the | |
4081 | % meantime: | |
4082 | \long\def\colophon{\hbox to0pt{}\vfill | |
4083 | \centerline{The body of this manual is set in} | |
4084 | \centerline{\fontname\tenrm,} | |
4085 | \centerline{with headings in {\bf\fontname\tenbf}} | |
4086 | \centerline{and examples in {\tt\fontname\tentt}.} | |
4087 | \centerline{{\it\fontname\tenit\/} and} | |
4088 | \centerline{{\sl\fontname\tensl\/}} | |
4089 | \centerline{are used for emphasis.}\vfill} | |
4090 | \page\colophon | |
c653b370 | 4091 | % Blame: doc@cygnus.com, 28mar91. |
2c5c0674 RP |
4092 | @end tex |
4093 | ||
4094 | ||
b4d4e8e3 | 4095 | @contents |
f22eee08 RP |
4096 | @bye |
4097 | ||
4098 |