bfd/doc/bfdsumm.texi

   1 @c This summary of BFD is shared by the BFD and LD docs.
   2 @c Copyright (C) 2012-2016 Free Software Foundation, Inc.
   3
   4 When an object file is opened, BFD subroutines automatically determine
   5 the format of the input object file.  They then build a descriptor in
   6 memory with pointers to routines that will be used to access elements of
   7 the object file's data structures.
   8
   9 As different information from the object files is required,
  10 BFD reads from different sections of the file and processes them.
  11 For example, a very common operation for the linker is processing symbol
  12 tables.  Each BFD back end provides a routine for converting
  13 between the object file's representation of symbols and an internal
  14 canonical format. When the linker asks for the symbol table of an object
  15 file, it calls through a memory pointer to the routine from the
  16 relevant BFD back end which reads and converts the table into a canonical
  17 form.  The linker then operates upon the canonical form. When the link is
  18 finished and the linker writes the output file's symbol table,
  19 another BFD back end routine is called to take the newly
  20 created symbol table and convert it into the chosen output format.
  21
  22 @menu
  23 * BFD information loss::        Information Loss
  24 * Canonical format::            The BFD canonical object-file format
  25 @end menu
  26
  27 @node BFD information loss
  28 @subsection Information Loss
  29
  30 @emph{Information can be lost during output.} The output formats
  31 supported by BFD do not provide identical facilities, and
  32 information which can be described in one form has nowhere to go in
  33 another format. One example of this is alignment information in
  34 @code{b.out}. There is nowhere in an @code{a.out} format file to store
  35 alignment information on the contained data, so when a file is linked
  36 from @code{b.out} and an @code{a.out} image is produced, alignment
  37 information will not propagate to the output file. (The linker will
  38 still use the alignment information internally, so the link is performed
  39 correctly).
  40
  41 Another example is COFF section names. COFF files may contain an
  42 unlimited number of sections, each one with a textual section name. If
  43 the target of the link is a format which does not have many sections (e.g.,
  44 @code{a.out}) or has sections without names (e.g., the Oasys format), the
  45 link cannot be done simply. You can circumvent this problem by
  46 describing the desired input-to-output section mapping with the linker command
  47 language.
  48
  49 @emph{Information can be lost during canonicalization.} The BFD
  50 internal canonical form of the external formats is not exhaustive; there
  51 are structures in input formats for which there is no direct
  52 representation internally.  This means that the BFD back ends
  53 cannot maintain all possible data richness through the transformation
  54 between external to internal and back to external formats.
  55
  56 This limitation is only a problem when an application reads one
  57 format and writes another.  Each BFD back end is responsible for
  58 maintaining as much data as possible, and the internal BFD
  59 canonical form has structures which are opaque to the BFD core,
  60 and exported only to the back ends. When a file is read in one format,
  61 the canonical form is generated for BFD and the application. At the
  62 same time, the back end saves away any information which may otherwise
  63 be lost. If the data is then written back in the same format, the back
  64 end routine will be able to use the canonical form provided by the
  65 BFD core as well as the information it prepared earlier.  Since
  66 there is a great deal of commonality between back ends,
  67 there is no information lost when
  68 linking or copying big endian COFF to little endian COFF, or @code{a.out} to
  69 @code{b.out}.  When a mixture of formats is linked, the information is
  70 only lost from the files whose format differs from the destination.
  71
  72 @node Canonical format
  73 @subsection The BFD canonical object-file format
  74
  75 The greatest potential for loss of information occurs when there is the least
  76 overlap between the information provided by the source format, that
  77 stored by the canonical format, and that needed by the
  78 destination format. A brief description of the canonical form may help
  79 you understand which kinds of data you can count on preserving across
  80 conversions.
  81 @cindex BFD canonical format
  82 @cindex internal object-file format
  83
  84 @table @emph
  85 @item files
  86 Information stored on a per-file basis includes target machine
  87 architecture, particular implementation format type, a demand pageable
  88 bit, and a write protected bit.  Information like Unix magic numbers is
  89 not stored here---only the magic numbers' meaning, so a @code{ZMAGIC}
  90 file would have both the demand pageable bit and the write protected
  91 text bit set.  The byte order of the target is stored on a per-file
  92 basis, so that big- and little-endian object files may be used with one
  93 another.
  94
  95 @item sections
  96 Each section in the input file contains the name of the section, the
  97 section's original address in the object file, size and alignment
  98 information, various flags, and pointers into other BFD data
  99 structures.
 100
 101 @item symbols
 102 Each symbol contains a pointer to the information for the object file
 103 which originally defined it, its name, its value, and various flag
 104 bits.  When a BFD back end reads in a symbol table, it relocates all
 105 symbols to make them relative to the base of the section where they were
 106 defined.  Doing this ensures that each symbol points to its containing
 107 section.  Each symbol also has a varying amount of hidden private data
 108 for the BFD back end.  Since the symbol points to the original file, the
 109 private data format for that symbol is accessible.  @code{ld} can
 110 operate on a collection of symbols of wildly different formats without
 111 problems.
 112
 113 Normal global and simple local symbols are maintained on output, so an
 114 output file (no matter its format) will retain symbols pointing to
 115 functions and to global, static, and common variables.  Some symbol
 116 information is not worth retaining; in @code{a.out}, type information is
 117 stored in the symbol table as long symbol names.  This information would
 118 be useless to most COFF debuggers; the linker has command line switches
 119 to allow users to throw it away.
 120
 121 There is one word of type information within the symbol, so if the
 122 format supports symbol type information within symbols (for example, COFF,
 123 IEEE, Oasys) and the type is simple enough to fit within one word
 124 (nearly everything but aggregates), the information will be preserved.
 125
 126 @item relocation level
 127 Each canonical BFD relocation record contains a pointer to the symbol to
 128 relocate to, the offset of the data to relocate, the section the data
 129 is in, and a pointer to a relocation type descriptor. Relocation is
 130 performed by passing messages through the relocation type
 131 descriptor and the symbol pointer. Therefore, relocations can be performed
 132 on output data using a relocation method that is only available in one of the
 133 input formats. For instance, Oasys provides a byte relocation format.
 134 A relocation record requesting this relocation type would point
 135 indirectly to a routine to perform this, so the relocation may be
 136 performed on a byte being written to a 68k COFF file, even though 68k COFF
 137 has no such relocation type.
 138
 139 @item line numbers
 140 Object formats can contain, for debugging purposes, some form of mapping
 141 between symbols, source line numbers, and addresses in the output file.
 142 These addresses have to be relocated along with the symbol information.
 143 Each symbol with an associated list of line number records points to the
 144 first record of the list.  The head of a line number list consists of a
 145 pointer to the symbol, which allows finding out the address of the
 146 function whose line number is being described. The rest of the list is
 147 made up of pairs: offsets into the section and line numbers. Any format
 148 which can simply derive this information can pass it successfully
 149 between formats (COFF, IEEE and Oasys).
 150 @end table