gdb/findvar.c

   1 /* Find a variable's value in memory, for GDB, the GNU debugger.
   2
   3    Copyright (C) 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995,
   4    1996, 1997, 1998, 1999, 2000, 2001, 2003, 2004, 2005, 2007
   5    Free Software Foundation, Inc.
   6
   7    This file is part of GDB.
   8
   9    This program is free software; you can redistribute it and/or modify
  10    it under the terms of the GNU General Public License as published by
  11    the Free Software Foundation; either version 2 of the License, or
  12    (at your option) any later version.
  13
  14    This program is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17    GNU General Public License for more details.
  18
  19    You should have received a copy of the GNU General Public License
  20    along with this program; if not, write to the Free Software
  21    Foundation, Inc., 51 Franklin Street, Fifth Floor,
  22    Boston, MA 02110-1301, USA.  */
  23
  24 #include "defs.h"
  25 #include "symtab.h"
  26 #include "gdbtypes.h"
  27 #include "frame.h"
  28 #include "value.h"
  29 #include "gdbcore.h"
  30 #include "inferior.h"
  31 #include "target.h"
  32 #include "gdb_string.h"
  33 #include "gdb_assert.h"
  34 #include "floatformat.h"
  35 #include "symfile.h"            /* for overlay functions */
  36 #include "regcache.h"
  37 #include "user-regs.h"
  38 #include "block.h"
  39
  40 /* Basic byte-swapping routines.  GDB has needed these for a long time...
  41    All extract a target-format integer at ADDR which is LEN bytes long.  */
  42
  43 #if TARGET_CHAR_BIT != 8 || HOST_CHAR_BIT != 8
  44   /* 8 bit characters are a pretty safe assumption these days, so we
  45      assume it throughout all these swapping routines.  If we had to deal with
  46      9 bit characters, we would need to make len be in bits and would have
  47      to re-write these routines...  */
  48 you lose
  49 #endif
  50
  51 LONGEST
  52 extract_signed_integer (const gdb_byte *addr, int len)
  53 {
  54   LONGEST retval;
  55   const unsigned char *p;
  56   const unsigned char *startaddr = addr;
  57   const unsigned char *endaddr = startaddr + len;
  58
  59   if (len > (int) sizeof (LONGEST))
  60     error (_("\
  61 That operation is not available on integers of more than %d bytes."),
  62            (int) sizeof (LONGEST));
  63
  64   /* Start at the most significant end of the integer, and work towards
  65      the least significant.  */
  66   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG)
  67     {
  68       p = startaddr;
  69       /* Do the sign extension once at the start.  */
  70       retval = ((LONGEST) * p ^ 0x80) - 0x80;
  71       for (++p; p < endaddr; ++p)
  72         retval = (retval << 8) | *p;
  73     }
  74   else
  75     {
  76       p = endaddr - 1;
  77       /* Do the sign extension once at the start.  */
  78       retval = ((LONGEST) * p ^ 0x80) - 0x80;
  79       for (--p; p >= startaddr; --p)
  80         retval = (retval << 8) | *p;
  81     }
  82   return retval;
  83 }
  84
  85 ULONGEST
  86 extract_unsigned_integer (const gdb_byte *addr, int len)
  87 {
  88   ULONGEST retval;
  89   const unsigned char *p;
  90   const unsigned char *startaddr = addr;
  91   const unsigned char *endaddr = startaddr + len;
  92
  93   if (len > (int) sizeof (ULONGEST))
  94     error (_("\
  95 That operation is not available on integers of more than %d bytes."),
  96            (int) sizeof (ULONGEST));
  97
  98   /* Start at the most significant end of the integer, and work towards
  99      the least significant.  */
 100   retval = 0;
 101   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG)
 102     {
 103       for (p = startaddr; p < endaddr; ++p)
 104         retval = (retval << 8) | *p;
 105     }
 106   else
 107     {
 108       for (p = endaddr - 1; p >= startaddr; --p)
 109         retval = (retval << 8) | *p;
 110     }
 111   return retval;
 112 }
 113
 114 /* Sometimes a long long unsigned integer can be extracted as a
 115    LONGEST value.  This is done so that we can print these values
 116    better.  If this integer can be converted to a LONGEST, this
 117    function returns 1 and sets *PVAL.  Otherwise it returns 0.  */
 118
 119 int
 120 extract_long_unsigned_integer (const gdb_byte *addr, int orig_len,
 121                                LONGEST *pval)
 122 {
 123   const gdb_byte *p;
 124   const gdb_byte *first_addr;
 125   int len;
 126
 127   len = orig_len;
 128   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG)
 129     {
 130       for (p = addr;
 131            len > (int) sizeof (LONGEST) && p < addr + orig_len;
 132            p++)
 133         {
 134           if (*p == 0)
 135             len--;
 136           else
 137             break;
 138         }
 139       first_addr = p;
 140     }
 141   else
 142     {
 143       first_addr = addr;
 144       for (p = addr + orig_len - 1;
 145            len > (int) sizeof (LONGEST) && p >= addr;
 146            p--)
 147         {
 148           if (*p == 0)
 149             len--;
 150           else
 151             break;
 152         }
 153     }
 154
 155   if (len <= (int) sizeof (LONGEST))
 156     {
 157       *pval = (LONGEST) extract_unsigned_integer (first_addr,
 158                                                   sizeof (LONGEST));
 159       return 1;
 160     }
 161
 162   return 0;
 163 }
 164
 165
 166 /* Treat the bytes at BUF as a pointer of type TYPE, and return the
 167    address it represents.  */
 168 CORE_ADDR
 169 extract_typed_address (const gdb_byte *buf, struct type *type)
 170 {
 171   if (TYPE_CODE (type) != TYPE_CODE_PTR
 172       && TYPE_CODE (type) != TYPE_CODE_REF)
 173     internal_error (__FILE__, __LINE__,
 174                     _("extract_typed_address: "
 175                     "type is not a pointer or reference"));
 176
 177   return gdbarch_pointer_to_address (current_gdbarch, type, buf);
 178 }
 179
 180
 181 void
 182 store_signed_integer (gdb_byte *addr, int len, LONGEST val)
 183 {
 184   gdb_byte *p;
 185   gdb_byte *startaddr = addr;
 186   gdb_byte *endaddr = startaddr + len;
 187
 188   /* Start at the least significant end of the integer, and work towards
 189      the most significant.  */
 190   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG)
 191     {
 192       for (p = endaddr - 1; p >= startaddr; --p)
 193         {
 194           *p = val & 0xff;
 195           val >>= 8;
 196         }
 197     }
 198   else
 199     {
 200       for (p = startaddr; p < endaddr; ++p)
 201         {
 202           *p = val & 0xff;
 203           val >>= 8;
 204         }
 205     }
 206 }
 207
 208 void
 209 store_unsigned_integer (gdb_byte *addr, int len, ULONGEST val)
 210 {
 211   unsigned char *p;
 212   unsigned char *startaddr = (unsigned char *) addr;
 213   unsigned char *endaddr = startaddr + len;
 214
 215   /* Start at the least significant end of the integer, and work towards
 216      the most significant.  */
 217   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG)
 218     {
 219       for (p = endaddr - 1; p >= startaddr; --p)
 220         {
 221           *p = val & 0xff;
 222           val >>= 8;
 223         }
 224     }
 225   else
 226     {
 227       for (p = startaddr; p < endaddr; ++p)
 228         {
 229           *p = val & 0xff;
 230           val >>= 8;
 231         }
 232     }
 233 }
 234
 235 /* Store the address ADDR as a pointer of type TYPE at BUF, in target
 236    form.  */
 237 void
 238 store_typed_address (gdb_byte *buf, struct type *type, CORE_ADDR addr)
 239 {
 240   if (TYPE_CODE (type) != TYPE_CODE_PTR
 241       && TYPE_CODE (type) != TYPE_CODE_REF)
 242     internal_error (__FILE__, __LINE__,
 243                     _("store_typed_address: "
 244                     "type is not a pointer or reference"));
 245
 246   gdbarch_address_to_pointer (current_gdbarch, type, buf, addr);
 247 }
 248
 249
 250
 251 /* Return a `value' with the contents of (virtual or cooked) register
 252    REGNUM as found in the specified FRAME.  The register's type is
 253    determined by register_type().  */
 254
 255 struct value *
 256 value_of_register (int regnum, struct frame_info *frame)
 257 {
 258   CORE_ADDR addr;
 259   int optim;
 260   struct value *reg_val;
 261   int realnum;
 262   gdb_byte raw_buffer[MAX_REGISTER_SIZE];
 263   enum lval_type lval;
 264
 265   /* User registers lie completely outside of the range of normal
 266      registers.  Catch them early so that the target never sees them.  */
 267   if (regnum >= gdbarch_num_regs (current_gdbarch)
 268                 + gdbarch_num_pseudo_regs (current_gdbarch))
 269     return value_of_user_reg (regnum, frame);
 270
 271   frame_register (frame, regnum, &optim, &lval, &addr, &realnum, raw_buffer);
 272
 273   reg_val = allocate_value (register_type (current_gdbarch, regnum));
 274
 275   memcpy (value_contents_raw (reg_val), raw_buffer,
 276           register_size (current_gdbarch, regnum));
 277   VALUE_LVAL (reg_val) = lval;
 278   VALUE_ADDRESS (reg_val) = addr;
 279   VALUE_REGNUM (reg_val) = regnum;
 280   set_value_optimized_out (reg_val, optim);
 281   VALUE_FRAME_ID (reg_val) = get_frame_id (frame);
 282   return reg_val;
 283 }
 284
 285 /* Given a pointer of type TYPE in target form in BUF, return the
 286    address it represents.  */
 287 CORE_ADDR
 288 unsigned_pointer_to_address (struct type *type, const gdb_byte *buf)
 289 {
 290   return extract_unsigned_integer (buf, TYPE_LENGTH (type));
 291 }
 292
 293 CORE_ADDR
 294 signed_pointer_to_address (struct type *type, const gdb_byte *buf)
 295 {
 296   return extract_signed_integer (buf, TYPE_LENGTH (type));
 297 }
 298
 299 /* Given an address, store it as a pointer of type TYPE in target
 300    format in BUF.  */
 301 void
 302 unsigned_address_to_pointer (struct type *type, gdb_byte *buf,
 303                              CORE_ADDR addr)
 304 {
 305   store_unsigned_integer (buf, TYPE_LENGTH (type), addr);
 306 }
 307
 308 void
 309 address_to_signed_pointer (struct type *type, gdb_byte *buf, CORE_ADDR addr)
 310 {
 311   store_signed_integer (buf, TYPE_LENGTH (type), addr);
 312 }
 313 \f
 314 /* Will calling read_var_value or locate_var_value on SYM end
 315    up caring what frame it is being evaluated relative to?  SYM must
 316    be non-NULL.  */
 317 int
 318 symbol_read_needs_frame (struct symbol *sym)
 319 {
 320   switch (SYMBOL_CLASS (sym))
 321     {
 322       /* All cases listed explicitly so that gcc -Wall will detect it if
 323          we failed to consider one.  */
 324     case LOC_COMPUTED:
 325     case LOC_COMPUTED_ARG:
 326       /* FIXME: cagney/2004-01-26: It should be possible to
 327          unconditionally call the SYMBOL_OPS method when available.
 328          Unfortunately DWARF 2 stores the frame-base (instead of the
 329          function) location in a function's symbol.  Oops!  For the
 330          moment enable this when/where applicable.  */
 331       return SYMBOL_OPS (sym)->read_needs_frame (sym);
 332
 333     case LOC_REGISTER:
 334     case LOC_ARG:
 335     case LOC_REF_ARG:
 336     case LOC_REGPARM:
 337     case LOC_REGPARM_ADDR:
 338     case LOC_LOCAL:
 339     case LOC_LOCAL_ARG:
 340     case LOC_BASEREG:
 341     case LOC_BASEREG_ARG:
 342     case LOC_HP_THREAD_LOCAL_STATIC:
 343       return 1;
 344
 345     case LOC_UNDEF:
 346     case LOC_CONST:
 347     case LOC_STATIC:
 348     case LOC_INDIRECT:
 349     case LOC_TYPEDEF:
 350
 351     case LOC_LABEL:
 352       /* Getting the address of a label can be done independently of the block,
 353          even if some *uses* of that address wouldn't work so well without
 354          the right frame.  */
 355
 356     case LOC_BLOCK:
 357     case LOC_CONST_BYTES:
 358     case LOC_UNRESOLVED:
 359     case LOC_OPTIMIZED_OUT:
 360       return 0;
 361     }
 362   return 1;
 363 }
 364
 365 /* Given a struct symbol for a variable,
 366    and a stack frame id, read the value of the variable
 367    and return a (pointer to a) struct value containing the value.
 368    If the variable cannot be found, return a zero pointer.
 369    If FRAME is NULL, use the selected frame.  */
 370
 371 struct value *
 372 read_var_value (struct symbol *var, struct frame_info *frame)
 373 {
 374   struct value *v;
 375   struct type *type = SYMBOL_TYPE (var);
 376   CORE_ADDR addr;
 377   int len;
 378
 379   if (SYMBOL_CLASS (var) == LOC_COMPUTED
 380       || SYMBOL_CLASS (var) == LOC_COMPUTED_ARG
 381       || SYMBOL_CLASS (var) == LOC_REGISTER
 382       || SYMBOL_CLASS (var) == LOC_REGPARM)
 383     /* These cases do not use V.  */
 384     v = NULL;
 385   else
 386     {
 387       v = allocate_value (type);
 388       VALUE_LVAL (v) = lval_memory;     /* The most likely possibility.  */
 389     }
 390
 391   len = TYPE_LENGTH (type);
 392
 393   /* FIXME drow/2003-09-06: this call to the selected frame should be
 394      pushed upwards to the callers.  */
 395   if (frame == NULL)
 396     frame = deprecated_safe_get_selected_frame ();
 397
 398   switch (SYMBOL_CLASS (var))
 399     {
 400     case LOC_CONST:
 401       /* Put the constant back in target format.  */
 402       store_signed_integer (value_contents_raw (v), len,
 403                             (LONGEST) SYMBOL_VALUE (var));
 404       VALUE_LVAL (v) = not_lval;
 405       return v;
 406
 407     case LOC_LABEL:
 408       /* Put the constant back in target format.  */
 409       if (overlay_debugging)
 410         {
 411           CORE_ADDR addr
 412             = symbol_overlayed_address (SYMBOL_VALUE_ADDRESS (var),
 413                                         SYMBOL_BFD_SECTION (var));
 414           store_typed_address (value_contents_raw (v), type, addr);
 415         }
 416       else
 417         store_typed_address (value_contents_raw (v), type,
 418                               SYMBOL_VALUE_ADDRESS (var));
 419       VALUE_LVAL (v) = not_lval;
 420       return v;
 421
 422     case LOC_CONST_BYTES:
 423       {
 424         memcpy (value_contents_raw (v), SYMBOL_VALUE_BYTES (var), len);
 425         VALUE_LVAL (v) = not_lval;
 426         return v;
 427       }
 428
 429     case LOC_STATIC:
 430       if (overlay_debugging)
 431         addr = symbol_overlayed_address (SYMBOL_VALUE_ADDRESS (var),
 432                                          SYMBOL_BFD_SECTION (var));
 433       else
 434         addr = SYMBOL_VALUE_ADDRESS (var);
 435       break;
 436
 437     case LOC_INDIRECT:
 438       {
 439         /* The import slot does not have a real address in it from the
 440            dynamic loader (dld.sl on HP-UX), if the target hasn't
 441            begun execution yet, so check for that. */
 442         CORE_ADDR locaddr;
 443         struct value *loc;
 444         if (!target_has_execution)
 445           error (_("\
 446 Attempt to access variable defined in different shared object or load module when\n\
 447 addresses have not been bound by the dynamic loader. Try again when executable is running."));
 448
 449         locaddr = SYMBOL_VALUE_ADDRESS (var);
 450         loc = value_at (lookup_pointer_type (type), locaddr);
 451         addr = value_as_address (loc);
 452         break;
 453       }
 454
 455     case LOC_ARG:
 456       if (frame == NULL)
 457         return 0;
 458       addr = get_frame_args_address (frame);
 459       if (!addr)
 460         return 0;
 461       addr += SYMBOL_VALUE (var);
 462       break;
 463
 464     case LOC_REF_ARG:
 465       {
 466         struct value *ref;
 467         CORE_ADDR argref;
 468         if (frame == NULL)
 469           return 0;
 470         argref = get_frame_args_address (frame);
 471         if (!argref)
 472           return 0;
 473         argref += SYMBOL_VALUE (var);
 474         ref = value_at (lookup_pointer_type (type), argref);
 475         addr = value_as_address (ref);
 476         break;
 477       }
 478
 479     case LOC_LOCAL:
 480     case LOC_LOCAL_ARG:
 481       if (frame == NULL)
 482         return 0;
 483       addr = get_frame_locals_address (frame);
 484       addr += SYMBOL_VALUE (var);
 485       break;
 486
 487     case LOC_BASEREG:
 488     case LOC_BASEREG_ARG:
 489     case LOC_HP_THREAD_LOCAL_STATIC:
 490       {
 491         struct value *regval;
 492
 493         regval = value_from_register (lookup_pointer_type (type),
 494                                       SYMBOL_BASEREG (var), frame);
 495         if (regval == NULL)
 496           error (_("Value of base register not available."));
 497         addr = value_as_address (regval);
 498         addr += SYMBOL_VALUE (var);
 499         break;
 500       }
 501
 502     case LOC_TYPEDEF:
 503       error (_("Cannot look up value of a typedef"));
 504       break;
 505
 506     case LOC_BLOCK:
 507       if (overlay_debugging)
 508         VALUE_ADDRESS (v) = symbol_overlayed_address
 509           (BLOCK_START (SYMBOL_BLOCK_VALUE (var)), SYMBOL_BFD_SECTION (var));
 510       else
 511         VALUE_ADDRESS (v) = BLOCK_START (SYMBOL_BLOCK_VALUE (var));
 512       return v;
 513
 514     case LOC_REGISTER:
 515     case LOC_REGPARM:
 516     case LOC_REGPARM_ADDR:
 517       {
 518         struct block *b;
 519         int regno = SYMBOL_VALUE (var);
 520         struct value *regval;
 521
 522         if (frame == NULL)
 523           return 0;
 524         b = get_frame_block (frame, 0);
 525
 526         if (SYMBOL_CLASS (var) == LOC_REGPARM_ADDR)
 527           {
 528             regval = value_from_register (lookup_pointer_type (type),
 529                                           regno,
 530                                           frame);
 531
 532             if (regval == NULL)
 533               error (_("Value of register variable not available."));
 534
 535             addr = value_as_address (regval);
 536             VALUE_LVAL (v) = lval_memory;
 537           }
 538         else
 539           {
 540             regval = value_from_register (type, regno, frame);
 541
 542             if (regval == NULL)
 543               error (_("Value of register variable not available."));
 544             return regval;
 545           }
 546       }
 547       break;
 548
 549     case LOC_COMPUTED:
 550     case LOC_COMPUTED_ARG:
 551       /* FIXME: cagney/2004-01-26: It should be possible to
 552          unconditionally call the SYMBOL_OPS method when available.
 553          Unfortunately DWARF 2 stores the frame-base (instead of the
 554          function) location in a function's symbol.  Oops!  For the
 555          moment enable this when/where applicable.  */
 556       if (frame == 0 && SYMBOL_OPS (var)->read_needs_frame (var))
 557         return 0;
 558       return SYMBOL_OPS (var)->read_variable (var, frame);
 559
 560     case LOC_UNRESOLVED:
 561       {
 562         struct minimal_symbol *msym;
 563
 564         msym = lookup_minimal_symbol (DEPRECATED_SYMBOL_NAME (var), NULL, NULL);
 565         if (msym == NULL)
 566           return 0;
 567         if (overlay_debugging)
 568           addr = symbol_overlayed_address (SYMBOL_VALUE_ADDRESS (msym),
 569                                            SYMBOL_BFD_SECTION (msym));
 570         else
 571           addr = SYMBOL_VALUE_ADDRESS (msym);
 572       }
 573       break;
 574
 575     case LOC_OPTIMIZED_OUT:
 576       VALUE_LVAL (v) = not_lval;
 577       set_value_optimized_out (v, 1);
 578       return v;
 579
 580     default:
 581       error (_("Cannot look up value of a botched symbol."));
 582       break;
 583     }
 584
 585   VALUE_ADDRESS (v) = addr;
 586   set_value_lazy (v, 1);
 587   return v;
 588 }
 589
 590 /* Install default attributes for register values.  */
 591
 592 struct value *
 593 default_value_from_register (struct type *type, int regnum,
 594                              struct frame_info *frame)
 595 {
 596   struct gdbarch *gdbarch = get_frame_arch (frame);
 597   int len = TYPE_LENGTH (type);
 598   struct value *value = allocate_value (type);
 599
 600   VALUE_LVAL (value) = lval_register;
 601   VALUE_FRAME_ID (value) = get_frame_id (frame);
 602   VALUE_REGNUM (value) = regnum;
 603
 604   /* Any structure stored in more than one register will always be
 605      an integral number of registers.  Otherwise, you need to do
 606      some fiddling with the last register copied here for little
 607      endian machines.  */
 608   if (gdbarch_byte_order (current_gdbarch) == BFD_ENDIAN_BIG
 609       && len < register_size (gdbarch, regnum))
 610     /* Big-endian, and we want less than full size.  */
 611     set_value_offset (value, register_size (gdbarch, regnum) - len);
 612   else
 613     set_value_offset (value, 0);
 614
 615   return value;
 616 }
 617
 618 /* Return a value of type TYPE, stored in register REGNUM, in frame FRAME.  */
 619
 620 struct value *
 621 value_from_register (struct type *type, int regnum, struct frame_info *frame)
 622 {
 623   struct gdbarch *gdbarch = get_frame_arch (frame);
 624   struct type *type1 = check_typedef (type);
 625   struct value *v;
 626
 627   if (gdbarch_convert_register_p (current_gdbarch, regnum, type1))
 628     {
 629       /* The ISA/ABI need to something weird when obtaining the
 630          specified value from this register.  It might need to
 631          re-order non-adjacent, starting with REGNUM (see MIPS and
 632          i386).  It might need to convert the [float] register into
 633          the corresponding [integer] type (see Alpha).  The assumption
 634          is that gdbarch_register_to_value populates the entire value
 635          including the location.  */
 636       v = allocate_value (type);
 637       VALUE_LVAL (v) = lval_register;
 638       VALUE_FRAME_ID (v) = get_frame_id (frame);
 639       VALUE_REGNUM (v) = regnum;
 640       gdbarch_register_to_value (current_gdbarch,
 641                                  frame, regnum, type1, value_contents_raw (v));
 642     }
 643   else
 644     {
 645       int len = TYPE_LENGTH (type);
 646
 647       /* Construct the value.  */
 648       v = gdbarch_value_from_register (gdbarch, type, regnum, frame);
 649
 650       /* Get the data.  */
 651       if (!get_frame_register_bytes (frame, regnum, value_offset (v), len,
 652                                      value_contents_raw (v)))
 653         set_value_optimized_out (v, 1);
 654     }
 655   return v;
 656 }
 657
 658 /* Return contents of register REGNUM in frame FRAME as address,
 659    interpreted as value of type TYPE.   Will abort if register
 660    value is not available.  */
 661
 662 CORE_ADDR
 663 address_from_register (struct type *type, int regnum, struct frame_info *frame)
 664 {
 665   struct value *value;
 666   CORE_ADDR result;
 667
 668   value = value_from_register (type, regnum, frame);
 669   gdb_assert (value);
 670
 671   result = value_as_address (value);
 672   release_value (value);
 673   value_free (value);
 674
 675   return result;
 676 }
 677
 678 \f
 679 /* Given a struct symbol for a variable or function,
 680    and a stack frame id,
 681    return a (pointer to a) struct value containing the properly typed
 682    address.  */
 683
 684 struct value *
 685 locate_var_value (struct symbol *var, struct frame_info *frame)
 686 {
 687   CORE_ADDR addr = 0;
 688   struct type *type = SYMBOL_TYPE (var);
 689   struct value *lazy_value;
 690
 691   /* Evaluate it first; if the result is a memory address, we're fine.
 692      Lazy evaluation pays off here. */
 693
 694   lazy_value = read_var_value (var, frame);
 695   if (lazy_value == 0)
 696     error (_("Address of \"%s\" is unknown."), SYMBOL_PRINT_NAME (var));
 697
 698   if (value_lazy (lazy_value)
 699       || TYPE_CODE (type) == TYPE_CODE_FUNC)
 700     {
 701       struct value *val;
 702
 703       addr = VALUE_ADDRESS (lazy_value);
 704       val = value_from_pointer (lookup_pointer_type (type), addr);
 705       return val;
 706     }
 707
 708   /* Not a memory address; check what the problem was.  */
 709   switch (VALUE_LVAL (lazy_value))
 710     {
 711     case lval_register:
 712       gdb_assert (gdbarch_register_name
 713                    (current_gdbarch, VALUE_REGNUM (lazy_value)) != NULL
 714                   && *gdbarch_register_name
 715                     (current_gdbarch, VALUE_REGNUM (lazy_value)) != '\0');
 716       error (_("Address requested for identifier "
 717                "\"%s\" which is in register $%s"),
 718             SYMBOL_PRINT_NAME (var),
 719             gdbarch_register_name (current_gdbarch, VALUE_REGNUM (lazy_value)));
 720       break;
 721
 722     default:
 723       error (_("Can't take address of \"%s\" which isn't an lvalue."),
 724              SYMBOL_PRINT_NAME (var));
 725       break;
 726     }
 727   return 0;                     /* For lint -- never reached */
 728 }