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18. GDB Files

GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file.

18.1 Commands to Specify Files  Commands to specify files
18.2 Debugging Information in Separate Files  Debugging information in separate files
18.3 Index Files Speed Up GDB  Index files speed up GDB
18.4 Errors Reading Symbol Files  Errors reading symbol files
18.5 GDB Data Files  GDB data files

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18.1 Commands to Specify Files

You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB's start-up commands (see section Getting In and Out of GDB).

Occasionally it is necessary to change to a different file during a GDB session. Or you may run GDB and forget to specify a file you want to use. Or you are debugging a remote target via gdbserver (see section Using the gdbserver Program). In these situations the GDB commands to specify new files are useful.

file filename
Use filename as the program to be debugged. It is read for its symbols and for the contents of pure memory. It is also the program executed when you use the run command. If you do not specify a directory and the file is not found in the GDB working directory, GDB uses the environment variable PATH as a list of directories to search, just as the shell does when looking for a program to run. You can change the value of this variable, for both GDB and your program, using the path command.

You can load unlinked object `.o' files into GDB using the file command. You will not be able to "run" an object file, but you can disassemble functions and inspect variables. Also, if the underlying BFD functionality supports it, you could use gdb -write to patch object files using this technique. Note that GDB can neither interpret nor modify relocations in this case, so branches and some initialized variables will appear to go to the wrong place. But this feature is still handy from time to time.

file
file with no argument makes GDB discard any information it has on both executable file and the symbol table.

exec-file [ filename ]
Specify that the program to be run (but not the symbol table) is found in filename. GDB searches the environment variable PATH if necessary to locate your program. Omitting filename means to discard information on the executable file.

symbol-file [ filename ]
Read symbol table information from file filename. PATH is searched when necessary. Use the file command to get both symbol table and program to run from the same file.

symbol-file with no argument clears out GDB information on your program's symbol table.

The symbol-file command causes GDB to forget the contents of some breakpoints and auto-display expressions. This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDB.

symbol-file does not repeat if you press RET again after executing it once.

When GDB is configured for a particular environment, it understands debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions. Best results are usually obtained from GNU compilers; for example, using GCC you can generate debugging information for optimized code.

For most kinds of object files, with the exception of old SVR3 systems using COFF, the symbol-file command does not normally read the symbol table in full right away. Instead, it scans the symbol table quickly to find which source files and which symbols are present. The details are read later, one source file at a time, as they are needed.

The purpose of this two-stage reading strategy is to make GDB start up faster. For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read. (The set verbose command can turn these pauses into messages if desired. See section Optional Warnings and Messages.)

We have not implemented the two-stage strategy for COFF yet. When the symbol table is stored in COFF format, symbol-file reads the symbol table data in full right away. Note that "stabs-in-COFF" still does the two-stage strategy, since the debug info is actually in stabs format.

symbol-file [ -readnow ] filename
file [ -readnow ] filename
You can override the GDB two-stage strategy for reading symbol tables by using the `-readnow' option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available.

core-file [filename]
core
Specify the whereabouts of a core dump file to be used as the "contents of memory". Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts.

core-file with no argument specifies that no core file is to be used.

Note that the core file is ignored when your program is actually running under GDB. So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running. To do this, use the kill command (see section Killing the Child Process).

add-symbol-file filename address
add-symbol-file filename address [ -readnow ]
add-symbol-file filename address -s section address ...
The add-symbol-file command reads additional symbol table information from the file filename. You would use this command when filename has been dynamically loaded (by some other means) into the program that is running. address should be the memory address at which the file has been loaded; GDB cannot figure this out for itself. You can additionally specify an arbitrary number of `-s section address' pairs, to give an explicit section name and base address for that section. You can specify any address as an expression.

The symbol table of the file filename is added to the symbol table originally read with the symbol-file command. You can use the add-symbol-file command any number of times; the new symbol data thus read keeps adding to the old. To discard all old symbol data instead, use the symbol-file command without any arguments.

Although filename is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable `.o' files, as long as:

Some embedded operating systems, like Sun Chorus and VxWorks, can load relocatable files into an already running program; such systems typically make the requirements above easy to meet. However, it's important to recognize that many native systems use complex link procedures (.linkonce section factoring and C++ constructor table assembly, for example) that make the requirements difficult to meet. In general, one cannot assume that using add-symbol-file to read a relocatable object file's symbolic information will have the same effect as linking the relocatable object file into the program in the normal way.

add-symbol-file does not repeat if you press RET after using it.

add-symbol-file-from-memory address
Load symbols from the given address in a dynamically loaded object file whose image is mapped directly into the inferior's memory. For example, the Linux kernel maps a syscall DSO into each process's address space; this DSO provides kernel-specific code for some system calls. The argument can be any expression whose evaluation yields the address of the file's shared object file header. For this command to work, you must have used symbol-file or exec-file commands in advance.

add-shared-symbol-files library-file
assf library-file
The add-shared-symbol-files command can currently be used only in the Cygwin build of GDB on MS-Windows OS, where it is an alias for the dll-symbols command (see section 21.1.5 Features for Debugging MS Windows PE Executables). GDB automatically looks for shared libraries, however if GDB does not find yours, you can invoke add-shared-symbol-files. It takes one argument: the shared library's file name. assf is a shorthand alias for add-shared-symbol-files.

section section addr
The section command changes the base address of the named section of the exec file to addr. This can be used if the exec file does not contain section addresses, (such as in the a.out format), or when the addresses specified in the file itself are wrong. Each section must be changed separately. The info files command, described below, lists all the sections and their addresses.

info files
info target
info files and info target are synonymous; both print the current target (see section Specifying a Debugging Target), including the names of the executable and core dump files currently in use by GDB, and the files from which symbols were loaded. The command help target lists all possible targets rather than current ones.

maint info sections
Another command that can give you extra information about program sections is maint info sections. In addition to the section information displayed by info files, this command displays the flags and file offset of each section in the executable and core dump files. In addition, maint info sections provides the following command options (which may be arbitrarily combined):

ALLOBJ
Display sections for all loaded object files, including shared libraries.
sections
Display info only for named sections.
section-flags
Display info only for sections for which section-flags are true. The section flags that GDB currently knows about are:
ALLOC
Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.
LOAD
Section will be loaded from the file into the child process memory. Set for pre-initialized code and data, clear for .bss sections.
RELOC
Section needs to be relocated before loading.
READONLY
Section cannot be modified by the child process.
CODE
Section contains executable code only.
DATA
Section contains data only (no executable code).
ROM
Section will reside in ROM.
CONSTRUCTOR
Section contains data for constructor/destructor lists.
HAS_CONTENTS
Section is not empty.
NEVER_LOAD
An instruction to the linker to not output the section.
COFF_SHARED_LIBRARY
A notification to the linker that the section contains COFF shared library information.
IS_COMMON
Section contains common symbols.
set trust-readonly-sections on
Tell GDB that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDB can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance.

The default is off.

set trust-readonly-sections off
Tell GDB not to trust readonly sections. This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed.

show trust-readonly-sections
Show the current setting of trusting readonly sections.

All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way.

GDB supports GNU/Linux, MS-Windows, HP-UX, SunOS, SVr4, Irix, and IBM RS/6000 AIX shared libraries.

On MS-Windows GDB must be linked with the Expat library to support shared libraries. See Expat.

GDB automatically loads symbol definitions from shared libraries when you use the run command, or when you examine a core file. (Before you issue the run command, GDB does not understand references to a function in a shared library, however--unless you are debugging a core file).

On HP-UX, if the program loads a library explicitly, GDB automatically loads the symbols at the time of the shl_load call.

There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them.

To control the automatic loading of shared library symbols, use the commands:

set auto-solib-add mode
If mode is on, symbols from all shared object libraries will be loaded automatically when the inferior begins execution, you attach to an independently started inferior, or when the dynamic linker informs GDB that a new library has been loaded. If mode is off, symbols must be loaded manually, using the sharedlibrary command. The default value is on.

If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDB memory footprint by preventing it from automatically loading the symbols from shared libraries. To that end, type set auto-solib-add off before running the inferior, then load each library whose debug symbols you do need with sharedlibrary regexp, where regexp is a regular expression that matches the libraries whose symbols you want to be loaded.

show auto-solib-add
Display the current autoloading mode.

To explicitly load shared library symbols, use the sharedlibrary command:

info share regex
info sharedlibrary regex
Print the names of the shared libraries which are currently loaded that match regex. If regex is omitted then print all shared libraries that are loaded.

sharedlibrary regex
share regex
Load shared object library symbols for files matching a Unix regular expression. As with files loaded automatically, it only loads shared libraries required by your program for a core file or after typing run. If regex is omitted all shared libraries required by your program are loaded.

nosharedlibrary
Unload all shared object library symbols. This discards all symbols that have been loaded from all shared libraries. Symbols from shared libraries that were loaded by explicit user requests are not discarded.

Sometimes you may wish that GDB stops and gives you control when any of shared library events happen. Use the set stop-on-solib-events command for this:

set stop-on-solib-events
This command controls whether GDB should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library.

show stop-on-solib-events
Show whether GDB stops and gives you control when shared library events happen.

Shared libraries are also supported in many cross or remote debugging configurations. GDB needs to have access to the target's libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDB to automatically retrieve the libraries from the target. If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not.

For remote debugging, you need to tell GDB where the target libraries are, so that it can load the correct copies--otherwise, it may try to load the host's libraries. GDB has two variables to specify the search directories for target libraries.

set sysroot path
Use path as the system root for the program being debugged. Any absolute shared library paths will be prefixed with path; many runtime loaders store the absolute paths to the shared library in the target program's memory. If you use set sysroot to find shared libraries, they need to be laid out in the same way that they are on the target, with e.g. a `/lib' and `/usr/lib' hierarchy under path.

If path starts with the sequence `remote:', GDB will retrieve the target libraries from the remote system. This is only supported when using a remote target that supports the remote get command (see section Sending files to a remote system). The part of path following the initial `remote:' (if present) is used as system root prefix on the remote file system. (12)

For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDB tries prefixing a few variants of the target absolute file name with path. But first, on Unix hosts, GDB converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix:

 
  c:\foo\bar.dll => c:/foo/bar.dll

Then, GDB attempts prefixing the target file name with path, and looks for the resulting file name in the host file system:

 
  c:/foo/bar.dll => /path/to/sysroot/c:/foo/bar.dll

If that does not find the shared library, GDB tries removing the `:' character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons:

 
  c:/foo/bar.dll => /path/to/sysroot/c/foo/bar.dll

This makes it possible to have a system root that mirrors a target with more than one drive. E.g., you may want to setup your local copies of the target system shared libraries like so (note `c' vs `z'):

 
 `/path/to/sysroot/c/sys/bin/foo.dll'
 `/path/to/sysroot/c/sys/bin/bar.dll'
 `/path/to/sysroot/z/sys/bin/bar.dll'

and point the system root at `/path/to/sysroot', so that GDB can find the correct copies of both `c:\sys\bin\foo.dll', and `z:\sys\bin\bar.dll'.

If that still does not find the shared library, GDB tries removing the whole drive spec from the target file name:

 
  c:/foo/bar.dll => /path/to/sysroot/foo/bar.dll

This last lookup makes it possible to not care about the drive name, if you don't want or need to.

The set solib-absolute-prefix command is an alias for set sysroot.

You can set the default system root by using the configure-time `--with-sysroot' option. If the system root is inside GDB's configured binary prefix (set with `--prefix' or `--exec-prefix'), then the default system root will be updated automatically if the installed GDB is moved to a new location.

show sysroot
Display the current shared library prefix.

set solib-search-path path
If this variable is set, path is a colon-separated list of directories to search for shared libraries. `solib-search-path' is used after `sysroot' fails to locate the library, or if the path to the library is relative instead of absolute. If you want to use `solib-search-path' instead of `sysroot', be sure to set `sysroot' to a nonexistent directory to prevent GDB from finding your host's libraries. `sysroot' is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols.

show solib-search-path
Display the current shared library search path.

set target-file-system-kind kind
Set assumed file system kind for target reported file names.

Shared library file names as reported by the target system may not make sense as is on the system GDB is running on. For example, when remote debugging a target that has MS-DOS based file system semantics, from a Unix host, the target may be reporting to GDB a list of loaded shared libraries with file names such as `c:\Windows\kernel32.dll'. On Unix hosts, there's no concept of drive letters, so the `c:\' prefix is not normally understood as indicating an absolute file name, and neither is the backslash normally considered a directory separator character. In that case, the native file system would interpret this whole absolute file name as a relative file name with no directory components. This would make it impossible to point GDB at a copy of the remote target's shared libraries on the host using set sysroot, and impractical with set solib-search-path. Setting target-file-system-kind to dos-based tells GDB to interpret such file names similarly to how the target would, and to map them to file names valid on GDB's native file system semantics. The value of kind can be "auto", in addition to one of the supported file system kinds. In that case, GDB tries to determine the appropriate file system variant based on the current target's operating system (see section Configuring the Current ABI). The supported file system settings are:

unix
Instruct GDB to assume the target file system is of Unix kind. Only file names starting the forward slash (`/') character are considered absolute, and the directory separator character is also the forward slash.

dos-based
Instruct GDB to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., `c:'), are considered absolute, and both the slash (`/') and the backslash (`\\') characters are considered directory separators.

auto
Instruct GDB to use the file system kind associated with the target operating system (see section Configuring the Current ABI). This is the default.

When processing file names provided by the user, GDB frequently needs to compare them to the file names recorded in the program's debug info. Normally, GDB compares just the base names of the files as strings, which is reasonably fast even for very large programs. (The base name of a file is the last portion of its name, after stripping all the leading directories.) This shortcut in comparison is based upon the assumption that files cannot have more than one base name. This is usually true, but references to files that use symlinks or similar filesystem facilities violate that assumption. If your program records files using such facilities, or if you provide file names to GDB using symlinks etc., you can set basenames-may-differ to true to instruct GDB to completely canonicalize each pair of file names it needs to compare. This will make file-name comparisons accurate, but at a price of a significant slowdown.

set basenames-may-differ
Set whether a source file may have multiple base names.

show basenames-may-differ
Show whether a source file may have multiple base names.

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18.2 Debugging Information in Separate Files

GDB allows you to put a program's debugging information in a file separate from the executable itself, in a way that allows GDB to find and load the debugging information automatically. Since debugging information can be very large--sometimes larger than the executable code itself--some systems distribute debugging information for their executables in separate files, which users can install only when they need to debug a problem.

GDB supports two ways of specifying the separate debug info file:

Depending on the way the debug info file is specified, GDB uses two different methods of looking for the debug file:

So, for example, suppose you ask GDB to debug `/usr/bin/ls', which has a debug link that specifies the file `ls.debug', and a build ID whose value in hex is abcdef1234. If the global debug directory is `/usr/lib/debug', then GDB will look for the following debug information files, in the indicated order:

You can set the global debugging info directory's name, and view the name GDB is currently using.

set debug-file-directory directories
Set the directories which GDB searches for separate debugging information files to directory. Multiple directory components can be set concatenating them by a directory separator.

show debug-file-directory
Show the directories GDB searches for separate debugging information files.

A debug link is a special section of the executable file named .gnu_debuglink. The section must contain:

Any executable file format can carry a debug link, as long as it can contain a section named .gnu_debuglink with the contents described above.

The build ID is a special section in the executable file (and in other ELF binary files that GDB may consider). This section is often named .note.gnu.build-id, but that name is not mandatory. It contains unique identification for the built files--the ID remains the same across multiple builds of the same build tree. The default algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the content for the build ID string. The same section with an identical value is present in the original built binary with symbols, in its stripped variant, and in the separate debugging information file.

The debugging information file itself should be an ordinary executable, containing a full set of linker symbols, sections, and debugging information. The sections of the debugging information file should have the same names, addresses, and sizes as the original file, but they need not contain any data--much like a .bss section in an ordinary executable.

The GNU binary utilities (Binutils) package includes the `objcopy' utility that can produce the separated executable / debugging information file pairs using the following commands:

 
objcopy --only-keep-debug foo foo.debug
strip -g foo

These commands remove the debugging information from the executable file `foo' and place it in the file `foo.debug'. You can use the first, second or both methods to link the two files:

The CRC used in .gnu_debuglink is the CRC-32 defined in IEEE 802.3 using the polynomial:

 
 x32 + x26 + x23 + x22 + x16 + x12 + x11
 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

The function is computed byte at a time, taking the least significant bit of each byte first. The initial pattern 0xffffffff is used, to ensure leading zeros affect the CRC and the final result is inverted to ensure trailing zeros also affect the CRC.

Note: This is the same CRC polynomial as used in handling the Remote Serial Protocol qCRC packet (see section GDB Remote Serial Protocol). However in the case of the Remote Serial Protocol, the CRC is computed most significant bit first, and the result is not inverted, so trailing zeros have no effect on the CRC value.

To complete the description, we show below the code of the function which produces the CRC used in .gnu_debuglink. Inverting the initially supplied crc argument means that an initial call to this function passing in zero will start computing the CRC using 0xffffffff.

 
unsigned long
gnu_debuglink_crc32 (unsigned long crc,
                     unsigned char *buf, size_t len)
{
  static const unsigned long crc32_table[256] =
    {
      0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
      0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
      0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
      0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
      0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
      0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
      0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
      0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
      0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
      0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
      0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
      0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
      0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
      0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
      0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
      0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
      0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
      0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
      0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
      0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
      0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
      0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
      0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
      0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
      0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
      0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
      0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
      0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
      0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
      0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
      0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
      0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
      0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
      0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
      0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
      0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
      0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
      0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
      0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
      0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
      0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
      0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
      0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
      0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
      0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
      0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
      0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
      0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
      0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
      0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
      0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
      0x2d02ef8d
    };
  unsigned char *end;

  crc = ~crc & 0xffffffff;
  for (end = buf + len; buf < end; ++buf)
    crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
  return ~crc & 0xffffffff;
}

This computation does not apply to the "build ID" method.

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18.3 Index Files Speed Up GDB

When GDB finds a symbol file, it scans the symbols in the file in order to construct an internal symbol table. This lets most GDB operations work quickly--at the cost of a delay early on. For large programs, this delay can be quite lengthy, so GDB provides a way to build an index, which speeds up startup.

The index is stored as a section in the symbol file. GDB can write the index to a file, then you can put it into the symbol file using objcopy.

To create an index file, use the save gdb-index command:

save gdb-index directory
Create an index file for each symbol file currently known by GDB. Each file is named after its corresponding symbol file, with `.gdb-index' appended, and is written into the given directory.

Once you have created an index file you can merge it into your symbol file, here named `symfile', using objcopy:

 
$ objcopy --add-section .gdb_index=symfile.gdb-index \
    --set-section-flags .gdb_index=readonly symfile symfile

There are currently some limitation on indices. They only work when for DWARF debugging information, not stabs. And, they do not currently work for programs using Ada.

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18.4 Errors Reading Symbol Files

While reading a symbol file, GDB occasionally encounters problems, such as symbol types it does not recognize, or known bugs in compiler output. By default, GDB does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers. If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDB to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDB to print more messages, to see how many times the problems occur, with the set complaints command (see section Optional Warnings and Messages).

The messages currently printed, and their meanings, include:

inner block not inside outer block in symbol

The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks.

GDB circumvents the problem by treating the inner block as if it had the same scope as the outer block. In the error message, symbol may be shown as "(don't know)" if the outer block is not a function.

block at address out of order

The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so.

GDB does not circumvent this problem, and has trouble locating symbols in the source file whose symbols it is reading. (You can often determine what source file is affected by specifying set verbose on. See section Optional Warnings and Messages.)

bad block start address patched

The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.

GDB circumvents the problem by treating the symbol scope block as starting on the previous source line.

bad string table offset in symbol n

Symbol number n contains a pointer into the string table which is larger than the size of the string table.

GDB circumvents the problem by considering the symbol to have the name foo, which may cause other problems if many symbols end up with this name.

unknown symbol type 0xnn

The symbol information contains new data types that GDB does not yet know how to read. 0xnn is the symbol type of the uncomprehended information, in hexadecimal.

GDB circumvents the error by ignoring this symbol information. This usually allows you to debug your program, though certain symbols are not accessible. If you encounter such a problem and feel like debugging it, you can debug gdb with itself, breakpoint on complain, then go up to the function read_dbx_symtab and examine *bufp to see the symbol.

stub type has NULL name

GDB could not find the full definition for a struct or class.

const/volatile indicator missing (ok if using g++ v1.x), got...
The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it.

info mismatch between compiler and debugger

GDB could not parse a type specification output by the compiler.

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18.5 GDB Data Files

GDB will sometimes read an auxiliary data file. These files are kept in a directory known as the data directory.

You can set the data directory's name, and view the name GDB is currently using.

set data-directory directory
Set the directory which GDB searches for auxiliary data files to directory.

show data-directory
Show the directory GDB searches for auxiliary data files.

You can set the default data directory by using the configure-time `--with-gdb-datadir' option. If the data directory is inside GDB's configured binary prefix (set with `--prefix' or `--exec-prefix'), then the default data directory will be updated automatically if the installed GDB is moved to a new location.

The data directory may also be specified with the --data-directory command line option. See section 2.1.2 Choosing Modes.

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