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.
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. In these situations the GDB commands to specify new files are useful.
file filename
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.
On systems with memory-mapped files, an auxiliary file named
`filename.syms' may hold symbol table information for
filename. If so, GDB maps in the symbol table from
`filename.syms', starting up more quickly. See the
descriptions of the file options `-mapped' and `-readnow'
(available on the command line, and with the commands file,
symbol-file, or add-symbol-file, described below),
for more information.
file
file with no argument makes GDB discard any information it
has on both executable file and the symbol table.
exec-file [ filename ]
PATH
if necessary to locate your program. Omitting filename means to
discard information on the executable file.
symbol-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 its convenience variables, the value history, and all 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 filename [ -readnow ] [ -mapped ]
file filename [ -readnow ] [ -mapped ]
mmap system call, you can use another option, `-mapped', to
cause GDB to write the symbols for your program into a reusable
file. Future GDB debugging sessions map in symbol information
from this auxiliary symbol file (if the program has not changed), rather
than spending time reading the symbol table from the executable
program. Using the `-mapped' option has the same effect as
starting GDB with the `-mapped' command-line option.
You can use both options together, to make sure the auxiliary symbol
file has all the symbol information for your program.
The auxiliary symbol file for a program called myprog is called
`myprog.syms'. Once this file exists (so long as it is newer
than the corresponding executable), GDB always attempts to use
it when you debug myprog; no special options or commands are
needed.
The `.syms' file is specific to the host machine where you run
GDB. It holds an exact image of the internal GDB
symbol table. It cannot be shared across multiple host platforms.
core-file [ filename ]
core
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 ] [ -mapped ]
add-symbol-file filename -ssection address ...
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 `-ssection 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:
add-symbol-file command.
.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.
You can use the `-mapped' and `-readnow' options just as with
the symbol-file command, to change how GDB manages the symbol
table information for filename.
add-shared-symbol-file
add-shared-symbol-file command can be used only under Harris' CXUX
operating system for the Motorola 88k. GDB automatically looks for
shared libraries, however if GDB does not find yours, you can run
add-shared-symbol-file. It takes no arguments.
section
section command changes the base address of section 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
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
sections
section-flags
ALLOC
LOAD
.bss sections.
RELOC
READONLY
CODE
DATA
ROM
CONSTRUCTOR
HAS_CONTENTS
NEVER_LOAD
COFF_SHARED_LIBRARY
IS_COMMON
set trust-readonly-sections on
set trust-readonly-sections off
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 HP-UX, SunOS, SVr4, Irix 5, and IBM RS/6000 shared libraries.
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
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.
show auto-solib-add
To explicitly load shared library symbols, use the sharedlibrary
command:
info share
info sharedlibrary
sharedlibrary regex
share regex
run. If
regex is omitted all shared libraries required by your program are
loaded.
On some systems, such as HP-UX systems, GDB supports autoloading shared library symbols until a limiting threshold size is reached. This provides the benefit of allowing autoloading to remain on by default, but avoids autoloading excessively large shared libraries, up to a threshold that is initially set, but which you can modify if you wish.
Beyond that threshold, symbols from shared libraries must be explicitly
loaded. To load these symbols, use the command sharedlibrary
filename. The base address of the shared library is determined
automatically by GDB and need not be specified.
To display or set the threshold, use the commands:
set auto-solib-limit threshold
sharedlibrary command. The default threshold is 100 (i.e. 100
Mb).
show auto-solib-limit
Shared libraries are also supported in many cross or remote debugging configurations. A copy of the target's libraries need to be present on the host system; 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.
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 solib-absolute-prefix path
show solib-absolute-prefix
set solib-search-path path
show solib-search-path
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.
If an executable's debugging information has been extracted to a separate file, the executable should contain a debug link giving the name of the debugging information file (with no directory components), and a checksum of its contents. (The exact form of a debug link is described below.) If the full name of the directory containing the executable is execdir, and the executable has a debug link that specifies the name debugfile, then GDB will automatically search for the debugging information file in three places:
GDB checks under each of these names for a debugging information file whose checksum matches that given in the link, and reads the debugging information from the first one it finds.
So, for example, if you ask GDB to debug `/usr/bin/ls', which has a link containing the name `ls.debug', and the global debug directory is `/usr/lib/debug', then GDB will look for debug information in `/usr/bin/ls.debug', `/usr/bin/.debug/ls.debug', and `/usr/lib/debug/usr/bin/ls.debug'.
You can set the global debugging info directory's name, and view the name GDB is currently using.
set debug-file-directory directory
show debug-file-directory
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 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.
As of December 2002, there is no standard GNU utility to produce
separated executable / debugging information file pairs. Ulrich
Drepper's `elfutils' package, starting with version 0.53,
contains a version of the strip command such that the command
strip foo -f foo.debug removes the debugging information from
the executable file `foo', places it in the file
`foo.debug', and leaves behind a debug link in `foo'.
Since there are many different ways to compute CRC's (different
polynomials, reversals, byte ordering, etc.), the simplest way to
describe the CRC used in .gnu_debuglink sections is to give the
complete code for a function that computes it:
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;
}
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
(don't know)" if the outer block is not a
function.
block at address out of order
set verbose on. See section Optional warnings and messages.)
bad block start address patched
bad string table offset in symbol n
foo, which may cause other problems if many symbols end up
with this name.
unknown symbol type 0xnn
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
const/volatile indicator missing (ok if using g++ v1.x), got...
info mismatch between compiler and debugger
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