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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 (*note Getting In and Out of GDB: Invocation.).
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
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.
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' 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 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
`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
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. *Note Optional warnings
and messages: Messages/Warnings.)
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 ]'
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.
If memory-mapped files are available on your system through the
`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 ]'
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
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 (*note Killing the child process: Kill Process.).
`add-symbol-file FILENAME ADDRESS'
`add-symbol-file FILENAME ADDRESS [ -readnow ] [ -mapped ]'
`add-symbol-file FILENAME -sSECTION 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 `-sSECTION ADDRESS' pairs, to give an explicit section name and
base address for that section. You can specify any ADDRESS as an
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:
* the file's symbolic information refers only to linker symbols
defined in that file, not to symbols defined by other object
* every section the file's symbolic information refers to has
actually been loaded into the inferior, as it appears in the
* you can determine the address at which every section was
loaded, and provide these to the `add-symbol-file' command.
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
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.
The `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
The `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' and `info target' are synonymous; both print the
current target (*note Specifying a Debugging Target: Targets.),
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):
Display sections for all loaded object files, including
Display info only for named SECTIONS.
Display info only for sections for which SECTION-FLAGS are
true. The section flags that GDB currently knows about are:
Section will have space allocated in the process when
loaded. Set for all sections except those containing
Section will be loaded from the file into the child
process memory. Set for pre-initialized code and data,
clear for `.bss' sections.
Section needs to be relocated before loading.
Section cannot be modified by the child process.
Section contains executable code only.
Section contains data only (no executable code).
Section will reside in ROM.
Section contains data for constructor/destructor lists.
Section is not empty.
An instruction to the linker to not output the section.
A notification to the linker that the section contains
COFF shared library information.
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
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.
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
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
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
`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'.
Display the current autoloading mode.
To explicitly load shared library symbols, use the `sharedlibrary'
Print the names of the shared libraries which are currently loaded.
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.
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
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'
Set the autoloading size threshold, in an integral number of
megabytes. If THRESHOLD is nonzero and shared library autoloading
is enabled, symbols from all shared object libraries will be
loaded until the total size of the loaded shared library symbols
exceeds this threshold. Otherwise, symbols must be loaded
manually, using the `sharedlibrary' command. The default
threshold is 100 (i.e. 100 Mb).
Display the current autoloading size threshold, in megabytes.
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
`set solib-absolute-prefix PATH'
If this variable is set, PATH will be used as a prefix for any
absolute shared library paths; many runtime loaders store the
absolute paths to the shared library in the target program's
memory. If you use `solib-absolute-prefix' to find shared
libraries, they need to be laid out in the same way that they are
on the target, with e.g. a `/usr/lib' hierarchy under PATH.
You can set the default value of `solib-absolute-prefix' by using
the configure-time `--with-sysroot' option.
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 `solib-absolute-prefix' 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
`solib-absolute-prefix', be sure to set `solib-absolute-prefix' to
a nonexistant directory to prevent GDB from finding your host's
Display the current shared library search path.
Created Mon Nov 8 17:42:38 2004 on tillpc with info_to_html version 0.9.6.