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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.
runcommand. If you do not specify a directory and the file is not found in the GDB working directory, GDB uses the environment variable
PATHas 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
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
add-symbol-file, described below),
for more information.
filewith no argument makes GDB discard any information it has on both executable file and the symbol table.
exec-file [ filename ]
PATHif necessary to locate your program. Omitting filename means to discard information on the executable file.
symbol-file [ filename ]
PATHis searched when necessary. Use the
filecommand 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.
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,
gcc you can generate debugging information for
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 ]
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 ]
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
(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-filecommand 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:
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
.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
add-symbol-file does not repeat if you press RET after using it.
You can use the `-mapped' and `-readnow' options just as with
symbol-file command, to change how GDB manages the symbol
table information for filename.
add-shared-symbol-filecommand 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.
sectioncommand 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 filescommand, described below, lists all the sections and their addresses.
info targetare 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 targetlists 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 sectionsprovides the following command options (which may be arbitrarily combined):
set trust-readonly-sections on
The default is off.
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
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
sharedlibrarycommand. The default value is
To explicitly load shared library symbols, use the
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
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
sharedlibrarycommand. The default threshold is 100 (i.e. 100 Mb).
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