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
`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.
On some kinds of object files, 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.
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-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).
load filename
load command may be available. Where it exists, it
is meant to make filename (an executable) available for debugging
on the remote system--by downloading, or dynamic linking, for example.
load also records the filename symbol table in GDB, like
the add-symbol-file command.
If your GDB does not have a load command, attempting to
execute it gets the error message "You can't do that when your
target is ..."
The file is loaded at whatever address is specified in the executable.
For some object file formats, you can specify the load address when you
link the program; for other formats, like a.out, the object file format
specifies a fixed address.
On VxWorks, load links filename dynamically on the
current target system as well as adding its symbols in GDB.
With the Nindy interface to an Intel 960 board, load
downloads filename to the 960 as well as adding its symbols in
GDB.
When you select remote debugging to a Hitachi SH, H8/300, or H8/500 board
(see section GDB and Hitachi microprocessors),
the load command downloads your program to the Hitachi board and also
opens it as the current executable target for GDB on your host
(like the file command).
load does not repeat if you press RET again after using it.
add-symbol-file filename address
add-symbol-file filename address [ -readnow ] [ -mapped ]
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 specify 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.
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 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.
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 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).
info share
info sharedlibrary
sharedlibrary regex
share regex
run. If
regex is omitted all shared libraries required by your program are
loaded.
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 misunderstood
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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