4.2 Starting your Program
- Use the
runcommand to start your program under gdb. You must first specify the program name (except on VxWorks) with an argument to gdb (see Getting In and Out of gdb), or by using the
exec-filecommand (see Commands to Specify Files).
If you are running your program in an execution environment that
run creates an inferior process and makes
that process run your program. In some environments without processes,
run jumps to the start of your program. Other targets,
like remote, are always running. If you get an error
message like this one:
The "remote" target does not support "run". Try "help target" or "continue".
continue to run your program. You may need
first (see load).
The execution of a program is affected by certain information it receives from its superior. gdb provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:
- The arguments.
- Specify the arguments to give your program as the arguments of the
runcommand. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In Unix systems, you can control which shell is used with the
SHELLenvironment variable. See Your Program's Arguments.
- The environment.
- Your program normally inherits its environment from gdb, but you can
use the gdb commands
unset environmentto change parts of the environment that affect your program. See Your Program's Environment.
- The working directory.
- Your program inherits its working directory from gdb. You can set
the gdb working directory with the
cdcommand in gdb. See Your Program's Working Directory.
- The standard input and output.
- Your program normally uses the same device for standard input and
standard output as gdb is using. You can redirect input and output
runcommand line, or you can use the
ttycommand to set a different device for your program. See Your Program's Input and Output.
Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, gdb is likely to wind up debugging the wrong program.
When you issue the
run command, your program begins to execute
immediately. See Stopping and Continuing, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the
call commands. See Examining Data.
If the modification time of your symbol file has changed since the last time gdb read its symbols, gdb discards its symbol table, and reads it again. When it does this, gdb tries to retain your current breakpoints.
- The name of the main procedure can vary from language to language.
With C or C++, the main procedure name is always
main, but other languages such as Ada do not require a specific name for their main procedure. The debugger provides a convenient way to start the execution of the program and to stop at the beginning of the main procedure, depending on the language used.
The start command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the run command.
Some programs contain an elaboration phase where some startup code is executed before the main procedure is called. This depends on the languages used to write your program. In C++, for instance, constructors for static and global objects are executed before
mainis called. It is therefore possible that the debugger stops before reaching the main procedure. However, the temporary breakpoint will remain to halt execution.
Specify the arguments to give to your program as arguments to the start command. These arguments will be given verbatim to the underlying run command. Note that the same arguments will be reused if no argument is provided during subsequent calls to start or run.
It is sometimes necessary to debug the program during elaboration. In these cases, using the
startcommand would stop the execution of your program too late, as the program would have already completed the elaboration phase. Under these circumstances, insert breakpoints in your elaboration code before running your program.
- When exec-wrapper is set, the specified wrapper is used to
launch programs for debugging. gdb starts your program
with a shell command of the form exec wrapper
program. Quoting is added to program and its
arguments, but not to wrapper, so you should add quotes if
appropriate for your shell. The wrapper runs until it executes
your program, and then gdb takes control.
You can use any program that eventually calls
execvewith its arguments as a wrapper. Several standard Unix utilities do this, e.g.
nohup. Any Unix shell script ending with
exec "$@"will also work.
For example, you can use
envto pass an environment variable to the debugged program, without setting the variable in your shell's environment:
(gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so' (gdb) run
This command is available when debugging locally on most targets, excluding djgpp, Cygwin, MS Windows, and QNX Neutrino.
set disable-randomization on
- This option (enabled by default in gdb) will turn off the native
randomization of the virtual address space of the started program. This option
is useful for multiple debugging sessions to make the execution better
reproducible and memory addresses reusable across debugging sessions.
This feature is implemented only on gnu/Linux. You can get the same behavior using
(gdb) set exec-wrapper setarch `uname -m` -R
set disable-randomization off
- Leave the behavior of the started executable unchanged. Some bugs rear their
ugly heads only when the program is loaded at certain addresses. If your bug
disappears when you run the program under gdb, that might be because
gdb by default disables the address randomization on platforms, such
as gnu/Linux, which do that for stand-alone programs. Use set
disable-randomization off to try to reproduce such elusive bugs.
The virtual address space randomization is implemented only on gnu/Linux. It protects the programs against some kinds of security attacks. In these cases the attacker needs to know the exact location of a concrete executable code. Randomizing its location makes it impossible to inject jumps misusing a code at its expected addresses.
Prelinking shared libraries provides a startup performance advantage but it makes addresses in these libraries predictable for privileged processes by having just unprivileged access at the target system. Reading the shared library binary gives enough information for assembling the malicious code misusing it. Still even a prelinked shared library can get loaded at a new random address just requiring the regular relocation process during the startup. Shared libraries not already prelinked are always loaded at a randomly chosen address.
Position independent executables (PIE) contain position independent code similar to the shared libraries and therefore such executables get loaded at a randomly chosen address upon startup. PIE executables always load even already prelinked shared libraries at a random address. You can build such executable using gcc -fPIE -pie.
Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled).
- Show the current setting of the explicit disable of the native randomization of the virtual address space of the started program.