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4.2 Starting your Program

run
r

Use the run command to start your program under GDB. You must first specify the program name with an argument to GDB (see Getting In and Out of GDB), or by using the file or exec-file command (see Commands to Specify Files).

If you are running your program in an execution environment that supports processes, 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".

then use continue to run your program. You may need load 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 run command. 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 SHELL environment variable. If you do not define SHELL, GDB uses the default shell (/bin/sh). You can disable use of any shell with the set startup-with-shell command (see below for details).

The environment.

Your program normally inherits its environment from GDB, but you can use the GDB commands set environment and unset environment to 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 cd command 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 in the run command line, or you can use the tty command 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 print or 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.

start

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 main is 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 start command 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.

set exec-wrapper wrapper
show exec-wrapper
unset exec-wrapper

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 execve with its arguments as a wrapper. Several standard Unix utilities do this, e.g. env and nohup. Any Unix shell script ending with exec "$@" will also work.

For example, you can use env to 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 startup-with-shell
set startup-with-shell on
set startup-with-shell off
show set startup-with-shell

On Unix systems, by default, if a shell is available on your target, GDB) uses it to start your program. Arguments of the run command are passed to the shell, which does variable substitution, expands wildcard characters and performs redirection of I/O. In some circumstances, it may be useful to disable such use of a shell, for example, when debugging the shell itself or diagnosing startup failures such as:

(gdb) run
Starting program: ./a.out
During startup program terminated with signal SIGSEGV, Segmentation fault.

which indicates the shell or the wrapper specified with ‘exec-wrapper’ crashed, not your program. Most often, this is caused by something odd in your shell’s non-interactive mode initialization file—such as .cshrc for C-shell, $.zshenv for the Z shell, or the file specified in the ‘BASH_ENV’ environment variable for BASH.

set auto-connect-native-target
set auto-connect-native-target on
set auto-connect-native-target off
show auto-connect-native-target

By default, if not connected to any target yet (e.g., with target remote), the run command starts your program as a native process under GDB, on your local machine. If you’re sure you don’t want to debug programs on your local machine, you can tell GDB to not connect to the native target automatically with the set auto-connect-native-target off command.

If on, which is the default, and if GDB is not connected to a target already, the run command automaticaly connects to the native target, if one is available.

If off, and if GDB is not connected to a target already, the run command fails with an error:

(gdb) run
Don't know how to run.  Try "help target".

If GDB is already connected to a target, GDB always uses it with the run command.

In any case, you can explicitly connect to the native target with the target native command. For example,

(gdb) set auto-connect-native-target off
(gdb) run
Don't know how to run.  Try "help target".
(gdb) target native
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 10421) exited normally]

In case you connected explicitly to the native target, GDB remains connected even if all inferiors exit, ready for the next run command. Use the disconnect command to disconnect.

Examples of other commands that likewise respect the auto-connect-native-target setting: attach, info proc, info os.

set disable-randomization
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 certain targets, including GNU/Linux. 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.

On targets where it is available, virtual address space randomization protects the programs against certain 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 disable-randomization

Show the current setting of the explicit disable of the native randomization of the virtual address space of the started program.


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