### 27.1 Running a Command ¶

The easy way to run another program is to use the `system` function. This function does all the work of running a subprogram, but it doesn’t give you much control over the details: you have to wait until the subprogram terminates before you can do anything else.

Function: `int` system `(const char *command)` ¶

  Preliminary: | MT-Safe | AS-Unsafe plugin heap lock | AC-Unsafe lock mem | See POSIX Safety Concepts.

  This function executes command as a shell command. In the GNU C Library, it always uses the default shell `sh` to run the command. In particular, it searches the directories in `PATH` to find programs to execute. The return value is `-1` if it wasn’t possible to create the shell process, and otherwise is the status of the shell process. See Process Completion, for details on how this status code can be interpreted.

  If the command argument is a null pointer, a return value of zero indicates that no command processor is available.

  This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `system` is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `system` should be protected using cancellation handlers.

  The `system` function is declared in the header file `stdlib.h`.

Portability Note: Some C implementations may not have any notion of a command processor that can execute other programs. You can determine whether a command processor exists by executing `system (NULL)`; if the return value is nonzero, a command processor is available.

The `popen` and `pclose` functions (see Pipe to a Subprocess) are closely related to the `system` function. They allow the parent process to communicate with the standard input and output channels of the command being executed.