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| <h3 class="section">26.1 Running a Command</h3> |
| |
| <p><a name="index-running-a-command-3150"></a> |
| The easy way to run another program is to use the <code>system</code> |
| 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. |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>system</b> (<var>const char *command</var>)<var><a name="index-system-3151"></a></var><br> |
| <blockquote><p><a name="index-sh-3152"></a>This function executes <var>command</var> as a shell command. In the GNU C |
| library, it always uses the default shell <code>sh</code> to run the command. |
| In particular, it searches the directories in <code>PATH</code> to find |
| programs to execute. The return value is <code>-1</code> if it wasn't |
| possible to create the shell process, and otherwise is the status of the |
| shell process. See <a href="Process-Completion.html#Process-Completion">Process Completion</a>, for details on how this |
| status code can be interpreted. |
| |
| <p>If the <var>command</var> argument is a null pointer, a return value of zero |
| indicates that no command processor is available. |
| |
| <p>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 <code>system</code> is |
| called. If the thread gets canceled these resources stay allocated |
| until the program ends. To avoid this calls to <code>system</code> should be |
| protected using cancellation handlers. |
| <!-- ref pthread_cleanup_push / pthread_cleanup_pop --> |
| |
| <p><a name="index-stdlib_002eh-3153"></a>The <code>system</code> function is declared in the header file |
| <samp><span class="file">stdlib.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p><strong>Portability Note:</strong> 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 |
| <code>system (NULL)</code><!-- /@w -->; if the return value is nonzero, a command |
| processor is available. |
| |
| <p>The <code>popen</code> and <code>pclose</code> functions (see <a href="Pipe-to-a-Subprocess.html#Pipe-to-a-Subprocess">Pipe to a Subprocess</a>) are closely related to the <code>system</code> function. They |
| allow the parent process to communicate with the standard input and |
| output channels of the command being executed. |
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