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| <h3 class="section">26.9 Process Creation Example</h3> |
| |
| <p>Here is an example program showing how you might write a function |
| similar to the built-in <code>system</code>. It executes its <var>command</var> |
| argument using the equivalent of ‘<samp><span class="samp">sh -c </span><var>command</var></samp>’. |
| |
| <pre class="smallexample"> #include <stddef.h> |
| #include <stdlib.h> |
| #include <unistd.h> |
| #include <sys/types.h> |
| #include <sys/wait.h> |
| |
| /* <span class="roman">Execute the command using this shell program.</span> */ |
| #define SHELL "/bin/sh" |
| |
| int |
| my_system (const char *command) |
| { |
| int status; |
| pid_t pid; |
| |
| pid = fork (); |
| if (pid == 0) |
| { |
| /* <span class="roman">This is the child process. Execute the shell command.</span> */ |
| execl (SHELL, SHELL, "-c", command, NULL); |
| _exit (EXIT_FAILURE); |
| } |
| else if (pid < 0) |
| /* <span class="roman">The fork failed. Report failure.</span> */ |
| status = -1; |
| else |
| /* <span class="roman">This is the parent process. Wait for the child to complete.</span> */ |
| if (waitpid (pid, &status, 0) != pid) |
| status = -1; |
| return status; |
| } |
| </pre> |
| <!-- Yes, this example has been tested. --> |
| <p>There are a couple of things you should pay attention to in this |
| example. |
| |
| <p>Remember that the first <code>argv</code> argument supplied to the program |
| represents the name of the program being executed. That is why, in the |
| call to <code>execl</code>, <code>SHELL</code> is supplied once to name the program |
| to execute and a second time to supply a value for <code>argv[0]</code>. |
| |
| <p>The <code>execl</code> call in the child process doesn't return if it is |
| successful. If it fails, you must do something to make the child |
| process terminate. Just returning a bad status code with <code>return</code> |
| would leave two processes running the original program. Instead, the |
| right behavior is for the child process to report failure to its parent |
| process. |
| |
| <p>Call <code>_exit</code> to accomplish this. The reason for using <code>_exit</code> |
| instead of <code>exit</code> is to avoid flushing fully buffered streams such |
| as <code>stdout</code>. The buffers of these streams probably contain data |
| that was copied from the parent process by the <code>fork</code>, data that |
| will be output eventually by the parent process. Calling <code>exit</code> in |
| the child would output the data twice. See <a href="Termination-Internals.html#Termination-Internals">Termination Internals</a>. |
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