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(libc.info)Process Creation Example

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Process Creation Example
========================

   Here is an example program showing how you might write a function
similar to the built-in `system'.  It executes its COMMAND argument
using the equivalent of `sh -c COMMAND'.

     #include <stddef.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/wait.h>
     
     /* Execute the command using this shell program.  */
     #define SHELL "/bin/sh"
     
     int
     my_system (const char *command)
     {
       int status;
       pid_t pid;
     
       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the shell command. */
           execl (SHELL, SHELL, "-c", command, NULL);
           _exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;
       return status;
     }

   There are a couple of things you should pay attention to in this
example.

   Remember that the first `argv' argument supplied to the program
represents the name of the program being executed.  That is why, in the
call to `execl', `SHELL' is supplied once to name the program to
execute and a second time to supply a value for `argv[0]'.

   The `execl' 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 `return'
would leave two processes running the original program.  Instead, the
right behavior is for the child process to report failure to its parent
process.

   Call `_exit' to accomplish this.  The reason for using `_exit'
instead of `exit' is to avoid flushing fully buffered streams such as
`stdout'.  The buffers of these streams probably contain data that was
copied from the parent process by the `fork', data that will be output
eventually by the parent process.  Calling `exit' in the child would
output the data twice.  Note: Termination Internals.