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<h4 class="subsection">27.6.3 Launching Jobs</h4>

<p><a name="index-launching-jobs-3216"></a>
Once the shell has taken responsibility for performing job control on
its controlling terminal, it can launch jobs in response to commands
typed by the user.

   <p>To create the processes in a process group, you use the same <code>fork</code>
and <code>exec</code> functions described in <a href="Process-Creation-Concepts.html#Process-Creation-Concepts">Process Creation Concepts</a>. 
Since there are multiple child processes involved, though, things are a
little more complicated and you must be careful to do things in the
right order.  Otherwise, nasty race conditions can result.

   <p>You have two choices for how to structure the tree of parent-child
relationships among the processes.  You can either make all the
processes in the process group be children of the shell process, or you
can make one process in group be the ancestor of all the other processes
in that group.  The sample shell program presented in this chapter uses
the first approach because it makes bookkeeping somewhat simpler.

   <p><a name="index-process-group-leader-3217"></a><a name="index-process-group-ID-3218"></a>As each process is forked, it should put itself in the new process group
by calling <code>setpgid</code>; see <a href="Process-Group-Functions.html#Process-Group-Functions">Process Group Functions</a>.  The first
process in the new group becomes its <dfn>process group leader</dfn>, and its
process ID becomes the <dfn>process group ID</dfn> for the group.

   <p><a name="index-race-conditions_002c-relating-to-job-control-3219"></a>The shell should also call <code>setpgid</code> to put each of its child
processes into the new process group.  This is because there is a
potential timing problem: each child process must be put in the process
group before it begins executing a new program, and the shell depends on
having all the child processes in the group before it continues
executing.  If both the child processes and the shell call
<code>setpgid</code>, this ensures that the right things happen no matter which
process gets to it first.

   <p>If the job is being launched as a foreground job, the new process group
also needs to be put into the foreground on the controlling terminal
using <code>tcsetpgrp</code>.  Again, this should be done by the shell as well
as by each of its child processes, to avoid race conditions.

   <p>The next thing each child process should do is to reset its signal
actions.

   <p>During initialization, the shell process set itself to ignore job
control signals; see <a href="Initializing-the-Shell.html#Initializing-the-Shell">Initializing the Shell</a>.  As a result, any child
processes it creates also ignore these signals by inheritance.  This is
definitely undesirable, so each child process should explicitly set the
actions for these signals back to <code>SIG_DFL</code> just after it is forked.

   <p>Since shells follow this convention, applications can assume that they
inherit the correct handling of these signals from the parent process. 
But every application has a responsibility not to mess up the handling
of stop signals.  Applications that disable the normal interpretation of
the SUSP character should provide some other mechanism for the user to
stop the job.  When the user invokes this mechanism, the program should
send a <code>SIGTSTP</code> signal to the process group of the process, not
just to the process itself.  See <a href="Signaling-Another-Process.html#Signaling-Another-Process">Signaling Another Process</a>.

   <p>Finally, each child process should call <code>exec</code> in the normal way. 
This is also the point at which redirection of the standard input and
output channels should be handled.  See <a href="Duplicating-Descriptors.html#Duplicating-Descriptors">Duplicating Descriptors</a>,
for an explanation of how to do this.

   <p>Here is the function from the sample shell program that is responsible
for launching a program.  The function is executed by each child process
immediately after it has been forked by the shell, and never returns.

<pre class="smallexample">     void
     launch_process (process *p, pid_t pgid,
                     int infile, int outfile, int errfile,
                     int foreground)
     {
       pid_t pid;
     
       if (shell_is_interactive)
         {
           /* <span class="roman">Put the process into the process group and give the process group</span>
              <span class="roman">the terminal, if appropriate.</span>
              <span class="roman">This has to be done both by the shell and in the individual</span>
              <span class="roman">child processes because of potential race conditions.</span>  */
           pid = getpid ();
           if (pgid == 0) pgid = pid;
           setpgid (pid, pgid);
           if (foreground)
             tcsetpgrp (shell_terminal, pgid);
     
           /* <span class="roman">Set the handling for job control signals back to the default.</span>  */
           signal (SIGINT, SIG_DFL);
           signal (SIGQUIT, SIG_DFL);
           signal (SIGTSTP, SIG_DFL);
           signal (SIGTTIN, SIG_DFL);
           signal (SIGTTOU, SIG_DFL);
           signal (SIGCHLD, SIG_DFL);
         }
     
       /* <span class="roman">Set the standard input/output channels of the new process.</span>  */
       if (infile != STDIN_FILENO)
         {
           dup2 (infile, STDIN_FILENO);
           close (infile);
         }
       if (outfile != STDOUT_FILENO)
         {
           dup2 (outfile, STDOUT_FILENO);
           close (outfile);
         }
       if (errfile != STDERR_FILENO)
         {
           dup2 (errfile, STDERR_FILENO);
           close (errfile);
         }
     
       /* <span class="roman">Exec the new process.  Make sure we exit.</span>  */
       execvp (p-&gt;argv[0], p-&gt;argv);
       perror ("execvp");
       exit (1);
     }
</pre>
   <p>If the shell is not running interactively, this function does not do
anything with process groups or signals.  Remember that a shell not
performing job control must keep all of its subprocesses in the same
process group as the shell itself.

   <p>Next, here is the function that actually launches a complete job. 
After creating the child processes, this function calls some other
functions to put the newly created job into the foreground or background;
these are discussed in <a href="Foreground-and-Background.html#Foreground-and-Background">Foreground and Background</a>.

<pre class="smallexample">     void
     launch_job (job *j, int foreground)
     {
       process *p;
       pid_t pid;
       int mypipe[2], infile, outfile;
     
       infile = j-&gt;stdin;
       for (p = j-&gt;first_process; p; p = p-&gt;next)
         {
           /* <span class="roman">Set up pipes, if necessary.</span>  */
           if (p-&gt;next)
             {
               if (pipe (mypipe) &lt; 0)
                 {
                   perror ("pipe");
                   exit (1);
                 }
               outfile = mypipe[1];
             }
           else
             outfile = j-&gt;stdout;
     
           /* <span class="roman">Fork the child processes.</span>  */
           pid = fork ();
           if (pid == 0)
             /* <span class="roman">This is the child process.</span>  */
             launch_process (p, j-&gt;pgid, infile,
                             outfile, j-&gt;stderr, foreground);
           else if (pid &lt; 0)
             {
               /* <span class="roman">The fork failed.</span>  */
               perror ("fork");
               exit (1);
             }
           else
             {
               /* <span class="roman">This is the parent process.</span>  */
               p-&gt;pid = pid;
               if (shell_is_interactive)
                 {
                   if (!j-&gt;pgid)
                     j-&gt;pgid = pid;
                   setpgid (pid, j-&gt;pgid);
                 }
             }
     
           /* <span class="roman">Clean up after pipes.</span>  */
           if (infile != j-&gt;stdin)
             close (infile);
           if (outfile != j-&gt;stdout)
             close (outfile);
           infile = mypipe[0];
         }
     
       format_job_info (j, "launched");
     
       if (!shell_is_interactive)
         wait_for_job (j);
       else if (foreground)
         put_job_in_foreground (j, 0);
       else
         put_job_in_background (j, 0);
     }
</pre>
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