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  for (p = j->first_process; p; p = p->next)
    if (!p->completed && !p->stopped)
      return 0;
  return 1;
}

/* Return true if all processes in the job have completed.  */
int
job_is_completed (job *j)
{
  process *p;
  
  for (p = j->first_process; p; p = p->next)
    if (!p->completed)
      return 0;
  return 1;
}
</PRE>
<P>
<A NAME="IDX1746"></A>
<A NAME="IDX1747"></A>
<H3><A NAME="SEC419" HREF="library_toc.html#SEC419" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC419">Initializing the Shell</A></H3>
<P>
When a shell program that normally performs job control is started, it
has to be careful in case it has been invoked from another shell that is
already doing its own job control.  
<P>
A subshell that runs interactively has to ensure that it has been placed
in the foreground by its parent shell before it can enable job control
itself.  It does this by getting its initial process group ID with the
<CODE>getpgrp</CODE> function, and comparing it to the process group ID of the
current foreground job associated with its controlling terminal (which
can be retrieved using the <CODE>tcgetpgrp</CODE> function).
<P>
If the subshell is not running as a foreground job, it must stop itself
by sending a <CODE>SIGTTIN</CODE> signal to its own process group.  It may not
arbitrarily put itself into the foreground; it must wait for the user to
tell the parent shell to do this.  If the subshell is continued again,
it should repeat the check and stop itself again if it is still not in
the foreground.
<A NAME="IDX1748"></A>
<P>
Once the subshell has been placed into the foreground by its parent
shell, it can enable its own job control.  It does this by calling
<CODE>setpgid</CODE> to put itself into its own process group, and then
calling <CODE>tcsetpgrp</CODE> to place this process group into the
foreground.
<P>
When a shell enables job control, it should set itself to ignore all the
job control stop signals so that it doesn't accidentally stop itself.
You can do this by setting the action for all the stop signals to
<CODE>SIG_IGN</CODE>.
<P>
A subshell that runs non-interactively cannot and should not support job
control.  It must leave all processes it creates in the same process
group as the shell itself; this allows the non-interactive shell and its
child processes to be treated as a single job by the parent shell.  This
is easy to do--just don't use any of the job control primitives--but
you must remember to make the shell do it.
<P>
Here is the initialization code for the sample shell that shows how to
do all of this.
<P>
<PRE>
/* Keep track of attributes of the shell.  */

#include &#60;sys/types.h&#62;
#include &#60;termios.h&#62;
#include &#60;unistd.h&#62;

pid_t shell_pgid;
struct termios shell_tmodes;
int shell_terminal;
int shell_is_interactive;


/* Make sure the shell is running interactively as the foreground job
   before proceeding. */

void
init_shell ()
{
  
  /* See if we are running interactively.  */
  shell_terminal = STDIN_FILENO;
  shell_is_interactive = isatty (shell_terminal);

  if (shell_is_interactive)
    {
      /* Loop until we are in the foreground.  */
      while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
        kill (- shell_pgid, SIGTTIN);

      /* Ignore interactive and job-control signals.  */
      signal (SIGINT, SIG_IGN);
      signal (SIGQUIT, SIG_IGN);
      signal (SIGTSTP, SIG_IGN);
      signal (SIGTTIN, SIG_IGN);
      signal (SIGTTOU, SIG_IGN);
      signal (SIGCHLD, SIG_IGN);

      /* Put ourselves in our own process group.  */
      shell_pgid = getpid ();
      if (setpgid (shell_pgid, shell_pgid) &#60; 0)
        {
          perror ("Couldn't put the shell in its own process group");
          exit (1);
        }

      /* Grab control of the terminal.  */
      tcsetpgrp (shell_terminal, shell_pgid);

      /* Save default terminal attributes for shell.  */
      tcgetattr (shell_terminal, &#38;shell_tmodes);
    }
}
</PRE>
<P>
<A NAME="IDX1749"></A>
<H3><A NAME="SEC420" HREF="library_toc.html#SEC420" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC420">Launching Jobs</A></H3>
<P>
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 section <A HREF="library_23.html#SEC403" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_23.html#SEC403">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.
<A NAME="IDX1750"></A>
<A NAME="IDX1751"></A>
<P>
As each process is forked, it should put itself in the new process group
by calling <CODE>setpgid</CODE>; see section <A HREF="library_24.html#SEC427" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_24.html#SEC427">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.
<A NAME="IDX1752"></A>
<P>
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 section <A HREF="library_24.html#SEC419" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_24.html#SEC419">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 section <A HREF="library_21.html#SEC365" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_21.html#SEC365">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 section <A HREF="library_12.html#SEC182" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_12.html#SEC182">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.
<P>
<PRE>
void
launch_process (process *p, pid_t pgid,
                int infile, int outfile, int errfile,
                int foreground)
{
  pid_t pid;

  if (shell_is_interactive)
    {
      /* Put the process into the process group and give the process group
         the terminal, if appropriate.
         This has to be done both by the shell and in the individual
         child processes because of potential race conditions.  */
      pid = getpid ();
      if (pgid == 0) pgid = pid;
      setpgid (pid, pgid);
      if (foreground)
        tcsetpgrp (shell_terminal, pgid);

      /* Set the handling for job control signals back to the default.  */
      signal (SIGINT, SIG_DFL);
      signal (SIGQUIT, SIG_DFL);
      signal (SIGTSTP, SIG_DFL);
      signal (SIGTTIN, SIG_DFL);
      signal (SIGTTOU, SIG_DFL);
      signal (SIGCHLD, SIG_DFL);
    }

  /* Set the standard input/output channels of the new process.  */
  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);
    }    
  
  /* Exec the new process.  Make sure we exit.  */ 
  execvp (p-&#62;argv[0], p-&#62;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 section <A HREF="library_24.html#SEC421" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_24.html#SEC421">Foreground and Background</A>.
<P>
<PRE>
void
launch_job (job *j, int foreground)
{
  process *p;
  pid_t pid;
  int mypipe[2], infile, outfile;
  
  infile = j-&#62;stdin;
  for (p = j-&#62;first_process; p; p = p-&#62;next)
    {
      /* Set up pipes, if necessary.  */
      if (p-&#62;next)
        {
          if (pipe (mypipe) &#60; 0)
            {
              perror ("pipe");
              exit (1);
            }
          outfile = mypipe[1];
        }
      else
        outfile = j-&#62;stdout;

      /* Fork the child processes.  */
      pid = fork ();
      if (pid == 0)
        /* This is the child process.  */
        launch_process (p, j-&#62;pgid, infile, outfile, j-&#62;stderr, foreground);
      else if (pid &#60; 0)
        {
          /* The fork failed.  */
          perror ("fork");