job.texi

一个C源代码分析器

  shell_terminal = STDIN_FILENO;
  shell_is_interactive = isatty (shell_terminal);

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

      /* @r{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);

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

      /* @r{Grab control of the terminal.}  */
      tcsetpgrp (shell_terminal, shell_pgid);

      /* @r{Save default terminal attributes for shell.}  */
      tcgetattr (shell_terminal, &shell_tmodes);
    @}
@}
@end smallexample


@node Launching Jobs, Foreground and Background, Initializing the Shell, Implementing a Shell
@subsection Launching Jobs
@cindex launching jobs

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.

To create the processes in a process group, you use the same @code{fork}
and @code{exec} functions described in @ref{Process Creation Concepts}.
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.

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.

@cindex process group leader
@cindex process group ID
As each process is forked, it should put itself in the new process group
by calling @code{setpgid}; see @ref{Process Group Functions}.  The first
process in the new group becomes its @dfn{process group leader}, and its
process ID becomes the @dfn{process group ID} for the group.

@cindex race conditions, relating to job control
The shell should also call @code{setpgid} 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}, this ensures that the right things happen no matter which
process gets to it first.

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}.  Again, this should be done by the shell as well
as by each of its child processes, to avoid race conditions.

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

During initialization, the shell process set itself to ignore job
control signals; see @ref{Initializing the Shell}.  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} just after it is forked.

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} signal to the process group of the process, not
just to the process itself.  @xref{Signaling Another Process}.

Finally, each child process should call @code{exec} in the normal way.
This is also the point at which redirection of the standard input and 
output channels should be handled.  @xref{Duplicating Descriptors},
for an explanation of how to do this.

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.

@smallexample
void
launch_process (process *p, pid_t pgid,
                int infile, int outfile, int errfile,
                int foreground)
@{
  pid_t pid;

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

      /* @r{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);
    @}

  /* @r{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);
    @}    
  
  /* @r{Exec the new process.  Make sure we exit.}  */ 
  execvp (p->argv[0], p->argv);
  perror ("execvp");
  exit (1);
@}
@end smallexample

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.

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 @ref{Foreground and Background}.

@smallexample
void
launch_job (job *j, int foreground)
@{
  process *p;
  pid_t pid;
  int mypipe[2], infile, outfile;
  
  infile = j->stdin;
  for (p = j->first_process; p; p = p->next)
    @{
      /* @r{Set up pipes, if necessary.}  */
      if (p->next)
        @{
          if (pipe (mypipe) < 0)
            @{
              perror ("pipe");
              exit (1);
            @}
          outfile = mypipe[1];
        @}
      else
        outfile = j->stdout;

      /* @r{Fork the child processes.}  */
      pid = fork ();
      if (pid == 0)
        /* @r{This is the child process.}  */
        launch_process (p, j->pgid, infile,
                        outfile, j->stderr, foreground);
      else if (pid < 0)
        @{
          /* @r{The fork failed.}  */
          perror ("fork");
          exit (1);
        @}
      else
        @{
          /* @r{This is the parent process.}  */
          p->pid = pid;
          if (shell_is_interactive)
            @{
              if (!j->pgid)
                j->pgid = pid;
              setpgid (pid, j->pgid);
            @}
        @}

      /* @r{Clean up after pipes.}  */
      if (infile != j->stdin)
        close (infile);
      if (outfile != j->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);
@}
@end smallexample


@node Foreground and Background, Stopped and Terminated Jobs, Launching Jobs, Implementing a Shell
@subsection Foreground and Background

Now let's consider what actions must be taken by the shell when it
launches a job into the foreground, and how this differs from what
must be done when a background job is launched.

@cindex foreground job, launching
When a foreground job is launched, the shell must first give it access
to the controlling terminal by calling @code{tcsetpgrp}.  Then, the
shell should wait for processes in that process group to terminate or
stop.  This is discussed in more detail in @ref{Stopped and Terminated
Jobs}.

When all of the processes in the group have either completed or stopped,
the shell should regain control of the terminal for its own process
group by calling @code{tcsetpgrp} again.  Since stop signals caused by
I/O from a background process or a SUSP character typed by the user
are sent to the process group, normally all the processes in the job
stop together.

The foreground job may have left the terminal in a strange state, so the
shell should restore its own saved terminal modes before continuing.  In
case the job is merely been stopped, the shell should first save the
current terminal modes so that it can restore them later if the job is
continued.  The functions for dealing with terminal modes are
@code{tcgetattr} and @code{tcsetattr}; these are described in
@ref{Terminal Modes}.

Here is the sample shell's function for doing all of this.

@smallexample
@group
/* @r{Put job @var{j} in the foreground.  If @var{cont} is nonzero,}
   @r{restore the saved terminal modes and send the process group a}
   @r{@code{SIGCONT} signal to wake it up before we block.}  */

void
put_job_in_foreground (job *j, int cont)
@{
  /* @r{Put the job into the foreground.}  */
  tcsetpgrp (shell_terminal, j->pgid);
@end group

@group
  /* @r{Send the job a continue signal, if necessary.}  */
  if (cont)
    @{
      tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
      if (kill (- j->pgid, SIGCONT) < 0)
        perror ("kill (SIGCONT)");
    @}
@end group
  
  /* @r{Wait for it to report.}  */
  wait_for_job (j);
    
  /* @r{Put the shell back in the foreground.}  */
  tcsetpgrp (shell_terminal, shell_pgid);
    
@group
  /* @r{Restore the shell's terminal modes.}  */
  tcgetattr (shell_terminal, &j->tmodes);
  tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
@}
@end group
@end smallexample

@cindex background job, launching
If the process group is launched as a background job, the shell should
remain in the foreground itself and continue to read commands from
the terminal.  

In the sample shell, there is not much that needs to be done to put
a job into the background.  Here is the function it uses:

@smallexample
/* @r{Put a job in the background.  If the cont argument is true, send}
   @r{the process group a @code{SIGCONT} signal to wake it up.}  */

void
put_job_in_background (job *j, int cont)
@{
  /* @r{Send the job a continue signal, if necessary.}  */
  if (cont)
    if (kill (-j->pgid, SIGCONT) < 0)
      perror ("kill (SIGCONT)");
@}
@end smallexample


@node Stopped and Terminated Jobs, Continuing Stopped Jobs, Foreground and Background, Implementing a Shell
@subsection Stopped and Terminated Jobs

@cindex stopped jobs, detecting
@cindex terminated jobs, detecting
When a foreground process is launched, the shell must block until all of
the processes in that job have either terminated or stopped.  It can do
this by calling the @code{waitpid} function; see @ref{Process
Completion}.  Use the @code{WUNTRACED} option so that status is reported
for processes that stop as well as processes that terminate.

The shell must also check on the status of background jobs so that it
can report terminated and stopped jobs to the user; this can be done by
calling @code{waitpid} with the @code{WNOHANG} option.  A good place to
put a such a check for terminated and stopped jobs is just before
prompting for a new command.

@cindex @code{SIGCHLD}, handling of
The shell can also receive asynchronous notification that there is
status information available for a child process by establishing a
handler for @code{SIGCHLD} signals.  @xref{Signal Handling}.

In the sample shell program, the @code{SIGCHLD} signal is normally
ignored.  This is to avoid reentrancy problems involving the global data
structures the shell manipulates.  But at specific times when the shell
is not using these data structures---such as when it is waiting for
input on the terminal---it makes sense to enable a handler for
@code{SIGCHLD}.  The same function that is used to do the synchronous
status checks (@code{do_job_notification}, in this case) can also be
called from within this handler.

Here are the parts of the sample shell program that deal with checking
the status of jobs and reporting the information to the user.

@smallexample
@group
/* @r{Store the status of the process @var{pid} that was returned by waitpid.}
   @r{Return 0 if all went well, nonzero otherwise.}  */

int
mark_process_status (pid_t pid, int status)
@{
  job *j;
  process *p;
@end group

@group
  if (pid > 0)
    @{
      /* @r{Update the record for the process.}  */
      for (j = first_job; j; j = j->next)
        for (p = j->first_process; p; p = p->next)
          if (p->pid == pid)
            @{
              p->status = status;
              if (WIFSTOPPED (status))
                p->stopped = 1;
              else
                @{
                  p->completed = 1;
                  if (WIFSIGNALED (status))
                    fprintf (stderr, "%d: Terminated by signal %d.\n",
                             (int) pid, WTERMSIG (p->status));
                @}
              return 0;
             @}
      fprintf (stderr, "No child process %d.\n", pid);
      return -1;
    @}
@end group
@group
  else if (pid == 0 || errno == ECHILD)
    /* @r{No processes ready to report.}  */
    return -1;
  else @{
    /* @r{Other weird errors.}  */
    perror ("waitpid");
    return -1;
  @}
@}
@end group

@group
/* @r{Check for processes that have status information available,}
   @r{without blocking.}  */

void
update_status (void)
@{
  int status;