Job Control
***********

Job control refers to the protocol for allowing a user to move between multiple process groups (or jobs) within a single login session.  The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control.  So you can probably ignore the material in this chapter unless you are writing a shell or login program.

You need to be familiar with concepts relating to process creation (see Process Creation Concepts) and signal handling (see Signal Handling) in order to understand this material presented in this chapter.

Concepts of Job Control
=======================

The fundamental purpose of an interactive shell is to read commands from the user's terminal and create processes to execute the programs specified by those commands.  It can do this using the fork (see Creating a Process) and exec (see Executing a File) functions.

A single command may run just one process---but often one command uses several processes.  If you use the | operator in a shell command, you explicitly request several programs in their own processes.  But even if you run just one program, it can use multiple processes internally.  For example, a single compilation command such as cc -c foo.c typically uses four processes (though normally only two at any given time).  If you run make, its job is to run other programs in separate processes.

The processes belonging to a single command are called a process group or job.  This is so that you can operate on all of them at once.  For example, typing C-c sends the signal SIGINT to terminate all the processes in the foreground process group.

A session is a larger group of processes.  Normally all the processes that stem from a single login belong to the same session.

Every process belongs to a process group.  When a process is created, it becomes a member of the same process group and session as its parent process.  You can put it in another process group using the setpgid function, provided the process group belongs to the same session.

The only way to put a process in a different session is to make it the initial process of a new session, or a session leader, using the setsid function.  This also puts the session leader into a new process group, and you can't move it out of that process group again.

Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell.

A shell that supports job control must arrange to control which job can use the terminal at any time.  Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user.  To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter.

The shell can give unlimited access to the controlling terminal to only one process group at a time.  This is called the foreground job on that controlling terminal.  Other process groups managed by the shell that are executing without such access to the terminal are called background jobs.

If a background job needs to read from its controlling terminal, it is stopped by the terminal driver; if the TOSTOP mode is set, likewise for writing.  The user can stop a foreground job by typing the SUSP character (see Special Characters) and a program can stop any job by sending it a SIGSTOP signal.  It's the responsibility of the shell to notice when jobs stop, to notify the user about them, and to provide mechanisms for allowing the user to interactively continue stopped jobs and switch jobs between foreground and background.

See Access to the Terminal, for more information about I/O to the controlling terminal,

Job Control is Optional
=======================

Not all operating systems support job control.  gnusystems do support job control, but if you are using theglibc on some other system, that system may not support job control itself.

You can use the _POSIX_JOB_CONTROL macro to test at compile-time whether the system supports job control.  See System Options.

If job control is not supported, then there can be only one process group per session, which behaves as if it were always in the foreground. The functions for creating additional process groups simply fail with the error code ENOSYS.

The macros naming the various job control signals (see Job Control Signals) are defined even if job control is not supported.  However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing.

Controlling Terminal of a Process
=================================

One of the attributes of a process is its controlling terminal.  Child processes created with fork inherit the controlling terminal from their parent process.  In this way, all the processes in a session inherit the controlling terminal from the session leader.  A session leader that has control of a terminal is called the controlling process of that terminal.

You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.

An individual process disconnects from its controlling terminal when it calls setsid to become the leader of a new session. See Process Group Functions.

Access to the Controlling Terminal
==================================

Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background processes do not.  This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.

When a process in a background job tries to read from its controlling terminal, the process group is usually sent a SIGTTIN signal. This normally causes all of the processes in that group to stop (unless they handle the signal and don't stop themselves).  However, if the reading process is ignoring or blocking this signal, then read fails with an EIO error instead.

Similarly, when a process in a background job tries to write to its controlling terminal, the default behavior is to send a SIGTTOU signal to the process group.  However, the behavior is modified by the TOSTOP bit of the local modes flags (see Local Modes).  If this bit is not set (which is the default), then writing to the controlling terminal is always permitted without sending a signal. Writing is also permitted if the SIGTTOU signal is being ignored or blocked by the writing process.

Most other terminal operations that a program can do are treated as reading or as writing.  (The description of each operation should say which.)

For more information about the primitive read and write functions, see I/O Primitives.

Orphaned Process Groups
=======================

When a controlling process terminates, its terminal becomes free and a new session can be established on it.  (In fact, another user could log in on the terminal.)  This could cause a problem if any processes from the old session are still trying to use that terminal.

To prevent problems, process groups that continue running even after the session leader has terminated are marked as orphaned process groups.

When a process group becomes an orphan, its processes are sent a SIGHUP signal.  Ordinarily, this causes the processes to terminate.  However, if a program ignores this signal or establishes a handler for it (see Signal Handling), it can continue running as in the orphan process group even after its controlling process terminates; but it still cannot access the terminal any more.

Implementing a Job Control Shell
================================

This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved.

Data Structures for the Shell
-----------------------------

All of the program examples included in this chapter are part of a simple shell program.  This section presents data structures and utility functions which are used throughout the example.

The sample shell deals mainly with two data structures.  The job type contains information about a job, which is a set of subprocesses linked together with pipes.  The process type holds information about a single subprocess.  Here are the relevant data structure declarations:

     /* A process is a single process.  */
     typedef struct process
     {
       struct process *next;       /* next process in pipeline */
       char **argv;                /* for exec */
       pid_t pid;                  /* process ID */
       char completed;             /* true if process has completed */
       char stopped;               /* true if process has stopped */
       int status;                 /* reported status value */
     } process;

     /* A job is a pipeline of processes.  */
     typedef struct job
     {
       struct job *next;           /* next active job */
       char *command;              /* command line, used for messages */
       process *first_process;     /* list of processes in this job */
       pid_t pgid;                 /* process group ID */
       char notified;              /* true if user told about stopped job */
       struct termios tmodes;      /* saved terminal modes */
       int stdin, stdout, stderr;  /* standard i/o channels */
     } job;

     /* The active jobs are linked into a list.  This is its head.   */
     job *first_job = NULL;

Here are some utility functions that are used for operating on job objects.

     /* Find the active job with the indicated PGID.  */
     job *
     find_job (pid_t pgid)
     {
       job *j;

       for (j = first_job; j; j = j->next)
         if (j->pgid == pgid)
           return j;
       return NULL;
     }

     /* Return true if all processes in the job have stopped or completed.  */
     int
     job_is_stopped (job *j)
     {
       process *p;

       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;
     }

Initializing the Shell
----------------------

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.

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 getpgrp 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 tcgetpgrp function).

If the subshell is not running as a foreground job, it must stop itself by sending a SIGTTIN 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.

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 setpgid to put itself into its own process group, and then calling tcsetpgrp to place this process group into the foreground.

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 SIG_IGN.

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.

Here is the initialization code for the sample shell that shows how to do all of this.

     /* Keep track of attributes of the shell.  */

     #include <sys/types.h>
     #include <termios.h>
     #include <unistd.h>

     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) < 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, &shell_tmodes);
         }
     }

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 fork and exec functions described in 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.

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

The shell should also call 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 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 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 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 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 SIGTSTP signal to the process group of the process, not just to the process itself.  See Signaling Another Process.

Finally, each child process should call exec in the normal way. This is also the point at which redirection of the standard input and output channels should be handled.  See 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.

     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->argv[0], p->argv);
       perror ("execvp");
       exit (1);
     }

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 Foreground and Background.

     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)
         {
           /* Set up pipes, if necessary.  */
           if (p->next)
             {
               if (pipe (mypipe) < 0)
                 {
                   perror ("pipe");
                   exit (1);
                 }
               outfile = mypipe[1];
             }
           else
             outfile = j->stdout;

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

           /* 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);
     }

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.

When a foreground job is launched, the shell must first give it access to the controlling terminal by calling tcsetpgrp.  Then, the shell should wait for processes in that process group to terminate or stop.  This is discussed in more detail in 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 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 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 tcgetattr and tcsetattr; these are described in Terminal Modes.

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

     /* Put job J in the foreground.  If CONT is nonzero,
        restore the saved terminal modes and send the process group a
        SIGCONT signal to wake it up before we block.  */

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

       /* 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)");
         }

       /* Wait for it to report.  */
       wait_for_job (j);

       /* Put the shell back in the foreground.  */
       tcsetpgrp (shell_terminal, shell_pgid);

       /* Restore the shell's terminal modes.  */
       tcgetattr (shell_terminal, &j->tmodes);
       tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
     }

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:

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

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

Stopped and Terminated Jobs
---------------------------

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 waitpid function; see Process Completion.  Use the 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 waitpid with the WNOHANG option.  A good place to put a such a check for terminated and stopped jobs is just before prompting for a new command.

The shell can also receive asynchronous notification that there is status information available for a child process by establishing a handler for SIGCHLD signals.  See Signal Handling.

In the sample shell program, the 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 SIGCHLD.  The same function that is used to do the synchronous status checks (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.

     /* Store the status of the process PID that was returned by waitpid.
        Return 0 if all went well, nonzero otherwise.  */

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

       if (pid > 0)
         {
           /* 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;
         }

       else if (pid == 0 || errno == ECHILD)
         /* No processes ready to report.  */
         return -1;
       else {
         /* Other weird errors.  */
         perror ("waitpid");
         return -1;
       }
     }

     /* Check for processes that have status information available,
        without blocking.  */

     void
     update_status (void)
     {
       int status;
       pid_t pid;

       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
       while (!mark_process_status (pid, status));
     }

     /* Check for processes that have status information available,
        blocking until all processes in the given job have reported.  */

     void
     wait_for_job (job *j)
     {
       int status;
       pid_t pid;

       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED);
       while (!mark_process_status (pid, status)
              && !job_is_stopped (j)
              && !job_is_completed (j));
     }

     /* Format information about job status for the user to look at.  */

     void
     format_job_info (job *j, const char *status)
     {
       fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
     }

     /* Notify the user about stopped or terminated jobs.
        Delete terminated jobs from the active job list.  */

     void
     do_job_notification (void)
     {
       job *j, *jlast, *jnext;
       process *p;

       /* Update status information for child processes.  */
       update_status ();

       jlast = NULL;
       for (j = first_job; j; j = jnext)
         {
           jnext = j->next;

           /* If all processes have completed, tell the user the job has
              completed and delete it from the list of active jobs.  */
           if (job_is_completed (j)) {
             format_job_info (j, "completed");
             if (jlast)
               jlast->next = jnext;
             else
               first_job = jnext;
             free_job (j);
           }

           /* Notify the user about stopped jobs,
              marking them so that we won't do this more than once.  */
           else if (job_is_stopped (j) && !j->notified) {
             format_job_info (j, "stopped");
             j->notified = 1;
             jlast = j;
           }

           /* Don't say anything about jobs that are still running.  */
           else
             jlast = j;
         }
     }

Continuing Stopped Jobs
-----------------------

The shell can continue a stopped job by sending a SIGCONT signal to its process group.  If the job is being continued in the foreground, the shell should first invoke tcsetpgrp to give the job access to the terminal, and restore the saved terminal settings.  After continuing a job in the foreground, the shell should wait for the job to stop or complete, as if the job had just been launched in the foreground.

The sample shell program handles both newly created and continued jobs with the same pair of functions, put_job_in_foreground and put_job_in_background.  The definitions of these functions were given in Foreground and Background.  When continuing a stopped job, a nonzero value is passed as the CONT argument to ensure that the SIGCONT signal is sent and the terminal modes reset, as appropriate.

This leaves only a function for updating the shell's internal bookkeeping about the job being continued:

     /* Mark a stopped job J as being running again.  */

     void
     mark_job_as_running (job *j)
     {
       Process *p;

       for (p = j->first_process; p; p = p->next)
         p->stopped = 0;
       j->notified = 0;
     }

     /* Continue the job J.  */

     void
     continue_job (job *j, int foreground)
     {
       mark_job_as_running (j);
       if (foreground)
         put_job_in_foreground (j, 1);
       else
         put_job_in_background (j, 1);
     }

The Missing Pieces
------------------

The code extracts for the sample shell included in this chapter are only a part of the entire shell program.  In particular, nothing at all has been said about how job and program data structures are allocated and initialized.

Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like.  All of this is far too complicated to explain here!  Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell.

Here is a table summarizing the major entry points we have presented:

void init_shell (void)
     Initialize the shell's internal state.  See Initializing the Shell.

void launch_job (job *J, int FOREGROUND)
     Launch the job J as either a foreground or background job. See Launching Jobs.

void do_job_notification (void)
     Check for and report any jobs that have terminated or stopped.  Can be called synchronously or within a handler for SIGCHLD signals. See Stopped and Terminated Jobs.

void continue_job (job *J, int FOREGROUND)
     Continue the job J.  See Continuing Stopped Jobs.

Of course, a real shell would also want to provide other functions for managing jobs.  For example, it would be useful to have commands to list all active jobs or to send a signal (such as SIGKILL) to a job.

Functions for Job Control
=========================

This section contains detailed descriptions of the functions relating to job control.

Identifying the Controlling Terminal
------------------------------------

You can use the ctermid function to get a file name that you can use to open the controlling terminal.  In theglibc, it returns the same string all the time: "/dev/tty".  That is a special ``magic'' file name that refers to the controlling terminal of the current process (if it has one).  To find the name of the specific terminal device, use ttyname; see Is It a Terminal.

The function ctermid is declared in the header file stdio.h.

 -- Function: char * ctermid (char *STRING)
     The ctermid function returns a string containing the file name of the controlling terminal for the current process.  If STRING is not a null pointer, it should be an array that can hold at least L_ctermid characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned, which might get overwritten on subsequent calls to this function.

     An empty string is returned if the file name cannot be determined for any reason.  Even if a file name is returned, access to the file it represents is not guaranteed.

 -- Macro: int L_ctermid
     The value of this macro is an integer constant expression that represents the size of a string large enough to hold the file name returned by ctermid.

See also the isatty and ttyname functions, in Is It a Terminal.

Process Group Functions
-----------------------

Here are descriptions of the functions for manipulating process groups. Your program should include the header files sys/types.h and unistd.h to use these functions.

 -- Function: pid_t setsid (void)
     The setsid function creates a new session.  The calling process becomes the session leader, and is put in a new process group whose process group ID is the same as the process ID of that process.  There are initially no other processes in the new process group, and no other process groups in the new session.

     This function also makes the calling process have no controlling terminal.

     The setsid function returns the new process group ID of the calling process if successful.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

     EPERM
          The calling process is already a process group leader, or there is already another process group around that has the same process group ID.

 -- Function: pid_t getsid (pid_t PID)

     The getsid function returns the process group ID of the session leader of the specified process.  If a PID is 0, the process group ID of the session leader of the current process is returned.

     In case of error -1 is returned and errno is set.  The following errno error conditions are defined for this function:

     ESRCH
          There is no process with the given process ID PID.

     EPERM
          The calling process and the process specified by PID are in different sessions, and the implementation doesn't allow to access the process group ID of the session leader of the process with ID PID from the calling process.

 -- Function: pid_t getpgrp (void)
     The getpgrp function returns the process group ID of the calling process.

 -- Function: int getpgid (pid_t PID)

     The getpgid function returns the process group ID of the process PID.  You can supply a value of 0 for the PID argument to get information about the calling process.

     In case of error -1 is returned and errno is set.  The following errno error conditions are defined for this function:

     ESRCH
          There is no process with the given process ID PID. The calling process and the process specified by PID are in different sessions, and the implementation doesn't allow to access the process group ID of the process with ID PID from the calling process.

 -- Function: int setpgid (pid_t PID, pid_t PGID)
     The setpgid function puts the process PID into the process group PGID.  As a special case, either PID or PGID can be zero to indicate the process ID of the calling process.

     This function fails on a system that does not support job control. See Job Control is Optional, for more information.

     If the operation is successful, setpgid returns zero.  Otherwise it returns -1.  The following errno error conditions are defined for this function:

     EACCES
          The child process named by PID has executed an exec function since it was forked.

     EINVAL
          The value of the PGID is not valid.

     ENOSYS
          The system doesn't support job control.

     EPERM
          The process indicated by the PID argument is a session leader, or is not in the same session as the calling process, or the value of the PGID argument doesn't match a process group ID in the same session as the calling process.

     ESRCH
          The process indicated by the PID argument is not the calling process or a child of the calling process.

 -- Function: int setpgrp (pid_t PID, pid_t PGID)
     This is the BSD Unix name for setpgid.  Both functions do exactly the same thing.

Functions for Controlling Terminal Access
-----------------------------------------

These are the functions for reading or setting the foreground process group of a terminal.  You should include the header files sys/types.h and unistd.h in your application to use these functions.

Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.

 -- Function: pid_t tcgetpgrp (int FILEDES)
     This function returns the process group ID of the foreground process group associated with the terminal open on descriptor FILEDES.

     If there is no foreground process group, the return value is a number greater than 1 that does not match the process group ID of any existing process group.  This can happen if all of the processes in the job that was formerly the foreground job have terminated, and no other job has yet been moved into the foreground.

     In case of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

     EBADF
          The FILEDES argument is not a valid file descriptor.

     ENOSYS
          The system doesn't support job control.

     ENOTTY
          The terminal file associated with the FILEDES argument isn't the controlling terminal of the calling process.

 -- Function: int tcsetpgrp (int FILEDES, pid_t PGID)
     This function is used to set a terminal's foreground process group ID. The argument FILEDES is a descriptor which specifies the terminal; PGID specifies the process group.  The calling process must be a member of the same session as PGID and must have the same controlling terminal.

     For terminal access purposes, this function is treated as output.  If it is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal.  The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.

     If successful, tcsetpgrp returns 0.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

     EBADF
          The FILEDES argument is not a valid file descriptor.

     EINVAL
          The PGID argument is not valid.

     ENOSYS
          The system doesn't support job control.

     ENOTTY
          The FILEDES isn't the controlling terminal of the calling process.

     EPERM
          The PGID isn't a process group in the same session as the calling process.

 -- Function: pid_t tcgetsid (int FILDES)
     This function is used to obtain the process group ID of the session for which the terminal specified by FILDES is the controlling terminal. If the call is successful the group ID is returned.  Otherwise the return value is (pid_t) -1 and the global variable ERRNO is set to the following value:

     EBADF
          The FILEDES argument is not a valid file descriptor.

     ENOTTY
          The calling process does not have a controlling terminal, or the file is not the controlling terminal.