NAME

       setpgid - set process group ID for job control

SYNOPSIS

       #include <unistd.h>

       int setpgid(pid_t pid, pid_t pgid);

DESCRIPTION

       The  setpgid()  function shall either join an existing process group or create a new process group within
       the session of the calling process. The process group ID of a  session  leader  shall  not  change.  Upon
       successful  completion,  the  process group ID of the process with a process ID that matches pid shall be
       set to pgid. As a special case, if pid is 0, the process ID of the calling process shall be  used.  Also,
       if pgid is 0, the process ID of the indicated process shall be used.

RETURN VALUE

       Upon  successful completion, setpgid() shall return 0; otherwise, -1 shall be returned and errno shall be
       set to indicate the error.

ERRORS

       The setpgid() function shall fail if:

       EACCES The value of the pid argument matches the process ID of a child process of the calling process and
              the child process has successfully executed one of the exec functions.

       EINVAL The value of the pgid argument is less than 0, or is not a value supported by the implementation.

       EPERM  The process indicated by the pid argument is a session leader.

       EPERM  The value of the pid argument matches the process ID of a child process of the calling process and
              the child process is not in the same session as the calling process.

       EPERM  The value of the pgid argument is valid but does not match the process ID of the process indicated
              by the pid argument and there is no process with a process group ID that matches the value of  the
              pgid argument in the same session as the calling process.

       ESRCH  The  value  of the pid argument does not match the process ID of the calling process or of a child
              process of the calling process.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       The setpgid() function shall group  processes  together  for  the  purpose  of  signaling,  placement  in
       foreground or background, and other job control actions.

       The  setpgid()  function is similar to the setpgrp() function of 4.2 BSD, except that 4.2 BSD allowed the
       specified new process group to assume any value. This presents certain  security  problems  and  is  more
       flexible than necessary to support job control.

       To provide tighter security, setpgid() only allows the calling process to join a process group already in
       use inside its session or create a new process group whose process group ID was equal to its process ID.

       When  a  job  control  shell spawns a new job, the processes in the job must be placed into a new process
       group via setpgid(). There are two timing constraints involved in this action:

        1. The new process must be placed in the new process group before the appropriate  program  is  launched
           via one of the exec functions.

        2. The  new  process must be placed in the new process group before the shell can correctly send signals
           to the new process group.

       To address these constraints, the following actions are performed.  The new processes call  setpgid()  to
       alter  their  own process groups after fork() but before exec. This satisfies the first constraint. Under
       4.3 BSD, the second constraint is satisfied by the synchronization property  of  vfork();  that  is,  the
       shell is suspended until the child has completed the exec, thus ensuring that the child has completed the
       setpgid().  A  new  version  of fork() with this same synchronization property was considered, but it was
       decided instead to merely allow the parent shell process  to  adjust  the  process  group  of  its  child
       processes  via  setpgid().  Both timing constraints are now satisfied by having both the parent shell and
       the child attempt to adjust the process group of the child process; it does  not  matter  which  succeeds
       first.

       Since  it  would be confusing to an application to have its process group change after it began executing
       (that is, after exec), and because the child process would already have adjusted its process group before
       this, the [EACCES] error was added to disallow this.

       One non-obvious use of setpgid() is to allow a job control shell to return itself to its original process
       group (the one in effect when the job control shell was executed). A job control shell does  this  before
       returning  control  back  to its parent when it is terminating or suspending itself as a way of restoring
       its job control "state" back to what its parent would expect. (Note that the original  process  group  of
       the  job  control  shell  typically  matches the process group of its parent, but this is not necessarily
       always the case.)

FUTURE DIRECTIONS

       None.

SEE ALSO

       exec() , getpgrp() , setsid() , tcsetpgrp()  ,  the  Base  Definitions  volume  of  IEEE Std 1003.1-2001,
       <sys/types.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc and The Open Group. In the event of any discrepancy between this version and the  original  IEEE  and
       The  Open  Group  Standard,  the  original  IEEE and The Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                            SETPGID(P)