SETPGID(3P)                                         POSIX Programmer's Manual                                        SETPGID(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

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 back-
       ground, 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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                   SETPGID(3P)