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



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

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 POSIX.1-2017, <sys_types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           SETPGID(3)
