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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 (consult 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  suc-
              cessfully 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 con-
       trol 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 com-
       pleted 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 suspend-
       ing 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.1-2008, <sys_types.h>, <unistd.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                          SETPGID(3P)