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 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 .