huschi.net

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  suc-
       cessful  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 prop-
       erty 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 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(3P)