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NAME
setpgid, getpgid, setpgrp, getpgrp - set/get process group
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int setpgid(pid_t pid, pid_t pgid);
pid_t getpgid(pid_t pid);
pid_t getpgrp(void); /* POSIX.1 version */
[[deprecated]] pid_t getpgrp(pid_t pid); /* BSD version */
int setpgrp(void); /* System V version */
[[deprecated]] int setpgrp(pid_t pid, pid_t pgid); /* BSD version */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getpgid():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
setpgrp() (POSIX.1):
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE
setpgrp() (BSD), getpgrp() (BSD):
[These are available only before glibc 2.19]
_BSD_SOURCE &&
! (_POSIX_SOURCE || _POSIX_C_SOURCE || _XOPEN_SOURCE
|| _GNU_SOURCE || _SVID_SOURCE)
DESCRIPTION
All of these interfaces are available on Linux, and are used for getting and setting the process group ID
(PGID) of a process. The preferred, POSIX.1-specified ways of doing this are: getpgrp(void), for
retrieving the calling process's PGID; and setpgid(), for setting a process's PGID.
setpgid() sets the PGID of the process specified by pid to pgid. If pid is zero, then the process ID of
the calling process is used. If pgid is zero, then the PGID of the process specified by pid is made the
same as its process ID. If setpgid() is used to move a process from one process group to another (as is
done by some shells when creating pipelines), both process groups must be part of the same session (see
setsid(2) and credentials(7)). In this case, the pgid specifies an existing process group to be joined
and the session ID of that group must match the session ID of the joining process.
The POSIX.1 version of getpgrp(), which takes no arguments, returns the PGID of the calling process.
getpgid() returns the PGID of the process specified by pid. If pid is zero, the process ID of the
calling process is used. (Retrieving the PGID of a process other than the caller is rarely necessary,
and the POSIX.1 getpgrp() is preferred for that task.)
The System V-style setpgrp(), which takes no arguments, is equivalent to setpgid(0, 0).
The BSD-specific setpgrp() call, which takes arguments pid and pgid, is a wrapper function that calls
setpgid(pid, pgid)
Since glibc 2.19, the BSD-specific setpgrp() function is no longer exposed by <unistd.h>; calls should be
replaced with the setpgid() call shown above.
The BSD-specific getpgrp() call, which takes a single pid argument, is a wrapper function that calls
getpgid(pid)
Since glibc 2.19, the BSD-specific getpgrp() function is no longer exposed by <unistd.h>; calls should be
replaced with calls to the POSIX.1 getpgrp() which takes no arguments (if the intent is to obtain the
caller's PGID), or with the getpgid() call shown above.
RETURN VALUE
On success, setpgid() and setpgrp() return zero. On error, -1 is returned, and errno is set to indicate
the error.
The POSIX.1 getpgrp() always returns the PGID of the caller.
getpgid(), and the BSD-specific getpgrp() return a process group on success. On error, -1 is returned,
and errno is set to indicate the error.
ERRORS
EACCES An attempt was made to change the process group ID of one of the children of the calling process
and the child had already performed an execve(2) (setpgid(), setpgrp()).
EINVAL pgid is less than 0 (setpgid(), setpgrp()).
EPERM An attempt was made to move a process into a process group in a different session, or to change
the process group ID of one of the children of the calling process and the child was in a
different session, or to change the process group ID of a session leader (setpgid(), setpgrp()).
EPERM The target process group does not exist. (setpgid(), setpgrp()).
ESRCH For getpgid(): pid does not match any process. For setpgid(): pid is not the calling process and
not a child of the calling process.
STANDARDS
getpgid()
setpgid()
getpgrp() (no args)
setpgrp() (no args)
POSIX.1-2008 (but see HISTORY).
setpgrp() (2 args)
getpgrp() (1 arg)
None.
HISTORY
getpgid()
setpgid()
getpgrp() (no args)
POSIX.1-2001.
setpgrp() (no args)
POSIX.1-2001. POSIX.1-2008 marks it as obsolete.
setpgrp() (2 args)
getpgrp() (1 arg)
4.2BSD.
NOTES
A child created via fork(2) inherits its parent's process group ID. The PGID is preserved across an
execve(2).
Each process group is a member of a session and each process is a member of the session of which its
process group is a member. (See credentials(7).)
A session can have a controlling terminal. At any time, one (and only one) of the process groups in the
session can be the foreground process group for the terminal; the remaining process groups are in the
background. If a signal is generated from the terminal (e.g., typing the interrupt key to generate
SIGINT), that signal is sent to the foreground process group. (See termios(3) for a description of the
characters that generate signals.) Only the foreground process group may read(2) from the terminal; if a
background process group tries to read(2) from the terminal, then the group is sent a SIGTTIN signal,
which suspends it. The tcgetpgrp(3) and tcsetpgrp(3) functions are used to get/set the foreground
process group of the controlling terminal.
The setpgid() and getpgrp() calls are used by programs such as bash(1) to create process groups in order
to implement shell job control.
If the termination of a process causes a process group to become orphaned, and if any member of the newly
orphaned process group is stopped, then a SIGHUP signal followed by a SIGCONT signal will be sent to each
process in the newly orphaned process group. An orphaned process group is one in which the parent of
every member of process group is either itself also a member of the process group or is a member of a
process group in a different session (see also credentials(7)).
SEE ALSO
getuid(2), setsid(2), tcgetpgrp(3), tcsetpgrp(3), termios(3), credentials(7)
Linux man-pages 6.7 2023-10-31 setpgid(2)