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# setpgid() - Unix, Linux System Call

# NAME

setpgid, getpgid, setpgrp, getpgrp - set/get process group

# SYNOPSIS

#include <unistd.h>

int setpgid(pid_t pid, pid_t pgid);
pid_t getpgid(pid_t pid);
int setpgrp(void);
pid_t getpgrp(void);

# DESCRIPTION

setpgid() sets the process group ID of the process specified by pid to pgid. If pid is zero, the process ID of the current process is used. If pgid is zero, the process ID of the process specified by pid is used. 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. 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.

getpgid() returns the process group ID of the process specified by pid. If pid is zero, the process ID of the current process is used.

The call setpgrp() is equivalent to setpgid(0,0).

Similarly, getpgrp() is equivalent to getpgid(0) . 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.

Process groups are used for distribution of signals, and by terminals to arbitrate requests for their input: Processes that have the same process group as the terminal are foreground and may read, while others will block with a signal if they attempt to read. These calls are thus used by programs such as csh(1) to create process groups in implementing job control. The TIOCGPGRP and TIOCSPGRP calls described in termios(3) are used to get/set the process group of the control terminal.

If a session has a controlling terminal, CLOCAL is not set and a hangup occurs, then the session leader is sent a SIGHUP. If the session leader exits, the SIGHUP signal will be sent to each process in the foreground process group of the controlling terminal.

If the exit of the 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.

# RETURN VALUE

On success, setpgid() and setpgrp() return zero. On error, -1 is returned, and errno is set appropriately.

getpgid() returns a process group on success. On error, -1 is returned, and errno is set appropriately.

getpgrp() always returns the current process group.

# ERRORS

Tag | Description
--- | ---
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() (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()).
ESRCH | For getpgid(): pid does not match any process. For setpgid(): pid is not the current process and not a child of the current process.

# CONFORMING TO

The functions setpgid() and getpgrp() conform to POSIX.1-2001. The function setpgrp() is from 4.2BSD. The function getpgid() conforms to SVr4.

# NOTES

A child created via fork(2) inherits its parent’s process group ID. The process group ID is preserved across an execve(2).

POSIX took setpgid() from the BSD function setpgrp(). Also System V has a function with the same name, but it is identical to setsid(2).

To get the prototypes under glibc, define both _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED, or use "#define _XOPEN_SOURCE n" for some integer n larger than or equal to 500.

# SEE ALSO

- getuid (2)

- setsid (2)

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