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
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.
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).
pgid is less than 0 ( setpgid,
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,
For getpgid: pid does not match any process.
For setpgid: pid is not the current process and not a child
of the current process.
POSIX took setpgid from the BSD function setpgrp. Also SysV 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.