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.
In the Linux DLL 4.4.1 library, setpgrp simply calls setpgid(0,0).
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(4)? 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.
On success, setpgid and setpgrp return zero. On error, -1 is returned, and errno is set appropriately.
pgid is less than 0.
Various permission violations.