setpgid

Setpgid is a POSIX-compatible system call used to set the process group ID (PGID) of a process. A process group is a collection of one or more processes that share terminal control and job-control semantics, enabling shells to manage pipelines and background tasks.

Prototype and headers: int setpgid(pid_t pid, pid_t pgid); typically declared in unistd.h. The function returns 0 on success and -1 on error, with errno set to indicate the failure reason.

Parameters and behavior: The target process is specified by pid. If pid is 0, the calling process is used as the target. If pgid is greater than 0, the target’s process group ID is set to that value. If pgid is 0, the target’s process group ID is set to the target’s PID, effectively creating a new process group led by that process. The call may fail for reasons such as the target process not existing, lacking permission to modify the target, or attempting to modify a process in an incompatible session.

Usage and context: setpgid is commonly used in job control and shell implementations. By assigning processes to distinct process groups, a shell can control which processes receive terminal input and can place groups in the foreground or background. It is also related to other session and terminal control mechanisms, such as setsid (to create a new session) and tcsetpgrp (to assign the controlling terminal to a process group).

See also: setsid, tcsetpgrp, getpgid, getpgrp, setpgrp (older variant).