Getpgrp

Getpgrp is a Unix-like system call that returns the process group ID (PGID) of a process. Process groups are a fundamental part of job control: a terminal has a foreground process group, and signals such as SIGINT are delivered to all processes in that group. The PGID is also used to organize related processes so that signals and terminal control can be managed as a unit.

POSIX defines a portable interface for obtaining a process group ID via getpgid(pid_t pid). This function returns the PGID of the specified process. On many systems, getpgrp is available as a compatibility form and, in some implementations, returns the PGID of the calling process, or of a given process, similar to getpgid. On Linux, getpgrp is typically equivalent to getpgid(0), i.e., it returns the PGID of the calling process.

Usage typically appears in C code by including unistd.h (and related headers): pid_t pgid = getpgrp(void); or pid_t pgid = getpgid(pid); The pid argument in getpgid specifies which process’s PGID to retrieve; a pid of 0 usually refers to the calling process, while positive values refer to that specific process if accessible.

Return values and errors: On success, the function returns the process group ID. On error, it returns -1 and sets errno. Common error codes include ESRCH (no such process for the given pid) and EINVAL (invalid or unsupported argument). Behavior and error reporting can vary slightly by system.

Notes: For portable code, prefer getpgid(pid) as defined by POSIX. getpgrp remains a historical alias on some systems and should be used only for compatibility. Understanding process groups is important for implementing and debugging job control and signal handling in shells and other systems software.