getpgid

getpgid is a POSIX function that returns the process group ID (PGID) of a process. Process groups are used for job control, allowing a group of related processes to be managed together, such as receiving signals or controlling terminal access.

The function is declared as pid_t getpgid(pid_t pid). If pid is 0, the PGID of the calling process is returned. If pid is greater than 0, the PGID of the process with that process ID is returned. If pid is less than -1, the PGID of the process group whose ID is the absolute value of pid is returned. The form with pid equal to -1 is not defined by the standard and may produce an error on some systems.

On success, getpgid returns the process group ID as a positive integer. On failure, it returns -1 and sets errno to indicate the error. Common error values include ESRCH when the specified process or process group does not exist, and EINVAL for an invalid pid value on some implementations. The exact error reporting can vary slightly across systems, but the success/failure contract is consistent.

Usage examples include querying the PGID of another process to determine group membership, or using a negative PGID with signaling functions to target an entire process group. For instance, a program might compare getpgid(0) with getpgid(some_pid) to see whether two processes belong to the same group.

getpgid is available on most Unix-like systems that implement POSIX, including Linux, BSD variants, and macOS. It is not part of the Windows API, though similar behavior can be accessed in some environments via compatibility layers. Related functions include getpgrp and setpgid.