This version of
getpgrp() differs from past Berkeley versions by not taking a
pid_t pid argument. This incompatibility is required by IEEE Std 1003.1-1990 (“POSIX.1”).
From the IEEE Std 1003.1-1990 (“POSIX.1”) Rationale:
4.3BSD provides a
getpgrp() function that returns the process group ID for a specified process. Although this function is used to support job control, all known job-control shells always specify the calling process with this function. Thus, the simpler System V
getpgrp() suffices, and the added complexity of the
4.3BSD getpgrp() has been omitted from POSIX.1. The old functionality is available from the
getpgid() function.