This version of getpgrp differs from past Berkeley versions by not taking a "pid_t pid" argument. This incompatibility is required by -p1003.1-90.
From the -p1003.1-90 Rationale:
BSD 4.3 provides a getpgrp system call 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 AT&T V getpgrp suffices, and the added complexity of the BSD 4.3 getpgrp has been omitted from POSIX.1. The old functionality is available from the getpgid system call.
The getpgid system call will succeed unless: