If the calling process is not already a session leader, the setpgrp() function makes it one by setting its process group ID and session ID to the value of its process ID, and releases its controlling terminal. See intro(2) for more information on process group IDs and session leaders.

The setpgrp() function returns the value of the new process group ID.

setpgrp(1), intro(2), exec(2), fork(2), getpid(2), getsid(2), kill(2), signal(3C)