Programming Perl, 4th Edition

semop

semop KEY, OPSTRING

This function calls the System V IPC syscall semop(2) to do semaphore operations such as signalling and waiting. Before calling, you should use IPC::SysV to get the correct constant definitions.

OPSTRING must be a packed array of semop structures. You can make each semop structure by saying pack("s*", $semnum, $semop, $semflag). The number of semaphore operations is implied by the length of OPSTRING. The function returns true if successful, or false if there is an error.

The following code waits on semaphore $semnum of semaphore id $semid:

$semop = pack "s*", $semnum, –1, 0;
semop($semid, $semop)     || die "Semaphore trouble: $!";

To signal the semaphore, simply replace –1 with 1.

See the section System V IPC in Chapter 15. See also the IPC::Semaphore module. This function is available only on machines supporting System V IPC.