semctl id, semnum, cmd, arg
Calls the System V IPC system call semctl(2). If
cmd is &IPC_STAT or &GETALL, then arg must
be a variable
which will hold the returned semid_ds structure or semaphore
value array. The function returns like ioctl: the undefined
value for error, 0 but true for zero, or the actual return
value otherwise. On error, it puts the error code into $!.
Before calling, you should say:
This function is available only on machines supporting System V IPC.require "ipc.ph"; require "sem.ph";