Calls the System V IPC function semctl(2). You'll probably have to say
first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like
ioctl: the undefined value for error, "
0 but true" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with
pack("s!",(0)x$nsem). See also "SysV IPC" in perlipc,
Portability issues: "semctl" in perlport.