Calls the System V IPC function shmctl. You'll probably have to say
first to get the correct constant definitions. If CMD is
IPC_STAT, then ARG must be a variable that will hold the returned
shmid_ds structure. Returns like ioctl: the undefined value for error, "
0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in perlipc and