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:
undef for error; "
0 but true" for zero; and the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for
Portability issues: "shmctl" in perlport.