use IPC::SysV qw/SEM_UNDO IPC_CREAT ftok/ ;
use IPC::Semaphore ;



# start script
$flags = PERMISSIONS ;
$sem = new IPC::Semaphore(ftok($0, 0), 1, $flags) ;
unless($sem) {
   # we must be the first one
   $sem = new IPC::Semaphore($id, 1, $flags | IPC_CREAT)
   $sem->setval(0, 100) ;
}

##
## Decrement the #0 semaphore by 1
##
$sem->op(0, -1, SEM_UNDO) ; # blocks if semaphore is zero

#
# DO SCRIPT STUFF
# 


##
## Increment #0 semaphore by 1
##
$sem->op(0, 1, SEM_UNDO) ; 

# once past this point, any script waiting can proceed

# done