PERL semget Function

Syntax

semget KEY, NSEMS, FLAGS

Definition and Usage

Returns the semaphore ID associated with KEY, using the system function semget( ) ie. Finds the semaphore associated with KEY.

Return Value

undef on failure
0 but true on success

Example

Try out following example: Creating a semaphore and incrementing its value

#!/usr/bin/perl -w

# Assume this file nam eis left.pl

use IPC::SysV;

#use these next two lines if the previous use fails.
eval 'sub IPC_CREAT {0001000}' unless defined &IPC_CREAT;
eval 'sub IPC_EXCL {0002000}'  unless defined &IPC_EXCL;
eval 'sub IPC_RMID {0}'        unless defined &IPC_RMID;

$key = 1066;

$| = 1;
$num = 0;
$flag = 0;

# Create the semaphor
$id = semget ( $key, 1, &IPC_EXCL|&IPC_CREAT|0777 ) or 
	die "Can't semget: $!";
foreach( 1..5) {
	$op  = 0;
	$operation = pack( "s*", $num, $op, $flags );
	semop( $id, $operation ) or die "Can't semop: $! ";
	print "Left....\n";
	sleep 1;
	$op = 2;
	$operation = pack( "s*", $num, $op, $flags );
	# add 2 to the semaphore ( now 2 )
	semop( $id, $operation ) or die "Can't semop $! ";
}
semctl (  $id, 0, &IPC_RMID, 0 );

Run the above program in background using $left.pl& and write following another program. Here Left sets the semaphore to 2 and Right prints Right and resets the semaphore to 0. This continues until Left finishes its loop after which it destroys the semaphore with semctl()

#!/usr/bin/perl -w

# Assume this file name is right.pl

$key = 1066;

$| = 1;
$num = 0;
$flags = 0;

# Identify the semaphore created by left.
$id = semget( $key, 1, 0 ) or die ("Can't semgt : $!" );

foreach( 1..5){
	$op = -1;
	$operation =  pack( "s*", $num, $op, $flags );
	# Add -1 to the semaphore (now 1)
	semop( $id, $operation ) or die " Can't semop $!";
	print "Right....\n";
	sleep 1;
	$operation = pack( "s*", $num, $op, $flags );
	 # Add -1 to the semaphore (now  0)
	semop( $id, $operation ) or die "Can't semop $! ";
}

Now run right.pl and It will produce following results:

Right....
Left....
Right....
Left....
Right....
Left....
Right....
Left....
Right....
Left....