Perl shmctl Function



This function controls the shared memory segment referred to by ID, using CMD with ARG. You will need to import the IPC::SysV module to get the command tokens defined below in Table.

Command 	Description
IPC_STAT 	Places the current value of each member of
   the data structure associated with ID into
   the scalar ARG

IPC_SET 	Sets the value of the following members o
   of the data structure associated with ID to
   the corresponding values found in the packed
   scalar ARG

IPC_RMID 	Removes the shared memory identifier specified
   by ID from the system and destroys the shared
   memory segment and data structure associated
   with it

SHM_LOCK 	Locks the shared memory segment specified by ID
   in memory

SHM_UNLOCK 	Unlocks the shared memory segment specified by ID


Following is the simple syntax for this function −

shmctl ID, CMD, ARG

Return Value

This function returns undef on failure and 0 but true if the return value from the shmctl( ) is 0.


Following is the example code showing its basic usage −


# Assume this file name is

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_RMID {0}'        unless defined &IPC_RMID;

$key  = 12345;
$size = 80;
$message = "Pennyfarthingale.";

# Create the shared memory segment

$key = shmget($key, $size, &IPC_CREAT | 0777 ) or 
      die "Can't shmget: $!";

# Place a string in itl
shmwrite( $id, $message, 0, 80 ) or die "Can't shmwrite: $!";

sleep 20;

# Delete it;

shmctl( $id, &OPC_RMID, 0 ) or die "Can't shmctl: $! ";

Write a reader program which retrieves the memory segment corresponding to $key and reads its contents using shmread();.


# Assume this file name is

$key = 12345;
$size = 80;

# Identify the shared memory segment
$id = shmget( $key, $size, 0777 ) or die "Can't shmget: $!";

# Read its contents itno a string
shmread($id, $var, 0, $size) or die "Can't shmread: $!";

print $var;

Now First run program in background and then then it will produces the following result.