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Last updated: Tue, 28 May 2002
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LXXXIX. Semaphore, Shared Memory and IPC Functions

This module provides wrappers for the System V IPC family of functions. It includes semaphores, shared memory and inter-process messaging (IPC).

Semaphores may be used to provide exclusive access to resources on the current machine, or to limit the number of processes that may simultaneously use a resource.

This module provides also shared memory functions using System V shared memory. Shared memory may be used to provide access to global variables. Different httpd-daemons and even other programs (such as Perl, C, ...) are able to access this data to provide a global data-exchange. Remember, that shared memory is NOT safe against simultaneous access. Use semaphores for synchronization.

Taulu 1. Limits of Shared Memory by the Unix OS

SHMMAXmax size of shared memory, normally 131072 bytes
SHMMINminimum size of shared memory, normally 1 byte
SHMMNI max amount of shared memory segments on a system, normally 100
SHMSEG max amount of shared memory segments per process, normally 6

The messaging functions may be used to send and receive messages to/from other processes. They provide a simple and effective means of exchanging data between processes, without the need for setting up an alternative using unix domain sockets.

Huomaa: These functions do not work on Windows systems.

Sis�llys
ftok --  Convert a pathname and a project identifier to a System V IPC key
msg_get_queue --  Create or attach to a message queue
msg_receive --  Receive a message from a message queue
msg_remove_queue --  Destroy a message queue
msg_send --  Send a message to a message queue
msg_set_queue --  Set information in the message queue data structure
msg_stat_queue --  Returns information from the message queue data structure
sem_acquire -- Acquire a semaphore
sem_get -- Get a semaphore id
sem_release -- Release a semaphore
sem_remove -- Remove a semaphore
shm_attach -- Creates or open a shared memory segment
shm_detach -- Disconnects from shared memory segment
shm_get_var -- Returns a variable from shared memory
shm_put_var -- Inserts or updates a variable in shared memory
shm_remove -- Removes shared memory from Unix systems
shm_remove_var -- Removes a variable from shared memory
User Contributed Notes
Semaphore, Shared Memory and IPC Functions
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24-Aug-2000 04:29

you have to compile in the options to have the system V stuff:
--enable-sysvsem --enable-sysvshm


21-Sep-2000 05:58

The integer keys for sem_get() and shm_attach() have to be systemwide
unique. There is no method to ensure that no other process on the system
will use your specific key (security! and possible malfunction). Also
shared memory is very seldom used there are possibilities for conflicts!
To see the used id's you can use the program 'ipcs' (at least under
SuseLinux;) ). Thanks Christian C.


01-Jun-2001 03:46

As for security,   please look at the perm argument to shm_get.   Shared
Memory blocks has the same permission semantics as unix user/group/other
file permissions.   As long as your webserver is running as a user that no
other users can script to..  and as long as the permissions are set to
600,  you should be fine and have no security concerns.


02-May-2002 12:30

If only Sys V message queues were supported, I would be happy.


22-Jun-2002 04:54

Samlpe code for using most of the functions here:

    $MEMSIZE    =   512;//  size of shared memory to allocate
    $SEMKEY     =   1;  //  Semaphore key
    $SHMKEY     =   2;  //  Shared memory key

    echo "Start.\n";
    // Get semaphore
    $sem_id = sem_get($SEMKEY, 1);
    if ($sem_id === false)
    {
        echo "Fail to get semaphore";
        exit;
    }
    else
        echo "Got semaphore $sem_id.\n";

    // Accuire semaphore
    if (! sem_acquire($sem_id))
    {
        echo "Fail to aquire semaphore $sem_id.\n";
        sem_remove($sem_id);
        exit;
    }
    else
        echo "Success aquire semaphore $sem_id.\n";

    $shm_id =   shm_attach($SHMKEY, $MEMSIZE);
    if ($shm_id === false)
    {
        echo "Fail to attach shared memory.\n";
        sem_remove($sem_id);
        exit;
    }
    else
        echo "Success to attach shared memory : $shm_id.\n";

    // Write variable 1
    if (!shm_put_var($shm_id, 1, "Variable 1"))
    {
        echo "Fail to put var 1 on shared memory $shm_id.\n";
        sem_remove($sem_id);
        shm_remove ($shm_id);
        exit;
    }
    else
        echo "Write var1 to shared memory.\n";

    // Write variable 2
    if (!shm_put_var($shm_id, 2, "Variable 2"))
    {
        echo "Fail to put var 2 on shared memory $shm_id.\n";
        sem_remove($sem_id);
        shm_remove ($shm_id);
        exit;
    }
    else
        echo "Write var2 to shared memory.\n";

    // Read variable 1
    $var1   =   shm_get_var ($shm_id, 1);
    if ($var1 === false)
    {
        echo "Fail to retrive Var 1 from Shared memory $shm_id,
return value=$var1.\n";
    }
    else
        echo "Read var1=$var1.\n";

    // Read variable 1
    $var2   =   shm_get_var ($shm_id, 2);
    if ($var1 === false)
    {
        echo "Fail to retrive Var 2 from Shared memory $shm_id,
return value=$var2.\n";
    }
    else
        echo "Read var2=$var2.\n";

    // Release semaphore
    if (!sem_release($sem_id))
        echo "Fail to release $sem_id semaphore.\n";
    else
        echo "Semaphore $sem_id released.\n";

    // remove shared memory segmant from SysV
    if (shm_remove ($shm_id))
        echo "Shared memory successfully removed from SysV.\n";
    else
        echo "Fail to remove $shm_id shared memory from
SysV.\n";

    // Remove semaphore
    if (sem_remove($sem_id))
        echo "semaphore removed successfully from SysV.\n";
    else
        echo "Fail to remove $sem_id semaphore from SysV.\n";
    echo "End.\n";

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