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5.7. Sharing Variables Between Processes

5.7.3. Discussion

A shared memory segment is a slice of your machine's RAM that different processes (such as the multiple web server processes that handle requests) can access. A semaphore makes sure that the different processes don't step on each other's toes when they access the shared memory segment. Before a process can use the segment, it needs to get control of the semaphore. When it's done with the segment, it releases the semaphore for another process to grab.

To get control of a semaphore, use sem_get( ) to find the semaphore's ID. The first argument to sem_get( ) is an integer semaphore key. You can make the key any integer you want, as long as all programs that need to access this particular semaphore use the same key. If a semaphore with the specified key doesn't already exist, it's created, the maximum number of processes that can access the semaphore is set to the second argument of sem_get( ) (in this case, 1), and the semaphore's permissions are set to sem_get( )'s third argument (0600). These permissions work just like file permissions, so 0600 means that the user that created the semaphore can read it and write to it. In this context, user doesn't just mean the process that created the semaphore but any process with the same user ID. Permissions of 0600 should be appropriate for most uses, in which web server processes run as the same user.

sem_get( ) returns an identifier that points to the underlying system semaphore. Use this ID to gain control of the semaphore with sem_acquire( ). This function waits until the semaphore can be acquired (perhaps waiting until other processes release the semaphore) and then returns true. It returns false on error. Errors include invalid permissions or not enough memory to create the semaphore. Once the semaphore is acquired, you can read from the shared memory segment.

First, establish a link to the particular shared memory segment with shm_attach( ) . As with sem_get( ), the first argument to shm_attach( ) is an integer key. This time, however it identifies the desired segment, not the semaphore. If the segment with the specified key doesn't exist, the other arguments create it. The second argument (16384) is the size in bytes of the segment, and the last argument (0600) are the permissions on the segment. shm_attach(200,16384,0600) creates a 16K shared memory segment that can be read from and written to only by the user who created it. The function returns the identifier you need to read from and write to the shared memory segment.

After attaching to the segment, pull variables out of it with shm_get_var($shm, 'population'). This looks in the shared memory segment identified by $shm and retrieves the value of the variable called population. You can store any type of variable in shared memory. Once the variable is retrieved, it can be operated on like other variables. shm_put_var($shm,'population',$population) puts the value of $population back into the shared memory segment as a variable called population.

You're now done with the shared memory statement. Detach from it with shm_detach( ) and release the semaphore with sem_release( ) so another process can use it.

Shared memory's chief advantage is that it's fast. But since it's stored in RAM, it can't hold too much data, and it doesn't persist when a machine is rebooted (unless you take special steps to write the information in shared memory to disk before shutdown and then load it into memory again at startup). Also, shared memory is not available on Windows.

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