Shared memory in shared library Post: 302118380

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Top Forums Programming Shared memory in shared library Post 302118380 by DreamWarrior on Monday 21st of May 2007 05:24:52 PM

05-21-2007

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Shared memory in shared library

I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between instances of the shared library. However, what I do need to worry about is initializing the memory once and never again.

I would love all the shared libraries to have a library scoped variable, but I don't think that's possible. If it is, let me know...but I know normally each process gets its own copy of the shared library's data segment. I don't think there is a way to flag a portion of that data segment as shared across all instances of the library.

So, I guess I'll need to have the library access the DB in shared memory. However, the first instance of the library to be started up needs to create the shared memory, attach to it, and load it all before any other possible instances can use it. How do I guarentee that this create/initialize happens once before the shared memory is used?

At first, I thought the non-existance of the shared memory segment would do it...but there exists a race condition between create and initialize where-in another instance of the library would see the shared memory but not be able to access it because it has not yet been inited. I know I can store posix mutex's in shared memory...so I guess I can use one of them. However, the other problem exists when the shared memory segment is "left over" from previous runs.

What happens then? So...all library's "detach" from the segment, but none of them destroy it. Now what...the next time the library starts fresh it should re-init the shared memory, but won't.

Hummm...any ideas? Some direction? What is the common method of providing shared memory syncronization? Should I use a system semaphore? Wouldn't it suffer from the same "warm" start problem (detached processes from shared library but memory/semaphores still resident).

This will be written for AIX 5, BTW, for those interested.

DreamWarrior

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LEARN ABOUT PHP

shmop_open

SHMOP_OPEN(3)								 1							     SHMOP_OPEN(3)

shmop_open - Create or open shared memory block

SYNOPSIS

       int shmop_open (int  $key, string  $flags, int  $mode, int  $size)

DESCRIPTION

       shmop_open(3) can create or open a shared memory block.

PARAMETERS

	      o $key
		- System's id for the shared memory block. Can be passed as a decimal or hex.

	      o $flags
		- The flags that you can use:

		     o	"a"  for access (sets SHM_RDONLY for shmat) use this flag when you need to open an existing shared memory segment for read
		     only

		     o "c" for create (sets IPC_CREATE) use this flag when you need to create a new shared memory segment or if a segment with the
		     same key exists, try to open it for read and write

		     o "w" for read & write access use this flag when you need to read and write to a shared memory segment, use this flag in most
		     cases.

		     o "n" create a new memory segment (sets IPC_CREATE|IPC_EXCL) use this flag when you want to create a new shared  memory  seg-
		     ment  but	if  one  already exists with the same flag, fail. This is useful for security purposes, using this you can prevent
		     race condition exploits.

	      o $mode
		- The permissions that you wish to assign to your memory segment, those are the same as permission for a file. Permissions need to
		be passed in octal form, like for example 0644

	      o $size
		- The size of the shared memory block you wish to create in bytes

       Note

	       Note: the 3rd and 4th should be entered as 0 if you are opening an existing memory segment.

RETURN VALUES

	On  success  shmop_open(3)  will  return  an id that you can use to access the shared memory segment you've created.  FALSE is returned on
       failure.

EXAMPLES

       Example #1

	      Create a new shared memory block

	      <?php
	      $shm_key = ftok(__FILE__, 't');
	      $shm_id = shmop_open($shm_key, "c", 0644, 100);
	      ?>

	This example opened a shared memory block with a system id returned by ftok(3).

SEE ALSO

       shmop_close(3), shmop_delete(3).

PHP Documentation Group 													     SHMOP_OPEN(3)

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