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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
Old 05-21-2007
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.
 

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IPCS(1) 						    BSD General Commands Manual 						   IPCS(1)

NAME
ipcs -- report System V interprocess communication facilities status SYNOPSIS
ipcs [-abcMmopQqSsTt] DESCRIPTION
The ipcs utility provides information on System V interprocess communication (IPC) facilities on the system. The options are as follows: -a Show the maximum amount of information possible when displaying active semaphores, message queues, and shared memory segments. (This is shorthand for specifying the -b, -c, -o, -p, and -t options.) -b Show the maximum allowed sizes for active semaphores, message queues, and shared memory segments. The ``maximum allowed size'' is the maximum number of bytes in a message on a message queue, the size of a shared memory segment, or the number of semaphores in a set of semaphores. -c Show the creator's name and group for active semaphores, message queues, and shared memory segments. -M Display system information about shared memory. -m Display information about active shared memory segments. -o Show outstanding usage for active message queues, and shared memory segments. The ``outstanding usage'' is the number of messages in a message queue, or the number of processes attached to a shared memory segment. -p Show the process ID information for active semaphores, message queues, and shared memory segments. The ``process ID information'' is the last process to send a message to or receive a message from a message queue, the process that created a semaphore, or the last process to attach or detach a shared memory segment. -Q Display system information about messages queues. -q Display information about active message queues. -S Display system information about semaphores. -s Display information about active semaphores. -T Display system information about shared memory, message queues and semaphores. -t Show access times for active semaphores, message queues, and shared memory segments. The access times is the time of the last con- trol operation on an IPC object, the last send or receive of a message, the last attach or detach of a shared memory segment, or the last operation on a semaphore. If none of the -M, -m, -Q, -q, -S, or -s options are specified, information about all active IPC facilities is listed. RESTRICTIONS
System data structures may change while ipcs is running; the output of ipcs is not guaranteed to be consistent. BUGS
This manual page is woefully incomplete, because it does not at all attempt to explain the information printed by ipcs. SEE ALSO
ipcrm(1) AUTHORS
Thorsten Lockert <tholo@sigmasoft.com> BSD
June 18, 1994 BSD
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