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PostgreSQL Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 408 pages
ISBN 0954612035
RRP £19.95 ($34.95)

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5.9.12 Shared Memory and LWLocks

Add-ins may reserve LWLocks and an allocation of shared memory on server startup. The add-in's shared library must be preloaded by specifying it in shared_preload_libraries . Shared memory is reserved by calling:

void RequestAddinShmemSpace(int size)

from your _PG_init function.

LWLocks are reserved by calling:

void RequestAddinLWLocks(int n)

from _PG_init.

To avoid possible race-conditions, each backend should use the LWLock AddinShmemInitLock when connecting to and initializing its allocation of shared memory, as shown here:

static mystruct *ptr = NULL;

if (!ptr)
{
        bool    found;

        LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
        ptr = ShmemInitStruct("my struct name", size, &found);
        if (!ptr)
                elog(ERROR, "out of shared memory");
        if (!found)
        {
                initialize contents of shmem area;
                acquire any requested LWLocks using:
                ptr->mylockid = LWLockAssign();
        }
        LWLockRelease(AddinShmemInitLock);
}
ISBN 0954612035PostgreSQL Reference Manual - Volume 2 - Programming GuideSee the print edition