Function PGSharedMemoryIsInUse

src/backend/port/sysv_shmem.c:319–339

* PGSharedMemoryIsInUse * * Is a previously-existing shmem segment still existing and in use? * * The point of this exercise is to detect the case where a prior postmaster * crashed, but it left child backends that are still running. Therefore * we only care about shmem segments that are associated with the intended * DataDir. This is an important consideration since accidental matches of

 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
	PGShmemHeader *memAddress;
	IpcMemoryState state;

	state = PGSharedMemoryAttach((IpcMemoryId) id2, NULL, &memAddress);
	if (memAddress && shmdt(memAddress) < 0)
		elog(LOG, "shmdt(%p) failed: %m", memAddress);
	switch (state)
	{
		case SHMSTATE_ENOENT:
		case SHMSTATE_FOREIGN:
		case SHMSTATE_UNATTACHED:
			return false;
		case SHMSTATE_ANALYSIS_FAILURE:
		case SHMSTATE_ATTACHED:
			return true;
	}
	return true;
}

/*
 * Test for a segment with id shmId; see comment at IpcMemoryState.