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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LEARN ABOUT OPENDARWIN
shmdt
SHMAT(2) BSD System Calls Manual SHMAT(2)
NAME
shmat, shmdt -- map/unmap shared memory
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
void *
shmat(int shmid, void *shmaddr, int shmflg);
int
shmdt(void *shmaddr);
DESCRIPTION
shmat() maps the shared memory segment associated with the shared memory identifier shmid into the address space of the calling process. The
address at which the segment is mapped is determined by the shmaddr parameter. If it is equal to 0, the system will pick an address itself.
Otherwise, an attempt is made to map the shared memory segment at the address shmaddr specifies. If SHM_RND is set in shmflg, the system will
round the address down to a multiple of SHMLBA bytes (SHMLBA is defined in <sys/shm.h> ). A shared memory segment can be mapped read-only by
specifying the SHM_RDONLY flag in shmflg. shmdt() unmaps the shared memory segment that is currently mapped at shmaddr from the calling
process' address space. shmaddr must be a value returned by a prior shmat() call. A shared memory segment will remain existant until it is
removed by a call to shmctl(2) with the IPC_RMID command.
RETURN VALUES
shmat() returns the address at which the shared memory segment has been mapped into the calling process' address space when successful,
shmdt() returns 0 on successful completion. Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the error.
ERRORS
shmat() will fail if:
[EACCES] The calling process has no permission to access this shared memory segment.
[ENOMEM] There is not enough available data space for the calling process to map the shared memory segment.
[EINVAL] shmid is not a valid shared memory identifier. shmaddr specifies an illegal address.
[EMFILE] The number of shared memory segments has reached the system-wide limit.
shmdt() will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared memory segment.
SEE ALSO
shmctl(2), shmget(2), mmap(2)
BSD
August 17, 1995 BSD