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    <p>Shared memory is probably one of the fastest ways of allowing for IPC as data need not be copied, the problem associated with it though is synchronizing access between multiple threads. You could do this using semaphores or record locks , we end up using the later in unix fro shared memory even though they are not as efficient as they are simple, the system cleans up well, and you don't need some of the bling that semaphores bring along. Lets look into how these work to understand why they are implemented as such.</p> <p>In comes the shmid_ds used by the linux kernel (<a href="http://www.tldp.org/LDP/lpg/node68.html" rel="nofollow">http://www.tldp.org/LDP/lpg/node68.html</a>) the shm_nattch is the unsigned int counter for current attaches. shmget gets you an shm id and sets stuff like the ipc_perm , dates, pid, atime ctime, request of the segment size (shm_segsz) next the shmctl kicks in and does stuff for ipc using IPC_STAT, IPC_RMID, IPC_SET like setting perms, getting or removing shm_id for a segment or even locking or unlocking it.</p> <p>Once the segment is ready shmat is used by a process to attach to its address space, depending on the flags and address parameters. Once it attaches the kernel increments the shm_nattch. When detaching we call shmdt to detach . Removal of the identifier and the associated data structure is not automated some process has to do this calling shmctl with the IPC_RMID and depending on shm_perm</p> <p>As you can see this is all very similar to how one would use semaphores and the implementation makes sense.</p>
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