Shared
Memory |
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Shared memory is reserved memory space for storing data structures
and data being shared between or among cooperating processes. Sharing
a common memory space eliminates the need for copying or moving
data to a separate location before it can be used by other processes,
reducing processor time and overhead as well as memory consumption.
Shared
Memory Access and Use |
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Shared memory management is similar in many respects to messages
and semaphores. A process requests a shared memory segment allocation
by means of the shmget() system call, specifying the segment size in one of the
function parameters (see shmget(2)). One or more processes
can then attach to the allocated segment by using the shmat() system call and detach when finished with the shmdt() system call (see shmop(2)). shmctl() is used to obtain information about the segment, and
to remove the segment when it is no longer needed (see shmctl(2)).
Semaphores can be used to prevent shared memory read/write
access collisions, but the more common method is for one process
to populate (write to) a given shared memory segment and other processes
read from the segment (write once, read many). In such arrangements,
when a subsequent write operation is to be performed, the writing process
allocates a new segment, and cooperating processes attach to that
new segment when ready to use it.
Shared memory is allocated in swappable shared memory space.
Data structures for managing shared memory are located in the kernel.
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