Shared Memory

Shared memory is memory that two or more applications can read from and write to. Shared memory is prepared in such a way that any application can receive a pointer to the memory and access the data. Applications must explicitly request access to shared memory; the shared memory is protected from applications that are not granted access.

There are two kinds of shared memory: named and unnamed. For named shared memory, any application that knows the name of the shared memory can access it. For unnamed shared memory, a pointer to the shared memory must be passed from the process that created the shared memory to the process being given access. Access can be granted to any application; it is not necessary that the process being granted access be related (parent-child) to the application that created the shared memory.

Since memory sharing is done by sharing linear addresses, the linear address range of the shared memory object is reserved in all process address spaces.

There are two basic methods of managing shared memory:

• In one method, two or more applications share the same memory at the same time. These applications read from and write to the memory object, usually controlling access to the memory by using a semaphore.

• In the other method of managing shared memory, one application prepares data in memory, then passes that memory to another application for further processing. The first application releases the memory after passing it along, so that only one application accesses the memory at a time.