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2003/10/05 Shared Memory


© October 2003 Tony Lawrence

Under normal conditions, one Unix process has absolutely no access to another's memory. However, sometimes it can be helpful (for reasons of speed, usually) to be able to read or write a shared memory area. Therefore, Unix and Linux systems provide system calls to setup and use such areas. See "man shmget" for an introduction. On some systems, there's a method of using mmap() (which maps a file into a memory buffer) to get a shared buffer. On SysV, that would involve mmapping /dev/zero, on BSD mmap has an ANON flag that tells it you want something that can be shared (though only with related processes).

When programmers talk about shared memory, it's probably this. However, hardware can also have memory mapped into system address space. This will usually be called "dual ported" memory (because both the system cpu and the add-on board can access it), but it's often just called "shared" also.

When you attempt to access memory that does not belong to you (usually because of a program bug), you'll get a "Memory Fault" error. People sometimes think this means defective ram; it doesn't.


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