what is a shared memory mapped file?

[Byron Myers]

What are the differences between shared memory, shared memory mapped files, and shared libraries?

[A. Clay Stephenson · Accepted solution]

Hi Byron,

Shared memory is just what is sounds like. An area of memory that can be shared among processes. The typical method is a group of processes agree on a protocol for building a 'key' usually using the ftok function.

Then a process creates a shared memory segment
using shmget with IPC_CREAT set. This process can also set the permissions as to who can read and write to this segment. The processes actually attach to the segment via the shmat system call.
man shmget, ftok, shmctl, shmat, shmdt for details.

Memory mapped files are an extension of this and use the mmap system call to completely map a disk based file to an area of shared memory. Once that is done, all the standard file i/o operations read, write, etc. work except that they are writing to memory and thus are very fast. This is typically used to store lookup tables and scratchpad files for related processes. Man mmap for details.

Shared libraries enable many processes that can be totally unrelated to use a single set of instructions (text) and thus save memory on a machine-wide basis. For example, the printf function's instructions might be shared by hundreds of processes. This sharing is possible because the instructions (text) do not change; if a shared library module also includes data (and the vast m ajority do) then
each process gets it own copy of the data space while sharing text space.

Hope this clears it up a bit, Clay

[Byron Myers]

Thanks, very good explanation. points comming. Please see follow up question on shared libraries to be posted soon.