Summary

Shared memory provides the user with an efficient means of communication via the sharing of data that resides in memory. Unlike pipe-based communications, this data can be accessed in a nonserial (random) manner. To prevent inconsistencies, semaphores are often used to coordinate access to shared memory segments. When using System V-based shared memory techniques, shared memory segments are generated with the shmget system call. If a shared memory segment has already been created, the shmget call provides the process with access to the segment. The shmctl system call is used to obtain the status of a memory segment, set permissions, and remove a shared memory segment. The shmat and shmdt system calls are used to attach (map the segment into the process's address space) and detach memory segments.

The mmap system call may also be used to map the virtual memory space of a process to a file. As files remain after a process has terminated , mmap ed files provide a means for communicating information between processes that exist at different times. Overall, mmap -based techniques are less complex and somewhat more portable than their System V-based counterparts.