Introduction

Shared memory allows multiple processes to share virtual memory space. This is the fastest but not necessarily the easiest (synchronization-wise) way for processes to communicate with one another. In general, one process creates or allocates the shared memory segment. The size and access permissions for the segment are set when it is created. The process then attaches the shared segment, causing it to be mapped [1] into its current data space. If needed, the creating process then initializes the shared memory. Once created, and if permissions permit, other processes can gain access to the shared memory segment and map it into their data space. Each process accesses the shared memory relative to its attachment address. While the data that these processes are referencing is in common , each process uses different attachment address values. For each process involved, the mapped memory appears to be no different from any other of its memory addresses. Figure 8.1 presents a diagrammatic way to envision three processes sharing a common memory segment. As there are no intrinsic shared memory synchronization constructs, semaphores are normally used to coordinate access to a shared memory segment. When a process is finished with the shared memory segment, it can detach from it. Additionally, the creator of the segment may grant ownership of the segment to another process. When all processes are finished with the shared memory segment, the process that created the segment is usually responsible for removing it.

[1] The actual mapping of the segment to virtual address space is dependent upon the memory management (MMU) hardware for the system.

Figure 8.1. Envisioning three processes sharing a common memory segment.