The chapter describes the NSPR API for named shared memory. Shared memory allows multiple processes to access one or more common shared memory regions, using it as an interprocess communication channel. The NSPR shared memory API provides a cross-platform named shared-memory interface that is modeled on similar constructs in the Unix and Windows operating systems.
Shared Memory Protocol
Using Named Shared Memory Functions
PR_OpenSharedMemory creates the shared memory segment, if it does not already exist, or opens a connection with the existing shared memory segment if it already exists.
PR_AttachSharedMemory should be called following PR_OpenSharedMemory to map the memory segment to an address in the application's address space. PR_AttachSharedMemory may also be called to remap a shared memory segment after detaching the same PRSharedMemory object. Be sure to detach it when you're finished.
PR_DetachSharedMemory should be called to unmap the shared memory segment from the application's address space.
PR_CloseSharedMemory should be called when no further use of the PRSharedMemory object is required within a process. Following a call to PR_CloseSharedMemory, the PRSharedMemory object is invalid and cannot be reused.
PR_DeleteSharedMemory should be called before process termination. After you call PR_DeleteSharedMemory, any further use of the shared memory associated with the name may cause unpredictable results.
Filenames
The name passed to PR_OpenSharedMemory should be a valid filename for a Unix platform. PR_OpenSharedMemory creates file using the name passed in. Some platforms may mangle the name before creating the file and the shared memory. The Unix implementation may use SysV IPC shared memory, Posix shared memory, or memory mapped files; the filename may be used to define the namespace. On Windows, the name is significant, but there is no file associated with the name.
No assumptions about the persistence of data in the named file should be made. Depending on platform, the shared memory may be mapped onto system paging space and be discarded at process termination.
All names provided to PR_OpenSharedMemory should be valid filename syntax or name syntax for shared memory for the target platform. Referenced directories should have permissions appropriate for writing.
Limits on Shared Memory Resources
Different platforms have limits on both the number and size of shared memory resources. The default system limits on some platforms may be smaller than your requirements. These limits may be adjusted on some platforms either via boot-time options or by setting the size of the system paging space to accommodate more and/or larger shared memory segment(s).
Security Considerations
On Unix platforms, depending on implementation, contents of the backing store for the shared memory can be exposed via the file system. Set permissions and or access controls at create and attach time to ensure you get the desired security.
On Windows platforms, no special security measures are provided.