Linux
2016-12-12
NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
| shm_open(3) | Create and open a new object, or open an existing object. This is analogous to open(2). The call returns a file descriptor for use by the other interfaces listed below. |
| ftruncate(2) | Set the size of the shared memory object. (A newly created shared memory object has a length of zero.) |
| mmap(2) | Map the shared memory object into the virtual address space of the calling process. |
| munmap(2) | Unmap the shared memory object from the virtual address space of the calling process. |
| shm_unlink(3) | Remove a shared memory object name. |
| close(2) | Close the file descriptor allocated by shm_open(3) when it is no longer needed. |
| fstat(2) | Obtain a stat structure that describes the shared memory object. Among the information returned by this call are the object’s size (st_size), permissions (st_mode), owner (st_uid), and group (st_gid). |
| fchown(2) | To change the ownership of a shared memory object. |
| fchmod(2) | To change the permissions of a shared memory object. |
Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
Persistence
POSIX shared memory objects have kernel persistence: a shared memory object will exist until the system is shut down, or until all processes have unmapped the object and it has been deleted with shm_unlink(3)
Linking
Programs using the POSIX shared memory API must be compiled with cc -lrt to link against the real-time library, librt.
Accessing shared memory objects via the filesystem
On Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, normally mounted under /dev/shm. Since kernel 2.6.19, Linux supports the use of access control lists (ACLs) to control the permissions of objects in the virtual filesystem.
NOTES
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
