{"id":3934,"date":"2022-12-20T17:28:27","date_gmt":"2022-12-20T20:28:27","guid":{"rendered":"http:\/\/lode.uno\/linux-man\/index.php\/2022\/12\/20\/shm_overview-man7\/"},"modified":"2022-12-20T17:28:27","modified_gmt":"2022-12-20T20:28:27","slug":"shm_overview-man7","status":"publish","type":"post","link":"https:\/\/lode.uno\/linux-man\/2022\/12\/20\/shm_overview-man7\/","title":{"rendered":"SHM_OVERVIEW (man7)"},"content":{"rendered":"

SHM_OVERVIEW<\/h1>\n

NAME<\/a>
DESCRIPTION<\/a>
NOTES<\/a>
SEE ALSO<\/a>
COLOPHON<\/a> <\/p>\n

\n

NAME <\/a> <\/h2>\n

shm_overview \u2212 overview of POSIX shared memory<\/p>\n

DESCRIPTION <\/a> <\/h2>\n

The POSIX shared memory API allows processes to communicate information by sharing a region of memory.<\/p>\n

The interfaces employed in the API are:<\/p>\n\n\n\n\n\n\n\n\n\n\n
<\/td>\n\n

shm_open<\/b>(3)<\/p>\n<\/td>\n

<\/td>\n\n

Create and open a new object, or open an existing object. This is analogous to open<\/b>(2). The call returns a file descriptor for use by the other interfaces listed below.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

ftruncate<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

mmap<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

Map the shared memory object into the virtual address space of the calling process.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

munmap<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

Unmap the shared memory object from the virtual address space of the calling process.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

shm_unlink<\/b>(3)<\/p>\n<\/td>\n

<\/td>\n\n

Remove a shared memory object name.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

close<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

Close the file descriptor allocated by shm_open<\/b>(3) when it is no longer needed.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

fstat<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

Obtain a stat<\/i> structure that describes the shared memory object. Among the information returned by this call are the object\u2019s size (st_size<\/i>), permissions (st_mode<\/i>), owner (st_uid<\/i>), and group (st_gid<\/i>).<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

fchown<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

To change the ownership of a shared memory object.<\/p>\n<\/td>\n<\/tr>\n

<\/td>\n\n

fchmod<\/b>(2)<\/p>\n<\/td>\n

<\/td>\n\n

To change the permissions of a shared memory object.<\/p>\n<\/td>\n<\/tr>\n<\/table>\n

Versions<\/b>
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.<\/p>\n

Persistence<\/b>
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<\/b>(3)<\/p>\n

Linking<\/b>
Programs using the POSIX shared memory API must be compiled with cc \u2212lrt<\/i> to link against the real-time library, librt<\/i>.<\/p>\n

Accessing shared memory objects via the filesystem<\/b>
On Linux, shared memory objects are created in a (tmpfs<\/i>(5)) virtual filesystem, normally mounted under \/dev\/shm<\/i>. Since kernel 2.6.19, Linux supports the use of access control lists (ACLs) to control the permissions of objects in the virtual filesystem.<\/p>\n

NOTES <\/a> <\/h2>\n

Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.<\/p>\n

System V shared memory (shmget<\/b>(2), shmop<\/b>(2), etc.) is an older shared memory API. POSIX shared memory provides a simpler, and better designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V shared memory.<\/p>\n

SEE ALSO <\/a> <\/h2>\n

fchmod<\/b>(2), fchown<\/b>(2), fstat<\/b>(2), ftruncate<\/b>(2), mmap<\/b>(2), mprotect<\/b>(2), munmap<\/b>(2), shmget<\/b>(2), shmop<\/b>(2), shm_open<\/b>(3), shm_unlink<\/b>(3), sem_overview<\/b>(7)<\/p>\n

COLOPHON <\/a> <\/h2>\n

This page is part of release 5.10 of the Linux man-pages<\/i> project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https:\/\/www.kernel.org\/doc\/man\u2212pages\/.<\/p>\n

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