SHM_OVERVIEW
SHM_OVERVIEW
NAMEDESCRIPTION
CONFORMING TO
NOTES
SEE ALSO
COLOPHON
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:
|
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. | |||
|
Set the size of the shared memory object. (A newly created shared memory object has a length of zero.) | |||
|
Map the shared memory object into the virtual address space of the calling process. | |||
|
Unmap the shared memory object from the virtual address space of the calling process. | |||
|
Remove a shared memory object name. | |||
|
Close the file descriptor allocated by shm_open(3) when it is no longer needed. | |||
|
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). | |||
|
To change the ownership of a shared memory object. | |||
|
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) 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.
CONFORMING TO
POSIX.1-2001.
NOTES
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
System V shared memory (shmget(2), shmop(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.
SEE ALSO
fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_overview(7)
COLOPHON
This page is part of release 3.69 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man−pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
