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Top Forums Programming Shared library with acces to shared memory. Post 302657999 by Corona688 on Monday 18th of June 2012 02:52:02 PM
Old 06-18-2012
Originally Posted by iamjag
Thanks Corona for your opinion.

I have been reading about mmap and I have understood
that you need a process running to keep the file in-memory.
Do not try to outsmart the operating system. Whether anything is in-memory is up to it -- things will get paged out at need. This includes shm segments too, so having it "purely in ram" is an illusion. Things which are frequently used will stay in memory.

Besides, with mmap, you get a file. With shm, you don't -- reboot and it's gone forever.

mmap may be actually more efficient since it doesn't need to back it with swap, only with file.
I like the idea of a process which makes the memory available for the first time and does not need to be running later. Also, this operation could be delegated to the own library at the first time it is called. Is this posible using mmap?
Do not try to outsmart the operating system. Whether anything is in-memory, be it code or data, is up to it. There are reasons for this. Just dumping everything into memory isn't always for the best.

On some systems you can tell mmap to preload the data, or you can mlock() the segment you want to keep in, but this is generally not necessary -- especially when the amount of data isn't huge. Frequeqntly-used data will stay in memory.

If the amount of data is huge, you don't want it all in memory, only the frequently used bits which will actually fit, for efficiency reasons. Large database systems often rely on mmap and the system deciding intelligently which things belong in memory.
Test Your Knowledge in Computers #549
Difficulty: Medium
a = 10; is an example of a unary operation.
True or False?

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SHM_OVERVIEW(7) 					     Linux Programmer's Manual						   SHM_OVERVIEW(7)

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 pro- cesses 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. 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_over- view(7) COLOPHON
This page is part of release 4.15 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 Linux 2016-12-12 SHM_OVERVIEW(7)

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