Why use shm_open?

c linux buffer shared-memory mmap
Trevor picture Trevor · Jul 21, 2014 · Viewed 17.4k times · Source

What's the advantage of doing: shm_open followed a mmap?
Why not create a regular file, and then pass that fd to mmap?
I can't see the advantage of shm_open - these are just references, are they not?

I've read the man of the whole family. It seems to me, that the "secret" is in the mmaping action - the file "type" seems to be meaningless.

Any pointers will be good, especially with performance account.
My context is a (cyclic over-writable) buffer (say 128MB) that will be constantly written to be one process, and constantly dumped from by another.

As an example: what's wrong with this open/mmap approach.

EDIT
To be precise, is one of the following better than the other:

fd = open("/dev/shm/myshm.file", O_CREAT|O_RDWR, S_IRUSR | S_IWUSR);
mem = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

vs.

fd = shm_open("/myshm.file", O_RDWR|O_CREATE, S_IRUSR | S_IWUSR);
mem = mmap(...same as before...);

When I created a file with regular open under the /dev/shm fs, and dumped a Gig of garbage to it, my available memory went down by 1G, and my avaiable disk space remained the same.
What's the difference between the two methods?

Answer

nos picture nos · Jul 22, 2014

If you open and mmap() a regular file, data will end up in that file.

If you just need to share a memory region, without the need to persist the data, which incurs extra I/O overhead, use shm_open().

Such a memory region would also allow you to store other kinds of objects such as mutexes or semaphores, which you can't store in a mmap()'ed regular file on most systems.

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