c - Do I need to keep a file open after calling mmap on it?

Question

I have a program that maps quite a few (100's) of sizable files 10-100MB each. I need them all mapped at the same time.

At the moment I am calling open followed by mmap at the beginning of the program, followed by munmap and close at the end.

Often I have to adjust the open files limit running ulimit -n before running the program.

Question is do I actually need to keep the files open, or can I open mmap close do some large data processing then munmap when I'm finished.

The man pages of mmap do not seem terribly clear to me on this one.

Answer 1

No, at least not on Linux it's fine to close the file.

The manual page clearly states:

On the other hand, closing the file descriptor does not unmap the region.

For portability, I also checked the POSIX manual, it says the same thing (although even more clearly):

The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.