Re: mmap, /dev/zero, and MAP_SHARED

Jamie Lokier (lkd@tantalophile.demon.co.uk)
Sat, 27 Nov 1999 01:29:37 +0100

ncherry@home.net wrote:
> a = mmap (0, 8192, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FILE, fd, 0);
>
> a always returns as -1 and errno is 22 (EINVAL). I've changed all the
> EINVAL's to EINVAL+n (n is a constant not a variable) I've replaced n by
> 1 - 22 in the mm directory and the above code still returns -1/EINVAL.
>
> Why?

Because shared anonymous mappings (MAP_ANON could be used instead of
/dev/zero) are not implemented.

Use MAP_PRIVATE instead of MAP_SHARED and it succeeds of course, but
probably doesn't do what you want.

You best solutions are to use a temporary file instead of /dev/zero
(unlink it straight after opening if you like), or use shared memory
(shmget et al.).

-- Jamie

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