I've been considering using mmap for file reading, and was wondering how portable that is.
I'm developing on a Linux platform, but would like my program to work on Mac OS X and Windows.
Can I assume mmap is working on these platforms?
The mmap() function is a POSIX call. It works fine on MacOS X (and Linux, and HP-UX, and AIX, and Solaris).
The problem area will be Windows. I'm not sure whether there is an _mmap() call in the POSIX 'compatibility' sub-system. It is likely to be there — but will have the name with the leading underscore because Microsoft has an alternative view on namespaces and considers mmap() to intrude on the user name space, even if you ask for POSIX functionality. You can find a definition of an alternative Windows interface MapViewOfFile() and discussion about performance in another SO question (mmap() vs reading blocks).
If you try to map large files on a 32-bit system, you may find there isn't enough contiguous space to allocate the whole file in memory, so the memory mapping will fail. Do not assume it will work; decide what your fallback strategy is if it fails.