random(3) and RAND_MAX

[John Marshall <John.W.Marshall@glasgow.ac.uk>]

Observed in CentOS 8's man-pages-4.15-6.el8.x86_64 and also on man-pages Git master:

Man-pages's rand.3 says rand() returns values in the range [0, RAND_MAX] and is very clear that this is inclusive. This is the same as the POSIX description of rand(3).

Man-pages's random.3 says random() returns values "in the range from 0 to RAND_MAX". However POSIX describes random() as returning values "in the range from 0 to 2^31-1".

In practice glibc and musl both fix RAND_MAX as a constant 2^31-1 so on these platforms it is the same thing. Similarly on macOS. It appears that FreeBSD used to have a slightly lower value of RAND_MAX but several months ago raised it to 2^31-1 similarly. OTOH it appears that Windows, Cygwin, etc still use a much smaller value for RAND_MAX (32767) but the full POSIX range for random(3).

So random.3 describing the range as 0..RAND_MAX is correct on Linux (unless you're using a very unusual libc) but misleading when used as a reference for writing code portable to other platforms. It would be good to change random.3 to refer to the hardcoded constant (2^31-1) instead of RAND_MAX (and perhaps add a note that on Linux this is the same as RAND_MAX), or at least to add a note saying that RAND_MAX may be an unrelated value on other platforms.

Thanks,

    John