Why is rand_r not defined in std?

Question

I was wondering why I can't use std::rand_r when including cstdlib? Or, more generally, why are some functions in cstdlib in the global namespace but not in the std namespace?

Answer 1

rand_r isn't part of the standard, it is a POSIX method.

rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE 

Conforming To

The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001. The function rand_r() is from POSIX.1-2001. POSIX.1-2008 marks rand_r() as obsolete.

Even better, forget about rand and rand_r and use std::mt19937 with the correct distribution (see <random>).