[rcoh]

The stdlib rand() function on unix has a global lock around it provided by many versions of Linux. As such, if rand() is called in performance critical parallel code, performance will tank as each thread or process attempts to acquire this lock. Even if this lock is not acquired, you will still have a race condition on the state of the random number generator and may produce bad (non-random) randomness.

Use rand_r(unsigned int *state) instead in parallel and concurrent applications.

Sources: man 3 rand [unix command] http://unixhelp.ed.ac.uk/CGI/man-cgi?rand+3

[ekimekim]

The problem you're describing is similar but not the same as the one in the article. What you describe is part of the libc implementation of rand(3), whereas the article is talking about reads from /dev/urandom, which has a lock inside the kernel code (for the same reasons as libc).