2.33 rand, srand—pseudo-random numbers

Synopsis

#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
int rand_r(unsigned int *seed);

Description
rand returns a different integer each time it is called; each integer is chosen by an algorithm designed to be unpredictable, so that you can use rand when you require a random number. The algorithm depends on a static variable called the “random seed”; starting with a given value of the random seed always produces the same sequence of numbers in successive calls to rand.

You can set the random seed using srand; it does nothing beyond storing its argument in the static variable used by rand. You can exploit this to make the pseudo-random sequence less predictable, if you wish, by using some other unpredictable value (often the least significant parts of a time-varying value) as the random seed before beginning a sequence of calls to rand; or, if you wish to ensure (for example, while debugging) that successive runs of your program use the same “random” numbers, you can use srand to set the same random seed at the outset.

Returns
rand returns the next pseudo-random integer in sequence; it is a number between 0 and RAND_MAX (inclusive).

srand does not return a result.

Notes
rand and srand are unsafe for multi-threaded applications. rand_r is thread-safe and should be used instead.

Portability
rand is required by ANSI, but the algorithm for pseudo-random number generation is not specified; therefore, even if you use the same random seed, you cannot expect the same sequence of results on two different systems.

rand requires no supporting OS subroutines.