#include <stdlib.h> int rand(void); void srand(unsigned int seed); int rand_r(unsigned int *seed);
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
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.
rand returns the next pseudo-random integer in sequence; it is a
srand does not return a result.
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.
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