libc.info: rand

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`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.
*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.