Provided by: manpages-pt-dev_20040726-4_all NAME

rand, srand - random number generator.

SYNOPSIS

#include <stdlib.h>

int rand(void);

void srand(unsigned int seed);

DESCRIPTION

The rand() function returns a pseudo-random integer between 0 and RAND_MAX.

The  srand()  function  sets  its argument as the seed for a new sequence of pseudo-random
integers to be returned by rand().  These sequences are repeatable by calling srand() with
the same seed value.

If  no seed value is provided, the rand() function is automatically seeded with a value of
1.

RETURNVALUE

The rand() function returns a value between 0 and RAND_MAX.  The srand() returns no value.

NOTES

The versions of rand() and srand() in the Linux C  Library  use  the  same  random  number
generator  as  random()  and srandom(), so the lower-order bits should be as random as the
higher-order bits.  However, on older rand() implementations,  the  lower-order  bits  are
much less random than the higher-order bits.

In  Numerical  Recipes  in  C: The Art of Scientific Computing (William H. Press, Brian P.
Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University  Press,
"If you want to generate a random integer between 1 and 10, you should always do it
by using high-order bits, as in

j=1+(int) (10.0*rand()/(RAND_MAX+1.0));

and never by anything resembling

j=1+(rand() % 10);

(which uses lower-order bits)."

Random-number generation is a complex  topic.   The  Numerical  Recipes  in  C  book  (see
reference  above)  provides  an excellent discussion of practical random-number generation
issues in Chapter 7 (Random Numbers).

For a more theoretical discussion which also covers many practical issues in depth, please
see  Chapter  3  (Random  Numbers)  in  Donald E. Knuth's The Art of Computer Programming,