RAND(3)

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.

RETURN VALUE
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, 1990 (1st ed, p. 207)), the following comments are made:
    "If you want to generate a random integer between 1 and 10, you should always do it by

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

    and never by anything resembling

        j=1+((int) (1000000.0*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, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.

CONFORMING TO
SVID 3, BSD 4.3, ISO 9899

SEE ALSO
random(3), srandom(3), initstate(3), setstate(3)