rand(3) -- Linux man page

NAME

SYNOPSIS

#include <stdlib.h>

int rand(void);

int rand_r(unsigned int *seedp);

void srand(unsigned int seed);

DESCRIPTION

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.

The function rand() is not reentrant or thread-safe, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behaviour in a threaded application, this state must be made explicit. The function rand_r() is supplied with a pointer to an unsigned int, to be used as state. This is a very small amount of state, so this function will be a weak pseudo-random generator. Try drand48_r(3) instead.  

RETURN VALUE

EXAMPLE

    static unsigned long next = 1;

    /* RAND_MAX assumed to be 32767 */
    int myrand(void) {
        next = next * 1103515245 + 12345;
        return((unsigned)(next/65536) % 32768);
    }

    void mysrand(unsigned seed) {
        next = seed;
    }

NOTES

FreeBSD adds a function

void sranddev(void);

that initializes the seed for their bad random generator rand() with a value obtained from their good random generator random(). Strange.

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, 1992 (2nd ed., p. 277)), the following comments are made:

"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, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.  

CONFORMING TO

SEE ALSO