From Linux Man Pages
rand, rand_r, srand - pseudo-random number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
int rand_r(unsigned int *seedp);
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.
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
The rand() and rand_r() functions return a value between 0 and RAND_MAX. The srand() function returns no value.
EXAMPLE
POSIX.1-2001 gives the following example of an implementation of rand() and srand(), possibly useful when one
needs the same sequence on two different machines.
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
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() imple-
mentations, and on current implementations on different systems, the lower-order bits are much less random than
the higher-order bits. Do not use this function in applications intended to be portable when good randomness is
needed.
In Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukol-
sky, 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, see Chapter 3 (Random Num-
bers) in Donald E. Knuth's The Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Read-
ing, Massachusetts: Addison-Wesley Publishing Company, 1981.
CONFORMING TO
The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C89, POSIX.1-2001. The function rand_r() is from
POSIX.1-2001.
RELATED
drand48(3), random(3)
CATEGORY