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 auto
matically 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 sran
dom(), so the lower-order bits should be as random as the
higher-order bits. However, on older rand() implementa
tions, 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, 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 Numeri
cal Recipes in C book (see reference above) provides an
excellent discussion of practical random-number generation
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 Program
ming, volume 2 (Seminumerical Algorithms), 2nd ed.; Read
ing, 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)
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