RAND(3) Linux Programmer's Manual RAND(3)
rand, srand - random number generator.
void srand(unsigned int seed);
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.
The rand() function returns a value between 0 and
RAND_MAX. The srand() returns no value.
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
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
"If you want to generate a random integer between 1
and 10, you should always do it by using high-order
bits, as in
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
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 Program-
ming, volume 2 (Seminumerical Algorithms), 2nd ed.; Read-
ing, Massachusetts: Addison-Wesley Publishing Company,
SVID 3, BSD 4.3, ISO 9899
random(3), srandom(3), initstate(3), setstate(3)