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
automatically 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 (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
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 excel-
lent 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.; Reading,
Massachusetts: Addison-Wesley Publishing Company, 1981.
SVID 3, BSD 4.3, ISO 9899
random(3), srandom(3), initstate(3),