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 automatically seeded with a value of
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 gen-
erator as random() and srandom(), so the lower-order bits should be as random as the
higher-order bits. However, on older rand() implementations, 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
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 refer-
ence 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, vol-
ume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publish-
ing Company, 1981.
SVID 3, BSD 4.3, ISO 9899
random(3), srandom(3), initstate(3), setstate(3)
GNU 1995-05-18 RAND(3)