rand, srand - random number generator.
#include <stdlib.h>
int rand(void);
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 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
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, please see Chapter 3 (Random Numbers) in Donald E.
Knuth's The Art of Computer Programming, 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), setstate(3)
GNU 1995-05-18 RAND(3)
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