rand, rand_r, srand — pseudo-random number generator
#include <stdlib.h>
int
rand( |
void) ; |
int
rand_r( |
unsigned int * | seedp) ; |
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 function rand
() is not
reentrant or thread-safe, since it uses hidden state that is
modified on each call. This might just be the seed value to
be used by the next call, or it might be something more
elaborate. In order to get reproducible behaviour in a
threaded application, this state must be made explicit. The
function rand_r
() is supplied
with a pointer to an unsigned int, to be used as state. This
is a very small amount of state, so this function will be a
weak pseudo-random generator. Try drand48_r(3) instead.
The rand
() and rand_r
() functions return a value between 0
and RAND_MAX. The srand
()
function returns no value.
POSIX.1-2001 gives the following example of an
implementation of rand
() and
srand
(), possibly useful when
one needs the same sequence on two different machines.
static unsigned long next = 1; /* RAND_MAX assumed to be 32767 */ int myrand(void) { next = next * 1103515245 + 12345; return((unsigned)(next/65536) % 32768); } void mysrand(unsigned seed) { next = seed; }
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, and on
current implementations on different systems, the lower-order
bits are much less random than the higher-order bits. Do not
use this function in applications intended to be portable
when good randomness is needed.
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, 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.
The functions rand
() and
srand
() conform to SVr4,
4.3BSD, C89, C99, POSIX.1-2001. The function rand_r
() is from POSIX.1-2001.
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