RAND(3C)RAND(3C)NAME
rand, srand, rand_r - simple random-number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
int rand_r(unsigned int *seed);
DESCRIPTION
The rand() function uses a multiplicative congruential random-number
generator with period 2^32 that returns successive pseudo-random num‐
bers in the range of 0 to RAND_MAX (defined in <stdlib.h>).
The srand() function uses the argument seed as a seed for a new
sequence of pseudo-random numbers to be returned by subsequent calls to
rand(). If srand() is then called with the same seed value, the
sequence of pseudo-random numbers will be repeated. If rand() is
called before any calls to srand() have been made, the same sequence
will be generated as when srand() is first called with a seed value of
1.
The rand_r() function has the same functionality as rand() except that
a pointer to a seed seed must be supplied by the caller. If rand_r()
is called with the same initial value for the object pointed to by seed
and that object is not modified between successive calls to rand_r(),
the same sequence as that produced by calls to rand() will be gener‐
ated.
The rand() and srand() functions provide per-process pseudo-random
streams shared by all threads. The same effect can be achieved if all
threads call rand_r() with a pointer to the same seed object. The
rand_r() function allows a thread to generate a private pseudo-random
stream by having the seed object be private to the thread.
USAGE
The spectral properties of rand() are limited. The drand48(3C) func‐
tion provides a better, more elaborate random-number generator.
When compiling multithreaded applications, the _REENTRANT flag must be
defined on the compile line. This flag should be used only in multi‐
threaded applications.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Standard │
├────────────────────┼─────────────────┤
│MT-Level │ Safe │
└────────────────────┴─────────────────┘
SEE ALSOdrand48(3C), attributes(5), standards(5)
May 19, 2004 RAND(3C)
