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# srand

Langue: *en*

Version: *2008-08-29 (ubuntu - 24/10/10)*

Section: *3 (Bibliothèques de fonctions)*

## NAME

rand, rand_r, srand - pseudo-random number generator## SYNOPSIS

#include <stdlib.h>int rand(void);int rand_r(unsigned int *seedp);void srand(unsigned intseed);

Feature Test Macro Requirements for glibc (see **feature_test_macros**(7)):

**rand_r**(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

## DESCRIPTION

The**rand**() function returns a pseudo-random integer in the range [0,

**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 behavior 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.

## RETURN VALUE

The**rand**() and

**rand_r**() functions return a value between 0 and

**RAND_MAX**. The

**srand**() function returns no value.

## CONFORMING TO

The functions**rand**() and

**srand**() conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001. The function

**rand_r**() is from POSIX.1-2001. POSIX.1-2008 marks

**rand_r**() as obsolete.

## NOTES

The versions of**rand**() and

**srand**() in the Linux C Library use the same random number generator as

**random**(3) and

**srandom**(3), 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. (Use

**random**(3) instead.)

## EXAMPLE

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; }

## SEE ALSO

**drand48**(3),

**random**(3)

## COLOPHON

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