rand, rand_r, srand - pseudo-random number generator

Synopsis

Description

Notes

Example

Colophon

#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

Therand() function returns a pseudo-random integer in the range 0 toRAND_MAXinclusive (i.e., the mathematical range [0,RAND_MAX]).The

srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned byrand(). These sequences are repeatable by callingsrand() 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; this can be done using the reentrant functionrand_r().Like

rand(),rand_r() returns a pseudo-random integer in the range [0,RAND_MAX]. Theseedpargument is a pointer to anunsigned intthat is used to store state between calls. Ifrand_r() is called with the same initial value for the integer pointed to byseedp, and that value is not modified between calls, then the same pseudo-random sequence will result.The value pointed to by the

seedpargument ofrand_r() provides only a very small amount of state, so this function will be a weak pseudo-random generator. Trydrand48_r(3) instead.

Therand() andrand_r() functions return a value between 0 andRAND_MAX(inclusive). Thesrand() function returns no value.

The functionsrand() andsrand() conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001. The functionrand_r() is from POSIX.1-2001. POSIX.1-2008 marksrand_r() as obsolete.

The versions ofrand() andsrand() in the Linux C Library use the same random number generator asrandom(3) andsrandom(3), so the lower-order bits should be as random as the higher-order bits. However, on olderrand() 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. (Userandom(3) instead.)

POSIX.1-2001 gives the following example of an implementation ofrand() andsrand(), 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 following program can be used to display the pseudo-random sequence produced by

rand() when given a particular seed.#include <stdlib.h> #include <stdio.h>

int main(int argc, char *argv[]) { int j, r, nloops; unsigned int seed;

if (argc != 3) { fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]); exit(EXIT_FAILURE); }

seed = atoi(argv[1]); nloops = atoi(argv[2]);

srand(seed); for (j = 0; j < nloops; j++) { r = rand(); printf("%d\n", r); }

exit(EXIT_SUCCESS); }

drand48(3),random(3)

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RAND (3) | 2010-10-01 |