go to> drand48(3)> random(3)
Homepage > Man Pages > Category > Subroutines
Homepage > Man Pages > Name > R

# rand_r

## man page of rand_r

### rand_r: pseudo-random number generator

```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 int seed);

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 to
RAND_MAX inclusive (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 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; this can be done using
the reentrant function rand_r()

Like rand(), rand_r() returns a  pseudo-random  integer  in  the  range
[0, RAND_MAX].  The seedp argument is a pointer to an unsigned int that
is used to store state between calls.  If rand_r() is called  with  the
same  initial value for the integer pointed to by seedp, and that value
is not modified between calls, then  the  same  pseudo-random  sequence
will result.

The  value pointed to by the seedp argument of rand_r() provides only a
very small amount of state, so this function will  be  a  weak  pseudo-

RETURN VALUE
The rand() and rand_r() functions return a value between 0 and RAND_MAX
(inclusive).  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;
}

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>
#include <stdlib.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)

COLOPHON
This  page  is  part of release 3.27 of the Linux man-pages project.  A
description of the project, and information about reporting  bugs,  can
be found at //www.kernel.org/doc/man-pages/.

2010-09-13                         RAND_R(3)
```