## 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**():

Since glibc 2.24:

_POSIX_C_SOURCE >= 199506L

Glibc 2.23 and earlier

_POSIX_C_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, 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-random generator. Try
drand48_r(3) instead.

## RETURN VALUE

The
**rand**() and **rand_r**() functions return a value
between 0 and **RAND_MAX** (inclusive). The
**srand**() function returns no value.

## ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

## 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 int 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); }

## SEE ALSO

## COLOPHON

This page is
part of release 4.16 of the Linux *man-pages* project.
A description of the project, information about reporting
bugs, and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.