rand(3) | Library Functions Manual | rand(3) |

# NAME

rand, rand_r, srand - pseudo-random number generator

# LIBRARY

Standard C library (*libc*, *-lc*)

# SYNOPSIS

#include <stdlib.h>

int rand(void);void srand(unsigned intseed);

[[deprecated]] int rand_r(unsigned int *seedp);

**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).

Interface |
Attribute |
Value |

rand (), rand_r (), srand () | Thread safety | MT-Safe |

# VERSIONS

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.)

# STANDARDS

# HISTORY

# EXAMPLES

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. When the
seed is *-1*, the program uses a random seed.

#include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { int r; unsigned int seed, nloops; if (argc != 3) { fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]); exit(EXIT_FAILURE); } seed = atoi(argv[1]); nloops = atoi(argv[2]); if (seed == -1) { seed = arc4random(); printf("seed: %u\n", seed); } srand(seed); for (unsigned int j = 0; j < nloops; j++) { r = rand(); printf("%d\n", r); } exit(EXIT_SUCCESS); }

# SEE ALSO

2024-06-15 | Linux man-pages 6.9.1 |