srandom - seed pseudorandom number generation

random, srandom, initstate, setstate - random number generator

Standard C library (`libc`

, `-lc`

)

`#define _DEFAULT_SOURCE`

#include <stdlib.h>

```
void srandom(unsigned int seed);
```

An `unsigned int`

must be non-negative.

Defining `_DEFAULT_SOURCE`

in this way enables srandom within `stdlib.h`

.

```
#include <stdlib.h>
long random(void);
void srandom(unsigned int seed);
char *initstate(unsigned int seed, char state[.n], size_t n);
char *setstate(char *state);
```

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

This function alters the sequence of pseudorandom numbers generated by random. It should be called (once) before making any calls to random. In other words, if you first call srandom with a `seed`

of `1`

, subsequent calls to random will return different values than if you first call srandom with a `seed`

of `2`

.

Rather than hardcode a value for `seed`

, it is common to pass the return value of time (which changes every second) to srandom.

The random() function uses a nonlinear additive
feedback random number generator employing a default table of size 31
long integers to return successive pseudo-random numbers in the range
from 0 to 2^31 - 1. The period of this random number generator is very
large, approximately `16 * ((2^31) - 1)`

.

The srandom() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by random(). These sequences are repeatable by calling srandom() with the same seed value. If no seed value is provided, the random() function is automatically seeded with a value of 1.

The initstate() function allows a state array
`state`

to be initialized for use by random().
The size of the state array `n`

is used by
initstate() to decide how sophisticated a random number
generator it should useāthe larger the state array, the better the
random numbers will be. Current "optimal" values for the size of the
state array `n`

are 8, 32, 64, 128, and 256 bytes; other amounts
will be rounded down to the nearest known amount. Using less than 8
bytes results in an error. `seed`

is the seed for the
initialization, which specifies a starting point for the random number
sequence, and provides for restarting at the same point.

The setstate() function changes the state array used
by the random() function. The state array
`state`

is used for random number generation until the next call
to initstate() or setstate().
`state`

must first have been initialized using
initstate() or be the result of a previous call of
setstate().

This function does not return a value.

The random() function returns a value between 0 and
`(2^31) - 1`

. The srandom() function returns no
value.

The initstate() function returns a pointer to the
previous state array. On failure, it returns NULL, and `errno`

is
set to indicate the error.

On success, setstate() returns a pointer to the
previous state array. On failure, it returns NULL, and `errno`

is
set to indicate the error.

`#define _DEFAULT_SOURCE`

#include <stdlib.h>

#include <stdio.h>

#include <time.h>

int main(void)
{
srandom(time(NULL));
printf("%lu\n", random());
printf("%lu\n", random());
printf("%lu\n", random());
}

Calling `time`

with an input of `NULL`

, a constant defined in `stdlib.h`

, returns the current time in seconds.

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

Interface | Attribute | Value |

Thread safety | MT-Safe |

POSIX.1-2008.

POSIX.1-2001, 4.3BSD.

Random-number generation is a complex topic. *Numerical Recipes in
C: The Art of Scientific Computing* (William H. Press, Brian P.
Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge
University Press, 2007, 3rd ed.) provides an excellent discussion of
practical random-number generation issues in Chapter 7 (Random
Numbers).

For a more theoretical discussion which also covers many practical
issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
`The Art of Computer Programming`

, volume 2 (Seminumerical
Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing
Company, 1981.

According to POSIX, initstate() should return NULL
on error. In the glibc implementation, `errno`

is (as specified)
set on error, but the function does not return NULL.