# srandom (3)

# NAME

random, srandom, initstate, setstate - random number generator# SYNOPSIS

#include <stdlib.h>

long int random(void);

void srandom(unsigned intseed);

char *initstate(unsigned intseed, char *state, size_tn);

char *setstate(char *state);

Feature Test Macro Requirements for glibc (see

**feature_test_macros**(7)):

**random**(),

**srandom**(),

**initstate**(),

**setstate**():

_XOPEN_SOURCE >= 500

|| /* Glibc since 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

|| /* Glibc since 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

# DESCRIPTION

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

**RAND_MAX**. 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 use—the 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**().

# RETURN VALUE

The**random**() function returns a value between 0 and

**RAND_MAX**. The

**srandom**() function returns no value. The

**initstate**() function returns a pointer to the previous state array. On error,

*errno*is set to indicate the cause. On success,

**setstate**() returns a pointer to the previous state array. On error, it returns NULL, with

*errno*set to indicate the cause of the error.

# ERRORS

**EINVAL**- The
*state*argument given to**setstate**() was NULL.

**EINVAL**- A state array of less than 8 bytes was specified to
**initstate**().

# ATTRIBUTES

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

Interface | Attribute | Value |

random (), srandom (), initstate (), setstate () | Thread safety | MT-Safe |

# CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.3BSD.# NOTES

The**random**() function should not be used in multithreaded programs where reproducible behavior is required. Use

**random_r**(3) for that purpose. 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.

# BUGS

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.

# SEE ALSO

**getrandom**(2),

**drand48**(3),

**rand**(3),

**random_r**(3),

**srand**(3)