random(3) | Library Functions Manual | random(3) |

# NAME

random, srandom, initstate, setstate - random number generator

# LIBRARY

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

# SYNOPSIS

#include <stdlib.h>

long random(void);void srandom(unsigned intseed);

char *initstate(unsigned intseed, charstate[.n], size_tn);char *setstate(char *state);

**random**(), **srandom**(), **initstate**(),
**setstate**():

_XOPEN_SOURCE >= 500

|| /* Glibc >= 2.19: */ _DEFAULT_SOURCE

|| /* Glibc <= 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
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 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
*(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.

# ERRORS

# 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 |

# STANDARDS

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

2022-12-15 | Linux man-pages 6.02 |