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RANDOM(3)                                                               Linux Programmer's Manual                                                              RANDOM(3)

NAME
       random, srandom, initstate, setstate - random number generator

SYNOPSIS
       #include <stdlib.h>

       long random(void);
       void srandom(unsigned int seed);

       char *initstate(unsigned int seed, char *state, size_t n);
       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 <= 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
       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 Pro‐
       gramming, 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)

GNU                                                                            2021-03-22                                                                      RANDOM(3)

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