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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |STANDARDS |HISTORY |NOTES |CAVEATS |BUGS |SEE ALSO |COLOPHON | |
random(3) Library Functions Manualrandom(3)random, srandom, initstate, setstate - random number generator
Standard C library (libc,-lc)
#include <stdlib.h>long random(void);void srandom(unsigned intseed);char *initstate(size_t n;unsigned intseed, charstate[n], size_tn);char *setstate(char *state); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):random(),srandom(),initstate(),setstate(): _XOPEN_SOURCE >= 500 || /* glibc >= 2.19: */ _DEFAULT_SOURCE || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
Therandom() 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, approximately16 * ((2^31) - 1). Thesrandom() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned byrandom(). These sequences are repeatable by callingsrandom() with the same seed value. If no seed value is provided, therandom() function is automatically seeded with a value of 1. Theinitstate() function allows a state arraystate to be initialized for use byrandom(). The size of the state arrayn is used byinitstate() 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 arrayn 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. Thesetstate() function changes the state array used by therandom() function. The state arraystate is used for random number generation until the next call toinitstate() orsetstate().state must first have been initialized usinginitstate() or be the result of a previous call ofsetstate().
Therandom() function returns a value between 0 and(2^31) - 1. Thesrandom() function returns no value. Theinitstate() function returns a pointer to the previous state array. On failure, it returns NULL, anderrno is set to indicate the error. On success,setstate() returns a pointer to the previous state array. On failure, it returns NULL, anderrno is set to indicate the error.
EINVALThestate argument given tosetstate() was NULL.EINVALA state array of less than 8 bytes was specified toinitstate().
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │random(),srandom(),initstate(), │ Thread safety │ MT-Safe │ │setstate() │ │ │ └──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001, 4.3BSD.
Random-number generation is a complex topic.Numerical Recipes inC: 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'sThe Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.
Therandom() function should not be used in multithreaded programs where reproducible behavior is required. Userandom_r(3) for that purpose.
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.
getrandom(2),drand48(3),rand(3),random_r(3),srand(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28random(3)Pages that refer to this page:drand48(3), drand48_r(3), rand(3), random_r(3)
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