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rand(3) [redhat man page]

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

NAME
rand, srand - random number generator. SYNOPSIS
#include <stdlib.h> int rand(void); void srand(unsigned int seed); DESCRIPTION
The rand() function returns a pseudo-random integer between 0 and RAND_MAX. The srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value. If no seed value is provided, the rand() function is automatically seeded with a value of 1. RETURN VALUE
The rand() function returns a value between 0 and RAND_MAX. The srand() returns no value. NOTES
The versions of rand() and srand() in the Linux C Library use the same random number generator as random() and srandom(), so the lower- order bits should be as random as the higher-order bits. However, on older rand() implementations, the lower-order bits are much less ran- dom than the higher-order bits. In 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, 1992 (2nd ed., p. 277)), the following comments are made: "If you want to generate a random integer between 1 and 10, you should always do it by using high-order bits, as in j=1+(int) (10.0*rand()/(RAND_MAX+1.0)); and never by anything resembling j=1+(rand() % 10); (which uses lower-order bits)." Random-number generation is a complex topic. The Numerical Recipes in C book (see reference above) 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, please 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. CONFORMING TO
SVID 3, BSD 4.3, ISO 9899 SEE ALSO
random(3), srandom(3), initstate(3), setstate(3) GNU
1995-05-18 RAND(3)

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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 int 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(): _SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED 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, approx- imately 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 automati- cally 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 init- state() to decide how sophisticated a random number generator it should use -- the larger the state array, the better the random numbers will be. 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. The setstate() function returns a pointer to the previous state array, or NULL on error. ERRORS
EINVAL A state array of less than 8 bytes was specified to initstate(). CONFORMING TO
4.3BSD, POSIX.1-2001. NOTES
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 will cause an error. This function should not be used in cases where multiple threads use random() and the behavior should be reproducible. 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 practi- cal 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. SEE ALSO
drand48(3), rand(3), random_r(3), srand(3) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. GNU
2010-09-20 RANDOM(3)

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