RAND(2) RAND(2) NAME rand, lrand, frand, nrand, lnrand, srand, truerand, n_truerand, fastrand - random number generator SYNOPSIS #include <u.h> #include <libc.h> int rand(void) long lrand(void) double frand(void) int nrand(int val) long lnrand(long val) void srand(long seed) ulong truerand(void) int n_truerand(int val) ulong fastrand(void) DESCRIPTION Rand returns a uniform pseudo-random number x, 0≤x<215. Lrand returns a uniform long x, 0≤x<231. Frand returns a uniform double x, 0.0≤x<1.0, This function calls lrand twice to generate a number with as many as 62 significant bits of mantissa. Nrand returns a uniform integer x, 0≤x<val. Lnrand is the same, but returns a long. The algorithm is additive feedback with: x[n] = (x[n-273] + x[n-607]) mod 2^31 giving a period of 2^30 × (2^607 - 1). The generators are initialized by calling srand with what- ever you like as argument. To get a different starting value each time, srand(time(0)) RAND(2) RAND(2) will work as long as it is not called more often than once per second. Calling srand(1) will initialize the generators to their starting state. Truerand returns a random unsigned long read from /dev/random. Due to the nature of /dev/random, truerand can only return a few hundred bits a second. N_truerand returns a uniform integer x, 0≤x<val. Fastrand is a pseudo random number generator which is seeded and periodically scrambled using truerand. It is approxi- mately 1000 times faster than truerand and is intended for security software that requires a larger stream of unguess- able data. SOURCE /sys/src/libc/port/rand.c /sys/src/libc/9sys/truerand.c SEE ALSO cons(3)