ref: f777743b7242e539a8ac806e4e15c4b527be4bb6
dir: /sys/man/2/rand/
.TH RAND 2 .SH NAME rand, lrand, frand, nrand, lnrand, srand, truerand, ntruerand, genrandom, prng, fastrand, nfastrand \- random number generators .SH SYNOPSIS .B #include <u.h> .br .B #include <libc.h> .PP .ta \w'\fLdouble 'u .B int rand(void) .PP .B long lrand(void) .PP .B double frand(void) .PP .B int nrand(int val) .PP .B long lnrand(long val) .PP .B void srand(long seed) .PP .B ulong truerand(void) .PP .B ulong ntruerand(ulong val) .sp .B #include <mp.h> .br .B #include <libsec.h> .PP .B void genrandom(uchar *buf, int nbytes) .PP .B void prng(uchar *buf, int nbytes) .PP .B ulong fastrand(void) .PP .B ulong nfastrand(ulong val) .SH DESCRIPTION .I Rand returns a uniform pseudo-random number .IR x , .if t 0≤ \fIx\fR <2\u\s715\s10\d. .if n 0≤ x <2^15. .PP .I Lrand returns a uniform .B long .IR x , .if t 0≤ \fIx\fR <2\u\s731\s10\d. .if n 0≤ x <2^31. .PP .I Frand returns a uniform .B double .IR x , .RI 0.0≤ x <1.0, This function calls .I lrand twice to generate a number with as many as 62 significant bits of mantissa. .PP .I Nrand returns a uniform integer .IR x , .RI 0≤ x < val. .I Lnrand is the same, but returns a .BR long . .PP The algorithm is additive feedback with: .IP x[n] = (x[n\(mi273] + x[n\(mi607]) mod .if t 2\u\s731\s0\d .if n 2^31 .LP giving a period of .if t 2\u\s730\s10\d \(mu (2\u\s7607\s10\d \- 1). .if n 2^30 × (2^607 - 1). .PP The generators are initialized by calling .I srand with whatever you like as argument. To get a different starting value each time, .IP .L srand(time(0)) .LP will work as long as it is not called more often than once per second. Calling .IP .L srand(1) .LP will initialize the generators to their starting state. .PP .I Truerand returns a random unsigned long read from .BR /dev/random . Due to the nature of .BR /dev/random , truerand can only return a few hundred bits a second. .PP .I Ntruerand returns a uniform random integer .IR x , .if t 0≤ \fIx\fR < \fIval\fR ≤ 2\u\s732\s10\d-1. .if n 0≤ x < val ≤ 2^32-1. .PP .I Genrandom fills a buffer with bytes from the X9.17 pseudo-random number generator. The X9.17 generator is seeded by 24 truly random bytes read from .BR /dev/random . .PP .I Prng uses the native .IR rand (2) pseudo-random number generator to fill the buffer. Used with .IR srand , this function can produce a reproducible stream of pseudo random numbers useful in testing. .PP Both .I genrandom and .I prng may be passed to .I mprand (see .IR mp (2)). .PP .I Fastrand uses .I genrandom to return a uniform .B "unsigned long .IR x , .if t 0≤ \fIx\fR <2\u\s732\s10\d-1. .if n 0≤ x <2^32-1. .PP .I Nfastrand uses .I genrandom to return a uniform .B "unsigned long .IR x , .if t 0≤ \fIx\fR < \fIval\fR ≤ 2\u\s732\s10\d-1. .if n 0≤ x < val ≤ 2^32-1. .SH SOURCE .B /sys/src/libc/port/*rand.c .br .B /sys/src/libc/9sys/truerand.c .br .B /sys/src/libsec/port/genrandom.c .br .B /sys/src/libsec/port/prng.c .br .B /sys/src/libsec/port/*fastrand.c .SH "SEE ALSO .IR cons (3), .IR mp (2) .SH BUGS .I Truerand and .I ntruerand maintain a static file descriptor.