'\" te
.\" Copyright (c) 2004, 2011, Oracle and/or its affiliates. All rights reserved.
.\" Copyright 1989 AT&T
.TH rand 3C "24 May 2011" "SunOS 5.11" "Standard C Library Functions"
.SH NAME
rand, srand, rand_r \- simple random-number generator
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>

\fBint\fR \fBrand\fR(\fBvoid\fR);
.fi

.LP
.nf
\fBvoid\fR \fBsrand\fR(\fBunsigned int\fR \fIseed\fR);
.fi

.LP
.nf
\fBint\fR \fBrand_r\fR(\fBunsigned int *\fR\fIseed\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBrand()\fR function uses a multiplicative congruential random-number generator with period 2^32 that returns successive pseudo-random numbers in the range of 0 to \fBRAND_MAX\fR (defined in <\fBstdlib.h\fR>).
.sp
.LP
The  \fBsrand()\fR function uses the argument \fIseed\fR as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to \fBrand()\fR. If \fBsrand()\fR is then called with the same \fIseed\fR value, the sequence of pseudo-random numbers will be repeated.  If \fBrand()\fR is called before any calls to \fBsrand()\fR have been made, the same sequence will be generated as when \fBsrand()\fR is first called with a \fIseed\fR value of 1.
.sp
.LP
The \fBrand_r()\fR function has the same functionality as \fBrand()\fR except that a pointer to a seed  \fIseed\fR must be supplied by the caller. If \fBrand_r()\fR is called with the same initial value for the object pointed to by seed and that object is not modified between successive calls to \fBrand_r()\fR, the same sequence as that produced by calls to \fBrand()\fR will be generated.
.sp
.LP
The \fBrand()\fR and \fBsrand()\fR functions provide per-process pseudo-random streams shared by all threads. The same effect can be achieved if all threads call \fBrand_r()\fR with a pointer to the same seed object. The \fBrand_r()\fR function allows a thread to generate a private pseudo-random stream by having the seed object be private to the thread.
.SH USAGE
.sp
.LP
The spectral properties of \fBrand()\fR are limited.  The \fBdrand48\fR(3C) function provides a better, more elaborate random-number generator.
.sp
.LP
When compiling multithreaded applications, the  \fB_REENTRANT\fR flag must be defined on the compile line.  This flag should be used only in multithreaded applications.
.sp
.LP
Programmers should use \fB/dev/urandom\fR or \fB/dev/random\fR for most random-number generation, especially for cryptographic purposes. See \fBrandom\fR(7D).
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
tab() box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPEATTRIBUTE VALUE
_
Interface StabilityCommitted
_
MT-LevelSafe
_
StandardSee \fBstandards\fR(5).
.TE

.SH SEE ALSO
.sp
.LP
\fBdrand48\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5), \fBrandom\fR(7D)