man3/random.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\" Modified Sun Mar 28 00:25:51 1993, David Metcalfe
  28 .\" Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faith@cs.unc.edu)
  29 .\" Modified Sun Aug 20 21:47:07 2000, aeb
  30 .\"
  31 .TH RANDOM 3  2000-08-20 "GNU" "Linux Programmer's Manual"
  32 .SH NAME
  33 random, srandom, initstate, setstate \- random number generator
  34 .SH SYNOPSIS
  35 .nf
  36 .B #include <stdlib.h>
  37 .sp
  38 .B long int random(void);
  39 .br
  40 .BI "void srandom(unsigned int " seed );
  41 .br
  42 .BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
  43 .br
  44 .BI "char *setstate(char *" state );
  45 .fi
  46 .SH DESCRIPTION
  47 The
  48 .BR random ()
  49 function uses a non-linear additive feedback random
  50 number generator employing a default table of size 31 long integers to
  51 return successive pseudo-random numbers in
  52 the range from 0 to \fBRAND_MAX\fR.
  53 The period of this random number generator is very large, approximately
  54 16*((2**31)\-1).
  55 .PP
  56 The
  57 .BR srandom ()
  58 function sets its argument as the seed for a new
  59 sequence of pseudo-random integers to be returned by
  60 .BR random ().
  61 These sequences are repeatable by calling
  62 .BR srandom ()
  63 with the same
  64 seed value.
  65 If no seed value is provided, the
  66 .BR random ()
  67 function
  68 is automatically seeded with a value of 1.
  69 .PP
  70 The
  71 .BR initstate ()
  72 function allows a state array \fIstate\fP to
  73 be initialized for use by
  74 .BR random ().
  75 The size of the state array
  76 \fIn\fP is used by
  77 .BR initstate ()
  78 to decide how sophisticated a
  79 random number generator it should use \(em the larger the state array,
  80 the better the random numbers will be.
  81 \fIseed\fP is the seed for the
  82 initialization, which specifies a starting point for the random number
  83 sequence, and provides for restarting at the same point.
  84 .PP
  85 The
  86 .BR setstate ()
  87 function changes the state array used by the
  88 .BR random ()
  89 function.
  90 The state array \fIstate\fP is used for
  91 random number generation until the next call to
  92 .BR initstate ()
  93 or
  94 .BR setstate ().
  95 \fIstate\fP must first have been initialized
  96 using
  97 .BR initstate ()
  98 or be the result of a previous call of
  99 .BR setstate ().
 100 .SH "RETURN VALUE"
 101 The
 102 .BR random ()
 103 function returns a value between 0 and
 104 .BR RAND_MAX .
 105 The
 106 .BR srandom ()
 107 function returns no value.
 108 The
 109 .BR initstate ()
 110 and
 111 .BR setstate ()
 112 functions return a pointer to the previous state
 113 array, or NULL on error.
 114 .SH ERRORS
 115 .TP
 116 .B EINVAL
 117 A state array of less than 8 bytes was specified to
 118 .BR initstate ().
 119 .SH "CONFORMING TO"
 120 4.3BSD, POSIX.1-2001.
 121 .SH NOTES
 122 Current "optimal" values for the size of the state array \fIn\fP are
 123 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
 124 the nearest known amount.
 125 Using less than 8 bytes will cause an
 126 error.
 127 .SH "SEE ALSO"
 128 .BR rand (3),
 129 .BR srand (3)