man3/drand48.3

   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 Sat Jul 24 19:46:03 1993 by Rik Faith (faith@cs.unc.edu)
  28 .TH DRAND48 3  1993-07-02 "" "Linux Programmer's Manual"
  29 .SH NAME
  30 drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
  31 lcong48 \- generate uniformly distributed pseudo-random numbers
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <stdlib.h>
  35 .sp
  36 .B double drand48(void);
  37 .sp
  38 .BI "double erand48(unsigned short " xsubi [3]);
  39 .sp
  40 .B long int lrand48(void);
  41 .sp
  42 .BI "long int nrand48(unsigned short " xsubi [3]);
  43 .sp
  44 .B long int mrand48(void);
  45 .sp
  46 .BI "long int jrand48(unsigned short " xsubi [3]);
  47 .sp
  48 .BI "void srand48(long int " seedval );
  49 .sp
  50 .BI "unsigned short *seed48(unsigned short " seed16v [3]);
  51 .sp
  52 .BI "void lcong48(unsigned short " param [7]);
  53 .fi
  54 .SH DESCRIPTION
  55 These functions generate pseudo-random numbers using the linear congruential
  56 algorithm and 48-bit integer arithmetic.
  57 .PP
  58 The \fBdrand48\fP() and \fBerand48\fP() functions return non-negative
  59 double-precision floating-point values uniformly distributed between
  60 [0.0, 1.0).
  61 .PP
  62 The \fBlrand48\fP() and \fBnrand48\fP() functions return non-negative
  63 long integers uniformly distributed between 0 and 2^31.
  64 .PP
  65 The \fBmrand48\fP() and \fBjrand48\fP() functions return signed long
  66 integers uniformly distributed between \-2^31 and 2^31.
  67 .PP
  68 The \fBsrand48\fP(), \fBseed48\fP() and \fBlcong48\fP() functions are
  69 initialization functions, one of which should be called before using
  70 \fBdrand48\fP(), \fBlrand48\fP() or \fBmrand48\fP().
  71 The functions
  72 \fBerand48\fP(), \fBnrand48\fP() and \fBjrand48\fP() do not require
  73 an initialization function to be called first.
  74 .PP
  75 All the functions work by generating a sequence of 48-bit integers,
  76 \fIXi\fP, according to the linear congruential formula:
  77 .sp
  78 .nf
  79 .RS
  80 .B Xn+1 = (aXn + c) mod m,   where n >= 0
  81 .RE
  82 .fi
  83 .sp
  84 The parameter \fIm\fP = 2^48, hence 48-bit integer arithmetic is performed.
  85 Unless \fBlcong48\fP() is called, \fIa\fP and \fIc\fP are given by:
  86 .sp
  87 .nf
  88 .RS
  89 .B a = 0x5DEECE66D
  90 .B c = 0xB
  91 .RE
  92 .fi
  93 .sp
  94 The value returned by any of the functions \fBdrand48\fP(), \fBerand48\fP(),
  95 \fBlrand48\fP(), \fBnrand48\fP(), \fBmrand48\fP() or \fBjrand48\fP() is
  96 computed by first generating the next 48-bit \fIXi\fP in the sequence.
  97 Then the appropriate number of bits, according to the type of data item to
  98 be returned, is copied from the high-order bits of \fIXi\fP and transformed
  99 into the returned value.
 100 .PP
 101 The functions \fBdrand48\fP(), \fBlrand48\fP() and \fBmrand48\fP() store
 102 the last 48-bit \fIXi\fP generated in an internal buffer.
 103 The functions
 104 \fBerand48\fP(), \fBnrand48\fP() and \fBjrand48\fP() require the calling
 105 program to provide storage for the successive \fIXi\fP values in the array
 106 argument \fIxsubi\fP.
 107 The functions are initialized by placing the initial
 108 value of \fIXi\fP into the array before calling the function for the first
 109 time.
 110 .PP
 111 The initializer function \fBsrand48\fP() sets the high order 32-bits of
 112 \fIXi\fP to the argument \fIseedval\fP.
 113 The low order 16-bits are set
 114 to the arbitrary value 0x330E.
 115 .PP
 116 The initializer function \fBseed48\fP() sets the value of \fIXi\fP to
 117 the 48-bit value specified in the array argument \fIseed16v\fP.
 118 The
 119 previous value of \fIXi\fP is copied into an internal buffer and a
 120 pointer to this buffer is returned by \fBseed48\fP().
 121 .PP
 122 The initialization function \fBlcong48\fP() allows the user to specify
 123 initial values for \fIXi\fP, \fIa\fP and \fIc\fP.
 124 Array argument
 125 elements \fIparam[0-2]\fP specify \fIXi\fP, \fIparam[3-5]\fP specify
 126 \fIa\fP, and \fIparam[6]\fP specifies \fIc\fP.
 127 After \fBlcong48\fP()
 128 has been called, a subsequent call to either \fBsrand48\fP() or
 129 \fBseed48\fP() will restore the standard values of \fIa\fP and \fIc\fP.
 130 .SH "CONFORMING TO"
 131 SVr4, POSIX.1-2001.
 132 .SH NOTES
 133 These functions are declared obsolete by SVID 3, which states that
 134 .BR rand(3)
 135 should be used instead.
 136 .SH "SEE ALSO"
 137 .BR rand (3),
 138 .BR random (3)