man3/random.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  29 .\" Modified Sun Mar 28 00:25:51 1993, David Metcalfe
  30 .\" Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faith@cs.unc.edu)
  31 .\" Modified Sun Aug 20 21:47:07 2000, aeb
  32 .\"
  33 .TH RANDOM 3  2016-10-08 "GNU" "Linux Programmer's Manual"
  34 .SH NAME
  35 random, srandom, initstate, setstate \- random number generator
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <stdlib.h>
  39 .sp
  40 .B long int random(void);
  41
  42 .BI "void srandom(unsigned int " seed );
  43
  44 .BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
  45 .br
  46 .BI "char *setstate(char *" state );
  47 .fi
  48 .sp
  49 .in -4n
  50 Feature Test Macro Requirements for glibc (see
  51 .BR feature_test_macros (7)):
  52 .in
  53 .sp
  54 .ad l
  55 .BR random (),
  56 .BR srandom (),
  57 .BR initstate (),
  58 .BR setstate ():
  59 .RS 4
  60 _XOPEN_SOURCE\ >=\ 500
  61 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  62     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  63     || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
  64 .RE
  65 .ad
  66 .SH DESCRIPTION
  67 The
  68 .BR random ()
  69 function uses a nonlinear additive feedback random
  70 number generator employing a default table of size 31 long integers to
  71 return successive pseudo-random numbers in
  72 the range from 0 to \fBRAND_MAX\fR.
  73 The period of this random number generator is very large, approximately
  74 .IR "16\ *\ ((2^31)\ \-\ 1)" .
  75 .PP
  76 The
  77 .BR srandom ()
  78 function sets its argument as the seed for a new
  79 sequence of pseudo-random integers to be returned by
  80 .BR random ().
  81 These sequences are repeatable by calling
  82 .BR srandom ()
  83 with the same
  84 seed value.
  85 If no seed value is provided, the
  86 .BR random ()
  87 function
  88 is automatically seeded with a value of 1.
  89 .PP
  90 The
  91 .BR initstate ()
  92 function allows a state array \fIstate\fP to
  93 be initialized for use by
  94 .BR random ().
  95 The size of the state array
  96 \fIn\fP is used by
  97 .BR initstate ()
  98 to decide how sophisticated a
  99 random number generator it should use\(emthe larger the state array,
 100 the better the random numbers will be.
 101 \fIseed\fP is the seed for the
 102 initialization, which specifies a starting point for the random number
 103 sequence, and provides for restarting at the same point.
 104 .PP
 105 The
 106 .BR setstate ()
 107 function changes the state array used by the
 108 .BR random ()
 109 function.
 110 The state array \fIstate\fP is used for
 111 random number generation until the next call to
 112 .BR initstate ()
 113 or
 114 .BR setstate ().
 115 \fIstate\fP must first have been initialized
 116 using
 117 .BR initstate ()
 118 or be the result of a previous call of
 119 .BR setstate ().
 120 .SH RETURN VALUE
 121 The
 122 .BR random ()
 123 function returns a value between 0 and
 124 .BR RAND_MAX .
 125 The
 126 .BR srandom ()
 127 function returns no value.
 128
 129 The
 130 .BR initstate ()
 131 function returns a pointer to the previous state array.
 132 On error,
 133 .I errno
 134 is set to indicate the cause.
 135
 136 On success,
 137 .BR setstate ()
 138 returns a pointer to the previous state array.
 139 On error, it returns NULL, with
 140 .I errno
 141 set to indicate the cause of the error.
 142 .SH ERRORS
 143 .TP
 144 .B EINVAL
 145 The
 146 .I state
 147 argument given to
 148 .BR setstate ()
 149 was NULL.
 150 .TP
 151 .B EINVAL
 152 A state array of less than 8 bytes was specified to
 153 .BR initstate ().
 154 .SH ATTRIBUTES
 155 For an explanation of the terms used in this section, see
 156 .BR attributes (7).
 157 .TS
 158 allbox;
 159 lbw23 lb lb
 160 l l l.
 161 Interface       Attribute       Value
 162 T{
 163 .BR random (),
 164 .BR srandom (),
 165 .br
 166 .BR initstate (),
 167 .BR setstate ()
 168 T}      Thread safety   MT-Safe
 169 .TE
 170 .SH CONFORMING TO
 171 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 172 .SH NOTES
 173 Current "optimal" values for the size of the state array \fIn\fP are
 174 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
 175 the nearest known amount.
 176 Using less than 8 bytes will cause an
 177 error.
 178 .PP
 179 This function should not be used in cases where multiple threads use
 180 .BR random ()
 181 and the behavior should be reproducible.
 182 Use
 183 .BR random_r (3)
 184 for that purpose.
 185 .PP
 186 Random-number generation is a complex topic.
 187 .I Numerical Recipes in C: The Art of Scientific Computing
 188 (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William
 189 T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.)
 190 provides an excellent discussion of practical random-number generation
 191 issues in Chapter 7 (Random Numbers).
 192 .PP
 193 For a more theoretical discussion which also covers many practical issues
 194 in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
 195 .IR "The Art of Computer Programming" ,
 196 volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
 197 Addison-Wesley Publishing Company, 1981.
 198 .SH BUGS
 199 According to POSIX,
 200 .BR initstate ()
 201 should return NULL on error.
 202 In the glibc implementation,
 203 .I errno
 204 is (as specified) set on error, but the function does not return NULL.
 205 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=15380
 206 .SH SEE ALSO
 207 .BR getrandom (2),
 208 .BR drand48 (3),
 209 .BR rand (3),
 210 .BR random_r (3),
 211 .BR srand (3)