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  2019-03-06 "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 .PP
  40 .B long int random(void);
  41 .PP
  42 .BI "void srandom(unsigned int " seed );
  43 .PP
  44 .BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
  45 .PP
  46 .BI "char *setstate(char *" state );
  47 .fi
  48 .PP
  49 .in -4n
  50 Feature Test Macro Requirements for glibc (see
  51 .BR feature_test_macros (7)):
  52 .in
  53 .PP
  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 Current "optimal" values for the size of the state array \fIn\fP are
 102 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
 103 the nearest known amount.
 104 Using less than 8 bytes results in an error.
 105 \fIseed\fP is the seed for the
 106 initialization, which specifies a starting point for the random number
 107 sequence, and provides for restarting at the same point.
 108 .PP
 109 The
 110 .BR setstate ()
 111 function changes the state array used by the
 112 .BR random ()
 113 function.
 114 The state array \fIstate\fP is used for
 115 random number generation until the next call to
 116 .BR initstate ()
 117 or
 118 .BR setstate ().
 119 \fIstate\fP must first have been initialized
 120 using
 121 .BR initstate ()
 122 or be the result of a previous call of
 123 .BR setstate ().
 124 .SH RETURN VALUE
 125 The
 126 .BR random ()
 127 function returns a value between 0 and
 128 .BR RAND_MAX .
 129 The
 130 .BR srandom ()
 131 function returns no value.
 132 .PP
 133 The
 134 .BR initstate ()
 135 function returns a pointer to the previous state array.
 136 On error,
 137 .I errno
 138 is set to indicate the cause.
 139 .PP
 140 On success,
 141 .BR setstate ()
 142 returns a pointer to the previous state array.
 143 On error, it returns NULL, with
 144 .I errno
 145 set to indicate the cause of the error.
 146 .SH ERRORS
 147 .TP
 148 .B EINVAL
 149 The
 150 .I state
 151 argument given to
 152 .BR setstate ()
 153 was NULL.
 154 .TP
 155 .B EINVAL
 156 A state array of less than 8 bytes was specified to
 157 .BR initstate ().
 158 .SH ATTRIBUTES
 159 For an explanation of the terms used in this section, see
 160 .BR attributes (7).
 161 .TS
 162 allbox;
 163 lbw23 lb lb
 164 l l l.
 165 Interface       Attribute       Value
 166 T{
 167 .BR random (),
 168 .BR srandom (),
 169 .br
 170 .BR initstate (),
 171 .BR setstate ()
 172 T}      Thread safety   MT-Safe
 173 .TE
 174 .SH CONFORMING TO
 175 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 176 .SH NOTES
 177 The
 178 .BR random ()
 179 function should not be used in multithreaded programs
 180 where reproducible behavior is required.
 181 Use
 182 .BR random_r (3)
 183 for that purpose.
 184 .PP
 185 Random-number generation is a complex topic.
 186 .I Numerical Recipes in C: The Art of Scientific Computing
 187 (William H.\& Press, Brian P.\& Flannery, Saul A.\& Teukolsky, William
 188 T.\& Vetterling; New York: Cambridge University Press, 2007, 3rd ed.)
 189 provides an excellent discussion of practical random-number generation
 190 issues in Chapter 7 (Random Numbers).
 191 .PP
 192 For a more theoretical discussion which also covers many practical issues
 193 in depth, see Chapter 3 (Random Numbers) in Donald E.\& Knuth's
 194 .IR "The Art of Computer Programming" ,
 195 volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
 196 Addison-Wesley Publishing Company, 1981.
 197 .SH BUGS
 198 According to POSIX,
 199 .BR initstate ()
 200 should return NULL on error.
 201 In the glibc implementation,
 202 .I errno
 203 is (as specified) set on error, but the function does not return NULL.
 204 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=15380
 205 .SH SEE ALSO
 206 .BR getrandom (2),
 207 .BR drand48 (3),
 208 .BR rand (3),
 209 .BR random_r (3),
 210 .BR srand (3)