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  2015-02-01 "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 _SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  61 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  62 .RE
  63 .ad
  64 .SH DESCRIPTION
  65 The
  66 .BR random ()
  67 function uses a nonlinear additive feedback random
  68 number generator employing a default table of size 31 long integers to
  69 return successive pseudo-random numbers in
  70 the range from 0 to \fBRAND_MAX\fR.
  71 The period of this random number generator is very large, approximately
  72 .IR "16\ *\ ((2^31)\ \-\ 1)" .
  73 .PP
  74 The
  75 .BR srandom ()
  76 function sets its argument as the seed for a new
  77 sequence of pseudo-random integers to be returned by
  78 .BR random ().
  79 These sequences are repeatable by calling
  80 .BR srandom ()
  81 with the same
  82 seed value.
  83 If no seed value is provided, the
  84 .BR random ()
  85 function
  86 is automatically seeded with a value of 1.
  87 .PP
  88 The
  89 .BR initstate ()
  90 function allows a state array \fIstate\fP to
  91 be initialized for use by
  92 .BR random ().
  93 The size of the state array
  94 \fIn\fP is used by
  95 .BR initstate ()
  96 to decide how sophisticated a
  97 random number generator it should use\(emthe larger the state array,
  98 the better the random numbers will be.
  99 \fIseed\fP is the seed for the
 100 initialization, which specifies a starting point for the random number
 101 sequence, and provides for restarting at the same point.
 102 .PP
 103 The
 104 .BR setstate ()
 105 function changes the state array used by the
 106 .BR random ()
 107 function.
 108 The state array \fIstate\fP is used for
 109 random number generation until the next call to
 110 .BR initstate ()
 111 or
 112 .BR setstate ().
 113 \fIstate\fP must first have been initialized
 114 using
 115 .BR initstate ()
 116 or be the result of a previous call of
 117 .BR setstate ().
 118 .SH RETURN VALUE
 119 The
 120 .BR random ()
 121 function returns a value between 0 and
 122 .BR RAND_MAX .
 123 The
 124 .BR srandom ()
 125 function returns no value.
 126
 127 The
 128 .BR initstate ()
 129 function returns a pointer to the previous state array.
 130 On error,
 131 .I errno
 132 is set to indicate the cause.
 133
 134 On success,
 135 .BR setstate ()
 136 returns a pointer to the previous state array.
 137 On error, it returns NULL, with
 138 .I errno
 139 set to indicate the cause of the error.
 140 .SH ERRORS
 141 .TP
 142 .B EINVAL
 143 The
 144 .I state
 145 argument given to
 146 .BR setstate ()
 147 was NULL.
 148 .TP
 149 .B EINVAL
 150 A state array of less than 8 bytes was specified to
 151 .BR initstate ().
 152 .SH ATTRIBUTES
 153 For an explanation of the terms used in this section, see
 154 .BR attributes (7).
 155 .TS
 156 allbox;
 157 lbw23 lb lb
 158 l l l.
 159 Interface       Attribute       Value
 160 T{
 161 .BR random (),
 162 .BR srandom (),
 163 .br
 164 .BR initstate (),
 165 .BR setstate ()
 166 T}      Thread safety   MT-Safe
 167 .TE
 168 .SH CONFORMING TO
 169 4.3BSD, POSIX.1-2001.
 170 .SH NOTES
 171 Current "optimal" values for the size of the state array \fIn\fP are
 172 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
 173 the nearest known amount.
 174 Using less than 8 bytes will cause an
 175 error.
 176 .PP
 177 This function should not be used in cases where multiple threads use
 178 .BR random ()
 179 and the behavior should be reproducible.
 180 Use
 181 .BR random_r (3)
 182 for that purpose.
 183 .PP
 184 Random-number generation is a complex topic.
 185 .I Numerical Recipes in C: The Art of Scientific Computing
 186 (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William
 187 T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.)
 188 provides an excellent discussion of practical random-number generation
 189 issues in Chapter 7 (Random Numbers).
 190 .PP
 191 For a more theoretical discussion which also covers many practical issues
 192 in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
 193 .IR "The Art of Computer Programming" ,
 194 volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
 195 Addison-Wesley Publishing Company, 1981.
 196 .SH BUGS
 197 According to POSIX,
 198 .BR initstate ()
 199 should return NULL on error.
 200 In the glibc implementation,
 201 .I errno
 202 is (as specified) set on error, but the function does not return NULL.
 203 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=15380
 204 .SH SEE ALSO
 205 .BR drand48 (3),
 206 .BR getrandom (2),
 207 .BR rand (3),
 208 .BR random_r (3),
 209 .BR srand (3)