1DRAND48(3P)                POSIX Programmer's Manual               DRAND48(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

14       drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48,  seed48,
15       srand48 — generate uniformly distributed pseudo-random numbers
16

SYNOPSIS

18       #include <stdlib.h>
19
20       double drand48(void);
21       double erand48(unsigned short xsubi[3]);
22       long jrand48(unsigned short xsubi[3]);
23       void lcong48(unsigned short param[7]);
24       long lrand48(void);
25       long mrand48(void);
26       long nrand48(unsigned short xsubi[3]);
27       unsigned short *seed48(unsigned short seed16v[3]);
28       void srand48(long seedval);
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DESCRIPTION

31       This  family  of functions shall generate pseudo-random numbers using a
32       linear congruential algorithm and 48-bit integer arithmetic.
33
34       The drand48() and erand48() functions shall return  non-negative,  dou‐
35       ble-precision,  floating-point  values,  uniformly distributed over the
36       interval [0.0,1.0).
37
38       The lrand48() and nrand48() functions shall return  non-negative,  long
39       integers, uniformly distributed over the interval [0,231).
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41       The mrand48() and jrand48() functions shall return signed long integers
42       uniformly distributed over the interval [−231,231).
43
44       The srand48(), seed48(), and  lcong48()  functions  are  initialization
45       entry  points,  one of which should be invoked before either drand48(),
46       lrand48(), or mrand48() is called.  (Although  it  is  not  recommended
47       practice,  constant  default initializer values shall be supplied auto‐
48       matically if drand48(), lrand48(), or mrand48()  is  called  without  a
49       prior call to an initialization entry point.) The erand48(), nrand48(),
50       and jrand48() functions do not require an initialization entry point to
51       be called first.
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53       All  the  routines work by generating a sequence of 48-bit integer val‐
54       ues, $X_ i" " ,$ according to the linear congruential formula:
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56              $X sub{n+1} " " = " " (aX_ n" "^+^c) sub{roman mod " " m} " "  "
57              "  " "  " "  " "  " "  " "  " " n>= " " 0$
58
59       The  parameter  $m^=^2"^"  48$; hence 48-bit integer arithmetic is per‐
60       formed. Unless lcong48() is invoked, the multiplier value $a$  and  the
61       addend value $c$ are given by:
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63              $a  "  "  mark  =  "  " roman "5DEECE66D"^sub 16 " " = " " roman
64              273673163155^sub 8$
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66              $c " " lineup = " " roman B^sub 16 " " = " " roman 13^sub 8$
67
68       The value returned by  any  of  the  drand48(),  erand48(),  jrand48(),
69       lrand48(),  mrand48(), or nrand48() functions is computed by first gen‐
70       erating the next 48-bit $X_ i$ in the sequence.  Then  the  appropriate
71       number  of bits, according to the type of data item to be returned, are
72       copied from the high-order (leftmost) bits of $X_  i$  and  transformed
73       into the returned value.
74
75       The drand48(), lrand48(), and mrand48() functions store the last 48-bit
76       $X_ i$ generated in an internal buffer; that  is  why  the  application
77       shall  ensure  that  these  are initialized prior to being invoked. The
78       erand48(), nrand48(), and jrand48() functions require the calling  pro‐
79       gram  to  provide storage for the successive $X_ i$ values in the array
80       specified as an argument when the functions are invoked.  That  is  why
81       these  routines  do  not  have  to  be initialized; the calling program
82       merely has to place the desired initial value of $X_ i$ into the  array
83       and  pass  it as an argument.  By using different arguments, erand48(),
84       nrand48(), and jrand48() allow separate modules of a large  program  to
85       generate several independent streams of pseudo-random numbers; that is,
86       the sequence of numbers in each stream shall not depend upon  how  many
87       times  the  routines  are  called  to  generate  numbers  for the other
88       streams.
89
90       The initializer function srand48() sets the high-order 32 bits  of  $X_
91       i$ to the low-order 32 bits contained in its argument. The low-order 16
92       bits of $X_ i$ are set to the arbitrary value $roman 330E_ 16" " .$
93
94       The initializer function seed48() sets the  value  of  $X_  i$  to  the
95       48-bit  value specified in the argument array. The low-order 16 bits of
96       $X_ i$ are set to the low-order 16 bits of seed16v[0].   The  mid-order
97       16  bits of $X_ i$ are set to the low-order 16 bits of seed16v[1].  The
98       high-order 16 bits of $X_ i$ are  set  to  the  low-order  16  bits  of
99       seed16v[2].  In addition, the previous value of $X_ i$ is copied into a
100       48-bit internal buffer, used only by seed48(), and a  pointer  to  this
101       buffer is the value returned by seed48().  This returned pointer, which
102       can just be ignored if not needed, is useful if  a  program  is  to  be
103       restarted from a given point at some future time—use the pointer to get
104       at and store the last $X_ i$ value, and then use this value  to  reini‐
105       tialize via seed48() when the program is restarted.
106
107       The  initializer function lcong48() allows the user to specify the ini‐
108       tial $X_ i" " ,$ the multiplier value $a,$ and the  addend  value  $c.$
109       Argument array elements param[0-2] specify $X_ i" " ,$ param[3-5] spec‐
110       ify the multiplier $a,$ and param[6] specifies the 16-bit  addend  $c.$
111       After  lcong48()  is  called,  a subsequent call to either srand48() or
112       seed48() shall restore the standard multiplier and addend values, a and
113       c, specified above.
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115       The  drand48(),  lrand48(), and mrand48() functions need not be thread-
116       safe.
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RETURN VALUE

119       As described in the DESCRIPTION above.
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ERRORS

122       No errors are defined.
123
124       The following sections are informative.
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EXAMPLES

127       None.
128

APPLICATION USAGE

130       None.
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RATIONALE

133       None.
134

FUTURE DIRECTIONS

136       None.
137

SEE ALSO

139       rand()
140
141       The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
142
144       Portions of this text are reprinted and reproduced in  electronic  form
145       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
146       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
147       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
148       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
149       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
150       event of any discrepancy between this version and the original IEEE and
151       The  Open Group Standard, the original IEEE and The Open Group Standard
152       is the referee document. The original Standard can be  obtained  online
153       at http://www.unix.org/online.html .
154
155       Any  typographical  or  formatting  errors that appear in this page are
156       most likely to have been introduced during the conversion of the source
157       files  to  man page format. To report such errors, see https://www.ker
158       nel.org/doc/man-pages/reporting_bugs.html .
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162IEEE/The Open Group                  2013                          DRAND48(3P)
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