1drand48(3C) Standard C Library Functions drand48(3C)
2
6 drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
7 lcong48 - generate uniformly distributed pseudo-random numbers
8
10 #include <stdlib.h>
11 double drand48(void)
13 double erand48(unsigned short x(i)[3]);
16 long lrand48(void)
19 long nrand48(unsigned short x(i)[3]);
22 long mrand48(void)
25 long jrand48(unsigned short x(i)[3]);
28 void srand48(long seedval);
31 unsigned short *seed48(unsigned short seed16v[3]);
34 void lcong48(unsigned short param[7]);
37
40 This family of functions generates pseudo-random numbers using the
41 well-known linear congruential algorithm and 48-bit integer arithmetic.
42 Functions drand48() and erand48() return non-negative double-precision
45 floating-point values uniformly distributed over the interval [0.0,
46 1.0).
47 Functions lrand48() and nrand48() return non-negative long integers
50 uniformly distributed over the interval [0, 2 ^31 ].
51 Functions mrand48() and jrand48() return signed long integers uniformly
54 distributed over the interval [-2 ^31 , 2 ^31 ].
55 Functions srand48(), seed48(), and lcong48() are initialization entry
58 points, one of which should be invoked before either drand48(),
59 lrand48(), or mrand48() is called. (Although it is not recommended
60 practice, constant default initializer values will be supplied automat‐
61 ically if drand48(), lrand48(), or mrand48() is called without a prior
62 call to an initialization entry point.) Functions erand48(), nrand48(),
63 and jrand48() do not require an initialization entry point to be called
64 first.
65 All the routines work by generating a sequence of 48-bit integer val‐
68 ues, X(i ), according to the linear congruential formula
69 X(n+1)= (aX (n)+c)(mod m) n>=0.
72 The parameter m = 2^48; hence 48-bit integer arithmetic is performed.
75 Unless lcong48() has been invoked, the multiplier value aand the addend
76 value care given by
77 a = 5DEECE66D(16) = 273673163155(8)
78 c = B(16) = 13(8)
79 The value returned by any of the functions drand48(), erand48(),
82 lrand48(), nrand48(), mrand48(), or jrand48() is computed by first gen‐
83 erating the next 48-bit X(i) in the sequence. Then the appropriate num‐
84 ber of bits, according to the type of data item to be returned, are
85 copied from the high-order (leftmost) bits of X(i) and transformed into
86 the returned value.
87 The functions drand48(), lrand48(), and mrand48() store the last 48-bit
90 X(i) generated in an internal buffer. X(i) must be initialized prior to
91 being invoked. The functions erand48(), nrand48(), and jrand48()
92 require the calling program to provide storage for the successive X(i)
93 values in the array specified as an argument when the functions are
94 invoked. These routines do not have to be initialized; the calling pro‐
95 gram must place the desired initial value of X(i) into the array and
96 pass it as an argument. By using different arguments, functions
97 erand48(), nrand48(), and jrand48() allow separate modules of a large
98 program to generate several independent streams of pseudo-random num‐
99 bers, that is, the sequence of numbers in each stream will not depend
100 upon how many times the routines have been called to generate numbers
101 for the other streams.
102 The initializer function srand48() sets the high-order 32 bits of X(i)
105 to the 32 bits contained in its argument. The low-order 16 bits of X(i)
106 are set to the arbitrary value 330E(16) .
107 The initializer function seed48() sets the value of X(i) to the 48-bit
110 value specified in the argument array. In addition, the previous value
111 of X(i) is copied into a 48-bit internal buffer, used only by seed48(),
112 and a pointer to this buffer is the value returned by seed48(). This
113 returned pointer, which can just be ignored if not needed, is useful if
114 a program is to be restarted from a given point at some future time —
115 use the pointer to get at and store the last X(i) value, and then use
116 this value to reinitialize using seed48() when the program is
117 restarted.
118 The initialization function lcong48() allows the user to specify the
121 initial X(i) the multiplier value a, and the addend value c. Argument
122 array elements param[0-2] specify X(i), param[3-5] specify the multi‐
123 plier a, and param[6] specifies the 16-bit addend c. After lcong48()
124 has been called, a subsequent call to either srand48() or seed48() will
125 restore the ``standard'' multiplier and addend values, a and c, speci‐
126 fied above.
127
129 See attributes(5) for descriptions of the following attributes:
130 ┌─────────────────────────────┬─────────────────────────────┐
135 │ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
136 ├─────────────────────────────┼─────────────────────────────┤
137 │Interface Stability │Standard │
138 ├─────────────────────────────┼─────────────────────────────┤
139 │MT-Level │Safe │
140 └─────────────────────────────┴─────────────────────────────┘
141
143 rand(3C), attributes(5), standards(5)
144SunOS 5.11 24 Jul 2002 drand48(3C)