drand48(3C) drand48(3C)
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48,
srand48, seed48, lcong48 - generate uniformly distributed
pseudo-random numbers
SYNOPSIS
double drand48 ( )
double erand48 (xsubi)
unsigned short xsubi[3];
long lrand48 ( )
long nrand48 (xsubi)
unsigned short xsubi[3];
long mrand48 ( )
long jrand48 (xsubi)
unsigned short xsubi[3];
void srand48 (seedval)
long seedval;
unsigned short *seed48 (seed16v)
unsigned short seed16v[3];
void lcong48 (param)
unsigned short param[7];
DESCRIPTION
This family of functions generates pseudo-random numbers
using the well-known linear congruential algorithm and 48-
bit integer arithmetic.
Functions drand48 and erand48 return non-negative double-
precision floating-point values uniformly distributed over
the interval [0.0, 1.0).
Functions lrand48 and nrand48 return non-negative long
integers uniformly distributed over the interval [0, 231).
Functions mrand48 and jrand48 return signed long integers
uniformly distributed over the interval [-231, 231).
[-231, 231). Functions srand48, seed48, and lcong48 are
initialization entry points, one of which should be invoked
before drand48,lrand48, or mrand48 is called. (Although it
is not recommended practice, constant default initializer
values are supplied automatically if drand48,lrand48, or
mrand48 is called without a prior call to an initialization
entry point.) Functions erand48, nrand48, and jrand48 do not
require an initialization entry point to be called first.
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drand48(3C) drand48(3C)
All the routines work by generating a sequence of 48-bit
integer values, Xi, according to the linear congruential
formula
Xn+1 = (aXn+c)mod m n>0.
The parameter m=248; hence 48-bit integer arithmetic is
performed. Unless lcong48 has been invoked, the multiplier
value a and the addend value c are given by
a = 5DEECE66D16 = 2736731631558
c = B16 = 138.
The value returned by any of the functions drand48,erand48,
lrand48,nrand48,mrand48, or jrand48 is computed by first
generating the next 48-bit Xi in the sequence. Then the
appropriate number of bits, according to the type of data
item to be returned, are copied from the high-order
(leftmost) bits of Xi and transformed into the returned
value.
The functions drand48,lrand48, and mrand48 store the last
48-bit Xi generated in an internal buffer; that is why they
must be initialized prior to being invoked. The functions
erand48 , nrand48 , and jrand48 require the calling program
to provide storage for the successive Xi values in the array
specified as an argument when the functions are invoked.
That is why these routines do not have to be initialized;
the calling program merely has to place the desired initial
value of Xi into the array and pass it as an argument. By
using different arguments, functions erand48,nrand48, and
jrand48 allow separate modules of a large program to
generate several independent streams of pseudo-random
numbers, i.e., the sequence of numbers in each stream does
not depend upon how many times the routines have been called
to generate numbers for the other streams.
The initializer function srand48 sets the high-order 32 bits
of Xi to the 32 bits contained in its argument. The low-
order 16 bits of Xi are set to the arbitrary value 330E16.
The initializer function seed48 sets the value of Xi to the
48-bit value specified in the argument array. The previous
value of Xi is copied into a 48-bit internal buffer, used
only by seed48. A pointer to this buffer is the value
returned by seed48 . The returned pointer, which can be
ignored if not needed, is useful if a program is to be
restarted from a given point at some future time. Use the
pointer to get and store the last Xi value; then use this
value to reinitialize via seed48 when the program is
restarted.
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drand48(3C) drand48(3C)
The initialization function lcong48 allows the user to
specify the initial Xi, the multiplier value a, and the
addend value c. Argument array elements param[0-2] specify
Xi, elements param[3-5] specify the multiplier a, and
param[6] specifies the 16-bit addend c. After lcong48 has
been called, a subsequent call to either srand48 or seed48
will restore the ``standard'' multiplier and addend values,
a and c, specified on the previous page.
NOTES
The routines are coded in portable C. The source code for
the portable version can even be used on computers which do
not have floating-point arithmetic. In such a situation,
functions drand48 and erand48 do not exist; instead, they
are replaced by the following two functions:
long irand48 (m)
unsigned short m;
long krand48 (xsubi,m)
unsigned short xsubi[3],m;
Functions irand48 and krand48 return non-negative long
integers uniformly distributed over the interval [0, m-1].
SEE ALSO
rand(3C).
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