drand48(3) CLIX drand48(3)
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48 - Generates uniformly distributed pseudo-random numbers
LIBRARY
Standard C Library (libc.a)
SYNOPSIS
double drand48(
void );
double erand48(
unsigned short xsubi[3] );
longlrand48(
void );
longnrand48(
unsigned short xsubi[3] );
longmrand48(
void );
longjrand48(
unsigned short xsubi[3] );
void srand48(
long seedval );
unsigned short *seed48(
unsigned short seed16v[3] );
void lcong48(
unsigned short param[7] );
PARAMETERS
xsubi For erand48, nrand48, and jrand48 the subi parameter specifies
the initial 48 bit value for xsubi in the equation used to
generate the random numbers.
seedval The value given in srand48 seedval sets the high order 32 bits
of the initial value of xsubi used in the random number
equation.
seed16v The seed seed16v sets the initial 48 bit value for xsubi used in
the random number.
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drand48(3) CLIX drand48(3)
param The lcong48 param[0] - param[2] specifies the xsubi for the
equation, param[3] - param[5] specify the multiplier 'a' to be
used in the equation. The param[6] specifies the addend 'c' to
be used.
DESCRIPTION
This family of functions generates pseudo-random numbers using the well-
known linear congruential algorithm and 48-bit integer arithmetic.
The functions drand48() and erand48() return non-negative double-precision
floating-point values uniformly distributed over the interval [0.0, 1.0).
The functions lrand48() and nrand48() return non-negative long integers
uniformly distributed over the interval [0,2^31).
The functions mrand48() and jrand48() return signed long integers
uniformly distributed over the interval [-2^31,2^31).
The functions srand48(), seed48() and lcong48() are initialization entry
points, one of which should be invoked before either drand48(), lrand48()
or mrand48() is called. (Although it is not recommended practice,
constant default initializer values will be 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.
All the functions work by generating a sequence of 48-bit integer values,
(xsubi, according to the linear congruential formula
Xn+1 = (aXn+c)mod m n>0.
The parameter m = 2^48; 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 xsubi 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 xsubi and transformed into
the returned value.
The functions drand48(), lrand48() and mrand48() store the last 48-bit
xsubi generated in an internal buffer, and must be initialized prior to
being invoked. The functions erand48(), nrand48() and jrand48() require
the calling program to provide storage for the successive xsubi values in
the array specified as an argument when the functions are invoked. These
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drand48(3) CLIX drand48(3)
functions do not have to be initialized; the calling program must place
the desired initial value of xsubi into the array and pass it as an
argument. By using different arguments, the functions erand48(),
nrand48() and jrand48() allow separate modules of a large program to
generate several independent streams of pseudo-random numbers, that is,
the sequence of numbers in each stream will not depend upon how many times
the functions have been called to generate numbers for the other streams.
The initializer function srand48() sets the high-order 32 bits of xsubi to
the 32 bits contained in its argument. The low-order 16 bits of xsubi are
set to the arbitrary value 0x330E.
The initializer function seed48() sets the value of xsubi to the 48-bit
value specified in the argument array. In addition, the previous value of
xsubi is copied into a 48-bit internal buffer, used only by seed48(), and
a pointer to this buffer is the value returned by seed48(). This returned
pointer, which can just 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 at and store the last xsubi value, and then use this value
to reinitialize with seed48() when the program is restarted.
The initialization function lcong48() allows the user to specify the
initial xsubi, the multiplier value a, and the addend value c. Argument
array elements param[0-2] specify xsubi, 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.
EXAMPLES
get_random (random)
{
double *random;
double erand48();
unsigned short xsubi[3];
/* setup initial value of xi */
xsubi[1] = 1024;
xsubi[2] = 962;
xsubi[3] = 2946;
/* get the random number */
*random = erand48(xsubi);
}
NOTES
The source code for the portable version can be used on computers which do
not have floating-point arithmetic. In such a situation, functions
drand48() and erand48() are replaced by the two new functions below.
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drand48(3) CLIX drand48(3)
long irand48(
unsigned short m );
long krand48(
unsigned short xsubi[3] ,
unsigned short m );
RETURN VALUES
The functions irand48() and krand48() return non-negative long integers
uniformly distributed over the interval [0,m-1]. The functions drand48
and erand48 return non-negative double precision floating-point values
uniformly distributed over the interval [0.0-1.0). The functions lrand48
and nrand48 return non-negative long integers uniformly distributed over
the interval [0,2^31). The functions mrand48 and jrand48 returns long
signed integers uniformly distributed over the interval [-2^31,2^31). The
functions srand48 and lcong48 do not return a value. The function seed48
returns a pointer to the buffer storing the value to be used as Xi in the
random number equation.
RELATED INFORMATION
Functions: rand(3)
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