DRAND48(3C) Standard C Library Functions DRAND48(3C)
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48 - generate uniformly distributed pseudo-random numbers
SYNOPSIS
#include <stdlib.h>
double drand48(void)
double erand48(
unsigned short x(i)[3]);
long lrand48(void)
long nrand48(
unsigned short x(i)[3]);
long mrand48(void)
long jrand48(
unsigned short x(i)[3]);
void srand48(
long seedval);
unsigned short *seed48(
unsigned short seed16v[3]);
void lcong48(
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, 2^31].
Functions
mrand48() and
jrand48() return signed long integers uniformly
distributed over the interval [-2^31, 2^31].
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 routines work by generating a sequence of 48-bit integer values,
X(i ), according to the linear congruential formula
X(n+1)= (aX (n)+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 = 5
DEECE66D(16) = 273673163155(8)
c = B(16) = 13(8)
The value returned by any of the functions
drand48(),
erand48(),
lrand48(),
nrand48(),
mrand48(), or
jrand48() is computed by first
generating the next 48-bit X(i) 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 X(i) and transformed into
the returned value.
The functions
drand48(),
lrand48(), and
mrand48() store the last 48-bit
X(i) generated in an internal buffer. X(i) must be initialized prior to
being invoked. The functions
erand48(),
nrand48(), and
jrand48() require
the calling program to provide storage for the successive X(i) values in
the array specified as an argument when the functions are invoked. These
routines do not have to be initialized; the calling program must place
the desired initial value of X(i) 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, that is, the
sequence of numbers in each stream will
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 X(i) to
the 32 bits contained in its argument. The low-order 16 bits of X(i) are
set to the arbitrary value 330E(16) .
The initializer function
seed48() sets the value of X(i) to the 48-bit
value specified in the argument array. In addition, the previous value of
X(i) 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 X(i) value, and then use this value
to reinitialize using
seed48() when the program is restarted.
The initialization function
lcong48() allows the user to specify the
initial X(i) the multiplier value
a, and the addend value
c. Argument
array elements
param[0-2] specify X(i),
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 above.
For a more powerful random number generator, see
arc4random(3C).
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | Safe |
+--------------------+-----------------+
SEE ALSO
arc4random(3C),
rand(3C),
attributes(7),
standards(7) June 20, 2021
DRAND48(3C)