drand48(3)drand48(3)Name
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48 - generate uniformly distributed pseudo-random numbers
Syntax
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
NOTE: Your terminal may not be able to accurately display the special
math symbols in this reference page. See the ULTRIX Programmer's Manual,
Unsupported for a complete description of the math symbols.
This family of functions generates pseudo-random numbers using the well-
known linear congruential algorithm and 48-bit integer arithmetic.
Functions and return non-negative double-precision floating-point values
uniformly distributed over the interval [0.0, 1.0).
Functions and return non-negative long integers uniformly distributed
over the interval [0, 231).
Functions and return signed long integers uniformly distributed
over the interval [−231, 231).
Functions and are initialization entry points, one of which should be
invoked before either or is called. Although it is not recommended
practice, constant default initializer values will be supplied automati‐
cally if or is called without a prior call to an initialization entry
point. Functions and do not require an initialization entry point to be
called first.
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 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 or is computed by first gen‐
erating 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 and store the last 48-bit Xi generated in an internal buf‐
fer; that is why they must be initialized prior to being invoked. The
functions and 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 ini‐
tial value of Xi into the array and pass it as an argument. By using
different arguments, functions and allow separate modules of a large
program to generate several independent streams of pseudo-random num‐
bers. 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 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 sets the value of Xi to the 48-bit value speci‐
fied in the argument array. In addition, the previous value of Xi is
copied into a 48-bit internal buffer, used only by and a pointer to this
buffer is the value returned by 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 Xi value, and then use this value to reinitialize via when the
program is restarted.
The initialization function 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, param[3-5] specify the multiplier a, and param[6]
specifies the 16-bit addend c. After has been called, a subsequent call
to either or will restore the ``standard'' multiplier and addend values,
a and c, specified on the previous page.
Notes
The source code for the portable version can even be used on computers
which do not have floating-point arithmetic. In such a situation, func‐
tions and do not exist. Instead, they are replaced by the two new func‐
tions below.
long irand48 (m)
unsigned short m;
long krand48 (xsubi, m)
unsigned short xsubi[3], m;
Functions and return non-negative long integers uniformly distributed
over the interval [0, m−1].
See Alsorand(3)
ULTRIX Programmer's Manual, Unsupported
drand48(3)