Guardian Native C Library Calls Reference Manual (G06.28+, H06.04+)

lrand48(3) Guardian Native C Library Calls Reference Manual
NAME
lrand48 - Generates uniformly distributed pseudorandom number sequences
LIBRARY
G-series native Guardian processes: $SYSTEM.SYSnn.ZCRTLSRL
G-series native OSS processes: /G/system/sysnn/zcrtlsrl
H-series native Guardian processes: $SYSTEM.ZDLLnnn.ZCRTLDLL
H-series OSS processes: /G/system/zdllnnn/zcrtldll
SYNOPSIS
#include <stdlib.h>
long lrand48(void);
DESCRIPTION
This function generates a sequence of pseudorandom numbers using the linear congruential algo-
rithm and 48-bit integer arithmetic.
The lrand48() function returns nonnegative long integers uniformly distributed over the range of
y values such that 0.0 (zero) is less than or equal to y which is less than 2 to the power 31.
The srand48(), seed48(), and lcong48() functions initialize the random-number generator. Pro-
grams should invoke one of them before calling the lrand48() function. (Although it is not
recommended practice, constant default initializer values are supplied automatically if the
lrand48() function is called without first calling an initialization function.)
The function generates a sequence of 48-bit integer values, X
i
, according to the linear congruen-
tial formula:
X
n+1
= (aX
n
+c)
mod m
n ≥ 0
The value of m equals 2 to the power 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 = 5DEECE66D
16
= 273673163155
8
c = B
16
= 13
8
The values returned by the lrand48() function is computed by first generating the next 48-bit X
i
in the sequence. Then the appropriate bits, according to the type of data item to be returned, are
copied from the high-order (most-significant) bits of X
i
and transformed into the returned value.
The *Llrand48( ) function stores the last 48-bit X
i
generated into an internal buffer, which is why
it must be initialized prior to being invoked.
NOTES
Prior to calling a random-number generator function, the init field of the struct drand48_data
structure (passed to the random-number generator functions) must be cleared.
This function supports both IEEE Std 754-1985 floating-point and Tandem floating-point values
in the native environment. IEEE values can include NaN and infinity, and the sign of 0.0 (zero)
can be either positive or negative. Refer to the fp_class(3) reference page for a description of
IEEE value classes.
Guardian functions are available to convert between floating-point formats. Refer to the Guar-
dian Programmer’s Guide for a discussion of floating-point conversions.
RETURN VALUES
The lrand48() function returns signed long integers uniformly distributed over the range
$0˜<=˜y<˜2 sup 31$.
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