drand48.3c (2010 09)

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drand48(3C) drand48(3C)

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] = (a*X[n]+c) modulo mn>=0

The parameter m = 2ˆ48; hence 48-bit integer arithmetic is performed.

Unless

lcong48() has been invoked, the default multiplier value a and the default addend value c are

given by

a = 0x5DEECE66D (base 16) = 0273673163155 (base 8)

c = 0xB (base 16) = 013 (base 8)

The value returned by any of the functions

drand48(), erand48(), lrand48(), nrand48(),

mrand48(),orjrand48() 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; 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 X[i]

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 X[i] into

the array and pass it as an argument. By using different arguments,

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 do 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 0x330E (base 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 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

via

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 multi-

plier a, and param [6] speciﬁes the 16-bit addend c. After

lcong48()

has been called, a subsequent call

to either

srand48() or seed48() restores the default multiplier and addend values for a and c,

speciﬁed above.

Obsolescent Interfaces

drand48_r(), erand48_r(), lrand48_r(), nrand48_r(), mrand48_r(), jrand48_r(),

srand48_r(), seed48_r(), lcong48_r() generate uniformly distributed pseudo-random numbers.

WARNINGS

drand48_r(), erand48_r(), lrand48_r(), nrand48_r(), mrand48_r(), jrand48_r(),

srand48_r(), seed48_r() and lcong48_r() are obsolescent interfaces supported only for compa-

tibility with existing DCE applications. New multithreaded applications should use

drand48(),

erand48(), lrand48(), nrand48(), mrand48(), jrand48(), srand48(), seed48(), and

lcong48().