This family of functions shall generate pseudo-random numbers using a linear
congruential algorithm and 48-bit integer arithmetic.
The drand48() and erand48() functions shall return non-negative,
double-precision, floating-point values, uniformly distributed over the
The lrand48() and nrand48() functions shall return non-negative,
long integers, uniformly distributed over the interval [0,2**31).
The mrand48() and jrand48() functions shall return signed long
integers uniformly distributed over the interval [-2**31,2**31).
The srand48(), seed48(), and lcong48() functions 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 shall be
supplied automatically if drand48(), lrand48(), or
mrand48() is called without a prior call to an initialization entry
point.) The erand48(), nrand48(), and jrand48() functions
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() is invoked, the multiplier value a and the
addend value c are given by: a = 5DEECE66D_16 = 273673163155_8
c = B_16 = 13_8
The value returned by any of the drand48(), erand48(),
jrand48(), lrand48(), mrand48(), or nrand48()
functions 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 drand48(), lrand48(), and mrand48() functions store the
last 48-bit X_i generated in an internal buffer; that is why the application
shall ensure that these are initialized prior to being invoked. The
erand48(), nrand48(), and jrand48() functions 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; that is, the
sequence of numbers in each stream shall not depend upon how many times
the routines are called to generate numbers for the other streams.
The initializer function srand48() sets the high-order 32 bits of X_i to
the low-order 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. The low-order 16 bits of X_i are set to
the low-order 16 bits of seed16v. The mid-order 16 bits of
X_i are set to the low-order 16 bits of seed16v. The
high-order 16 bits of X_i are set to the low-order 16 bits of
seed16v. 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 via
seed48() when the program is restarted.
The initializer 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 specifies the
16-bit addend c. After lcong48() is called, a subsequent call to
either srand48() or seed48() shall restore the standard
multiplier and addend values, a and c, specified above.
The drand48(), lrand48(), and mrand48() functions need not
be reentrant. A function that is not required to be reentrant is not required
to be thread-safe.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html