path: root/man3p/drand48.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "DRAND48" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" drand48 
.SH NAME
drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48, seed48,
srand48 \- generate uniformly distributed
pseudo\-random numbers
.SH SYNOPSIS
.LP
\fB#include <stdlib.h>
.br
.sp
double drand48(void);
.br
double erand48(unsigned short\fP \fIxsubi\fP\fB[3]);
.br
long jrand48(unsigned short\fP \fIxsubi\fP\fB[3]);
.br
void lcong48(unsigned short\fP \fIparam\fP\fB[7]);
.br
long lrand48(void);
.br
long mrand48(void);
.br
long nrand48(unsigned short\fP \fIxsubi\fP\fB[3]);
.br
unsigned short *seed48(unsigned short\fP \fIseed16v\fP\fB[3]);
.br
void srand48(long\fP \fIseedval\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
This family of functions shall generate pseudo-random numbers using
a linear congruential algorithm and 48-bit integer
arithmetic.
.LP
The \fIdrand48\fP() and \fIerand48\fP() functions shall return non-negative,
double-precision, floating-point values,
uniformly distributed over the interval [0.0,1.0).
.LP
The \fIlrand48\fP() and \fInrand48\fP() functions shall return non-negative,
long integers, uniformly distributed over the
interval [0,2**31).
.LP
The \fImrand48\fP() and \fIjrand48\fP() functions shall return signed
long integers uniformly distributed over the interval
[-2**31,2**31).
.LP
The \fIsrand48\fP(), \fIseed48\fP(), and \fIlcong48\fP() functions
are initialization entry points, one of which should be
invoked before either \fIdrand48\fP(), \fIlrand48\fP(), or \fImrand48\fP()
is called. (Although it is not recommended practice,
constant default initializer values shall be supplied automatically
if \fIdrand48\fP(), \fIlrand48\fP(), or \fImrand48\fP() is
called without a prior call to an initialization entry point.) The
\fIerand48\fP(), \fInrand48\fP(), and \fIjrand48\fP()
functions do not require an initialization entry point to be called
first.
.LP
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
.LP
The parameter \fIm\fP = 2**48; hence 48-bit integer arithmetic is
performed. Unless \fIlcong48\fP() is invoked, the multiplier
value \fIa\fP and the addend value \fIc\fP are given by:
\fIa\fP = 5DEECE66D_16 = 273673163155_8
.LP
\fIc\fP = B_16 = 13_8
.LP
The value returned by any of the \fIdrand48\fP(), \fIerand48\fP(),
\fIjrand48\fP(), \fIlrand48\fP(), \fImrand48\fP(), or
\fInrand48\fP() 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.
.LP
The \fIdrand48\fP(), \fIlrand48\fP(), and \fImrand48\fP() 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 \fIerand48\fP(),
\fInrand48\fP(), and \fIjrand48\fP() 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, \fIerand48\fP(), \fInrand48\fP(), and \fIjrand48\fP()
allow separate modules of a large program to generate
several \fIindependent\fP streams of pseudo-random numbers; that is,
the sequence of numbers in each stream shall \fInot\fP
depend upon how many times the routines are called to generate numbers
for the other streams.
.LP
The initializer function \fIsrand48\fP() 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.
.LP
The initializer function \fIseed48\fP() 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 \fIseed16v\fP[\fI0\fP].
The mid-order 16 bits of X_i are
set to the low-order 16 bits of \fIseed16v\fP[\fI1\fP]. The high-order
16 bits of X_i are set to the low-order 16 bits of
\fIseed16v\fP[\fI2\fP]. In addition, the previous value of X_i is
copied into a 48-bit internal buffer, used only by
\fIseed48\fP(), and a pointer to this buffer is the value returned
by \fIseed48\fP(). 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 \fIseed48\fP() when the program is restarted.
.LP
The initializer function \fIlcong48\fP() allows the user to specify
the initial X_i , the multiplier value \fIa\fP, and the
addend value \fIc\fP. 
Argument array elements \fIparam\fP[\fI0-2\fP] specify X_i , \fIparam\fP[\fI3-5\fP]
specify the
multiplier \fIa\fP, and \fIparam\fP[\fI6\fP] specifies the 16-bit
addend \fIc\fP. After \fIlcong48\fP() is called, a subsequent call
to
either \fIsrand48\fP() or \fIseed48\fP() shall restore the standard
multiplier and addend values, \fIa\fP and \fIc,\fP
specified above.
.LP
The \fIdrand48\fP(), \fIlrand48\fP(), and \fImrand48\fP() functions
need not be reentrant. A function that is not required to
be reentrant is not required to be thread-safe.
.SH RETURN VALUE
.LP
As described in the DESCRIPTION above.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIrand\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<stdlib.h>\fP
.SH COPYRIGHT
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 .