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drand48.3p (7103B)

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.TH DRAND48 "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.EQ
delim $$
.EN
.SH NAME
drand48,
erand48,
jrand48,
lcong48,
lrand48,
mrand48,
nrand48,
seed48,
srand48
\(em generate uniformly distributed pseudo-random numbers
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>
.P
double drand48(void);
double erand48(unsigned short \fIxsubi\fP[3]);
long jrand48(unsigned short \fIxsubi\fP[3]);
void lcong48(unsigned short \fIparam\fP[7]);
long lrand48(void);
long mrand48(void);
long nrand48(unsigned short \fIxsubi\fP[3]);
unsigned short *seed48(unsigned short \fIseed16v\fP[3]);
void srand48(long \fIseedval\fP);
.fi
.SH DESCRIPTION
This family of functions shall generate pseudo-random numbers using
a linear congruential algorithm and 48-bit integer arithmetic.
.P
The
\fIdrand48\fR()
and
\fIerand48\fR()
functions shall return non-negative, double-precision, floating-point
values, uniformly distributed over the interval [0.0,1.0).
.P
The
\fIlrand48\fR()
and
\fInrand48\fR()
functions shall return non-negative, long integers, uniformly
distributed over the interval [0,2\u\s-331\s+3\d).
.P
The
\fImrand48\fR()
and
\fIjrand48\fR()
functions shall return signed long integers uniformly distributed over
the interval [\-2\u\s-331\s+3\d,2\u\s-331\s+3\d).
.P
The
\fIsrand48\fR(),
\fIseed48\fR(),
and
\fIlcong48\fR()
functions are initialization entry points, one of which should be
invoked before either
\fIdrand48\fR(),
\fIlrand48\fR(),
or
\fImrand48\fR()
is called. (Although it is not recommended practice, constant default
initializer values shall be supplied automatically if
\fIdrand48\fR(),
\fIlrand48\fR(),
or
\fImrand48\fR()
is called without a prior call to an initialization entry point.) The
\fIerand48\fR(),
\fInrand48\fR(),
and
\fIjrand48\fR()
functions do not require an initialization entry point to be called
first.
.P
All the routines work by generating a sequence of 48-bit integer
values, $X_ i" " ,$ according to the linear congruential formula:
.sp
.RS
$X sub{n+1} " " = " " (aX_ n" "^+^c) sub{roman mod " " m} " "  " "  " "  " "  " "  " "  " "  " " n>= " " 0$
.RE
.P
The parameter $m^=^2"^" 48$; hence 48-bit integer arithmetic is
performed. Unless
\fIlcong48\fR()
is invoked, the multiplier value $a$ and the addend value $c$ are given
by:
.sp
.RS
$a " " mark = " " roman "5DEECE66D"^sub 16 " " = " " roman 273673163155^sub 8$
.P
$c " " lineup = " " roman B^sub 16 " " = " " roman 13^sub 8$
.RE
.P
The value returned by any of the
\fIdrand48\fR(),
\fIerand48\fR(),
\fIjrand48\fR(),
\fIlrand48\fR(),
\fImrand48\fR(),
or
\fInrand48\fR()
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.
.P
The
\fIdrand48\fR(),
\fIlrand48\fR(),
and
\fImrand48\fR()
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\fR(),
\fInrand48\fR(),
and
\fIjrand48\fR()
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\fR(),
\fInrand48\fR(),
and
\fIjrand48\fR()
allow separate modules of a large program to generate several
.IR independent
streams of pseudo-random numbers; that is, the sequence of numbers in
each stream shall
.IR not
depend upon how many times the routines are called to generate numbers
for the other streams.
.P
The initializer function
\fIsrand48\fR()
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 $roman 330E_ 16" " .$
.P
The initializer function
\fIseed48\fR()
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
.IR seed16v [ 0 ].
The mid-order 16 bits of $X_ i$ are set to the low-order 16 bits of
.IR seed16v [ 1 ].
The high-order 16 bits of $X_ i$ are set to the low-order 16 bits of
.IR seed16v [ 2 ].
In addition, the previous value of $X_ i$ is copied into a 48-bit
internal buffer, used only by
\fIseed48\fR(),
and a pointer to this buffer is the value returned by
\fIseed48\fR().
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\(emuse the pointer to get at and store the last $X_ i$
value, and then use this value to reinitialize via
\fIseed48\fR()
when the program is restarted.
.P
The initializer function
\fIlcong48\fR()
allows the user to specify the initial $X_ i" " ,$ the multiplier value
$a,$ and the addend value $c.$ Argument array elements
.IR param [ 0-2 ]
specify $X_ i" " ,$
.IR param [ 3-5 ]
specify the multiplier $a,$ and
.IR param [ 6 ]
specifies the 16-bit addend $c.$ After
\fIlcong48\fR()
is called, a subsequent call to either
\fIsrand48\fR()
or
\fIseed48\fR()
shall restore the standard multiplier and addend values,
.IR a
and
.IR c,
specified above.
.P
The
\fIdrand48\fR(),
\fIlrand48\fR(),
and
\fImrand48\fR()
functions need not be thread-safe.
.SH "RETURN VALUE"
As described in the DESCRIPTION above.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
These functions should be avoided whenever non-trivial
requirements (including safety) have to be fulfilled.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIinitstate\fR\^(\|)",
.IR "\fIrand\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdlib.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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 .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .