path: root/man3/drand48.3

'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 19:46:03 1993 by Rik Faith (faith@cs.unc.edu)
.TH drand48 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48 \- generate uniformly distributed pseudo-random numbers
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.B double drand48(void);
.BI "double erand48(unsigned short " xsubi [3]);
.PP
.B long lrand48(void);
.BI "long nrand48(unsigned short " xsubi [3]);
.PP
.B long mrand48(void);
.BI "long jrand48(unsigned short " xsubi [3]);
.PP
.BI "void srand48(long " seedval );
.BI "unsigned short *seed48(unsigned short " seed16v [3]);
.BI "void lcong48(unsigned short " param [7]);
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
All functions shown above:
.\" .BR drand48 (),
.\" .BR erand48 (),
.\" .BR lrand48 (),
.\" .BR nrand48 (),
.\" .BR mrand48 (),
.\" .BR jrand48 (),
.\" .BR srand48 (),
.\" .BR seed48 (),
.\" .BR lcong48 ():
.nf
    _XOPEN_SOURCE
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE
.fi
.SH DESCRIPTION
These functions generate pseudo-random numbers using the linear congruential
algorithm and 48-bit integer arithmetic.
.PP
The
.BR drand48 ()
and
.BR erand48 ()
functions return nonnegative
double-precision floating-point values uniformly distributed over the interval
[0.0,\ 1.0).
.PP
The
.BR lrand48 ()
and
.BR nrand48 ()
functions return nonnegative
long integers uniformly distributed over the interval [0,\ 2\[ha]31).
.PP
The
.BR mrand48 ()
and
.BR jrand48 ()
functions return signed long
integers uniformly distributed over the interval [\-2\[ha]31,\ 2\[ha]31).
.PP
The
.BR srand48 (),
.BR seed48 (),
and
.BR lcong48 ()
functions are
initialization functions, one of which should be called before using
.BR drand48 (),
.BR lrand48 (),
or
.BR mrand48 ().
The functions
.BR erand48 (),
.BR nrand48 (),
and
.BR jrand48 ()
do not require
an initialization function to be called first.
.PP
All the functions work by generating a sequence of 48-bit integers,
.IR Xi ,
according to the linear congruential formula:
.PP
.in +4n
.EX
.B Xn+1 = (aXn + c) mod m,   where n >= 0
.EE
.in
.PP
The parameter
.I m
= 2\[ha]48, hence 48-bit integer arithmetic is performed.
Unless
.BR lcong48 ()
is called,
.I a
and
.I c
are given by:
.PP
.in +4n
.EX
.B a = 0x5DEECE66D
.B c = 0xB
.EE
.in
.PP
The value returned by any of the functions
.BR drand48 (),
.BR erand48 (),
.BR lrand48 (),
.BR nrand48 (),
.BR mrand48 (),
or
.BR jrand48 ()
is
computed by first generating the next 48-bit
.I Xi
in the sequence.
Then the appropriate number of bits, according to the type of data item to
be returned, is copied from the high-order bits of
.I Xi
and transformed
into the returned value.
.PP
The functions
.BR drand48 (),
.BR lrand48 (),
and
.BR mrand48 ()
store
the last 48-bit
.I Xi
generated in an internal buffer.
The functions
.BR erand48 (),
.BR nrand48 (),
and
.BR jrand48 ()
require the calling
program to provide storage for the successive
.I Xi
values in the array
argument
.IR xsubi .
The functions are initialized by placing the initial
value of
.I Xi
into the array before calling the function for the first
time.
.PP
The initializer function
.BR srand48 ()
sets the high order 32-bits of
.I Xi
to the argument
.IR seedval .
The low order 16-bits are set
to the arbitrary value 0x330E.
.PP
The initializer function
.BR seed48 ()
sets the value of
.I Xi
to
the 48-bit value specified in the array argument
.IR seed16v .
The
previous value of
.I Xi
is copied into an internal buffer and a
pointer to this buffer is returned by
.BR seed48 ().
.PP
The initialization function
.BR lcong48 ()
allows the user to specify
initial values for
.IR Xi ,
.IR a ,
and
.IR c .
Array argument
elements
.I param[0\-2]
specify
.IR Xi ,
.I param[3\-5]
specify
.IR a ,
and
.I param[6]
specifies
.IR c .
After
.BR lcong48 ()
has been called, a subsequent call to either
.BR srand48 ()
or
.BR seed48 ()
will restore the standard values of
.I a
and
.IR c .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR drand48 (),
.BR erand48 (),
.BR lrand48 (),
.BR nrand48 (),
.BR mrand48 (),
.BR jrand48 (),
.BR srand48 (),
.BR seed48 (),
.BR lcong48 ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:drand48
T}
.TE
.sp 1
.PP
The above
functions record global state information for the random number generator,
so they are not thread-safe.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.SH SEE ALSO
.BR rand (3),
.BR random (3)