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.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
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.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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2004-11-03 13:51:07 +00:00
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-09-19 17:19:13 +00:00
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.TH LOGB 3 2010-09-20 "" "Linux Programmer's Manual"
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.SH NAME
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logb, logbf, logbl \- get exponent of a floating-point value
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.SH SYNOPSIS
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.B #include <math.h>
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.sp
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.BI "double logb(double " x );
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.br
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.BI "float logbf(float " x );
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.br
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.BI "long double logbl(long double " x );
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.sp
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Link with \fI\-lm\fP.
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.ad l
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.BR logb ():
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.RS 4
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_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
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_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE ||
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_POSIX_C_SOURCE\ >=\ 200112L;
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.br
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or
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.I cc\ -std=c99
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.RE
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.br
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.BR logbf (),
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.BR logbl ():
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.RS 4
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_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
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_POSIX_C_SOURCE\ >=\ 200112L;
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.br
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or
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.I cc\ -std=c99
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.RE
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.ad b
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.SH DESCRIPTION
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These functions extract the exponent from the
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internal floating-point representation of
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.I x
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and return it as a floating-point value.
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The integer constant
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.BR FLT_RADIX ,
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defined in
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.IR <float.h> ,
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indicates the radix used for the system's floating-point representation.
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If
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.B FLT_RADIX
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is 2,
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.BI logb( x )
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is equal to
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.BI floor(log2( x ))\fR,
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except that it is probably faster.
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.LP
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If
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.I x
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is subnormal,
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.BR logb ()
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returns the exponent
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.I x
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would have if it were normalized.
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.SH RETURN VALUE
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On success, these functions return the exponent of
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.IR x .
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If
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.I x
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is a NaN,
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a NaN is returned.
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If
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.I x
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is zero, then a pole error occurs, and the functions return
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.RB - HUGE_VAL ,
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.RB - HUGE_VALF ,
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or
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.RB - HUGE_VALL ,
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respectively.
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If
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.I x
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is negative infinity or positive infinity, then
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positive infinity is returned.
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.SH ERRORS
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See
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.BR math_error (7)
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for information on how to determine whether an error has occurred
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when calling these functions.
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.PP
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The following errors can occur:
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.TP
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Pole error: \fIx\fP is 0
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.\" .I errno
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.\" is set to
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.\" .BR ERANGE .
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A divide-by-zero floating-point exception
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.RB ( FE_DIVBYZERO )
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is raised.
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.PP
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These functions do not set
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.IR errno .
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.\" FIXME . Is it intentional that these functions do not set errno?
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.\" log(), log2(), log10() do set errno
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.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6793
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.\"
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2007-05-18 11:59:14 +00:00
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.\" .SH HISTORY
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.\" The
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.\" .BR logb ()
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.\" function occurs in 4.3BSD.
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2005-07-18 15:05:56 +00:00
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.\" see IEEE.3 in the 4.3BSD manual
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.SH "CONFORMING TO"
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C99, POSIX.1-2001.
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.SH "SEE ALSO"
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.BR ilogb (3),
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.BR log (3)
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