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.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
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.\" Inspired by a page by Walter Harms created 2002-08-10
.\"
.TH ILOGB 3 2010-09-20 "" "Linux Programmer's Manual"
.SH NAME
ilogb, ilogbf, ilogbl \- get integer exponent of a floating-point value
.SH SYNOPSIS
.B #include <math.h>
.sp
.BI "int ilogb(double " x );
.br
.BI "int ilogbf(float " x );
.br
.BI "int ilogbl(long double " x );
.sp
Link with \fI\-lm\fP.
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR ilogb ():
.RS 4
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED ||
_ISOC99_SOURCE ||
_POSIX_C_SOURCE\ >=\ 200112L;
.br
or
.I cc\ -std=c99
.RE
.br
.BR ilogbf (),
.BR ilogbl ():
.RS 4
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE\ >=\ 200112L;
.br
or
.I cc\ -std=c99
.RE
.ad b
.SH DESCRIPTION
These functions return the exponent part of their argument
as a signed integer.
When no error occurs, these functions
are equivalent to the corresponding
.BR logb (3)
functions, cast to
.IR int .
.SH RETURN VALUE
On success, these functions return the exponent of
.IR x ,
as a signed integer.

If
.I x
is zero, then a domain error occurs, and the functions return
.\" the POSIX.1 spec for logb() says logb() gives pole error for this
.\" case, but for ilogb() it says domain error.
.BR FP_ILOGB0 .
.\" glibc: The numeric value is either `INT_MIN' or `-INT_MAX'.

If
.I x
is a NaN, then a domain error occurs, and the functions return
.BR FP_ILOGBNAN .
.\" glibc: The numeric value is either `INT_MIN' or `INT_MAX'.
.\" On i386, FP_ILOGB0 and FP_ILOGBNAN have the same value.

If
.I x
is negative infinity or positive infinity, then
a domain error occurs, and the functions return
.BR INT_MAX .
.\"
.\" POSIX.1-2001 also says:
.\" If the correct value is greater than {INT_MAX}, {INT_MAX}
.\" shall be returned and a domain error shall occur.
.\"
.\" If the correct value is less than {INT_MIN}, {INT_MIN}
.\" shall be returned and a domain error shall occur.
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.PP
The following errors can occur:
.TP
Domain error: \fIx\fP is 0 or a NaN
.\" .I errno
.\" is set to
.\" .BR EDOM .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.IP
These functions do not set
.IR errno
for this case.
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6794
.TP
Domain error: \fIx\fP is an infinity
.\" .I errno
.\" is set to
.\" .BR EDOM .
.\" An invalid floating-point exception
.\" .RB ( FE_INVALID )
.\" is raised.
.IP
These functions do not set
.IR errno
or raise an exception for this case.
.\" FIXME . Is it intentional that these functions do not set errno,
.\" or raise an exception?
.\" log(), log2(), log10() do set errno
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6794
.SH CONFORMING TO
C99, POSIX.1-2001.
.SH SEE ALSO
.BR log (3),
.BR logb (3),
.BR significand (3)