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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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ILOGB(3P) POSIX Programmer's Manual ILOGB(3P)
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.
ilogb, ilogbf, ilogbl — return an unbiased exponent
#include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
These functions shall return the exponent part of their argument
x. Formally, the return value is the integral part of logr|x| as
a signed integral value, for non-zero x, where r is the radix of
the machine's floating-point arithmetic, which is the value of
FLT_RADIX defined in <float.h>.
An application wishing to check for error situations should set
errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling
these functions. On return, if errno is non-zero or
fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
FE_UNDERFLOW) is non-zero, an error has occurred.
Upon successful completion, these functions shall return the
exponent part of x as a signed integer value. They are equivalent
to calling the corresponding logb() function and casting the
returned value to type int.
If x is 0, the value FP_ILOGB0 shall be returned. On XSI-
conformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If x is ±Inf, the value {INT_MAX} shall be returned. On XSI-
conformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If x is a NaN, the value FP_ILOGBNAN shall be returned. On XSI-
conformant systems, a domain error shall occur;
otherwise, a domain error may occur.
If the correct value is greater than {INT_MAX}, a domain error
shall occur and an unspecified value shall be returned. On XSI-
conformant systems, a domain error shall occur and {INT_MAX} shall
be returned.
If the correct value is less than {INT_MIN}, a domain error shall
occur and an unspecified value shall be returned. On XSI-
conformant systems, a domain error shall occur and {INT_MIN} shall
be returned.
These functions shall fail if:
Domain Error
The correct value is not representable as an integer.
The x argument is zero, NaN, or ±Inf.
If the integer expression (math_errhandling &
MATH_ERRNO) is non-zero, then errno shall be set to
[EDOM]. If the integer expression (math_errhandling &
MATH_ERREXCEPT) is non-zero, then the invalid
floating-point exception shall be raised.
These functions may fail if:
Domain Error
The x argument is zero, NaN, or ±Inf.
If the integer expression (math_errhandling &
MATH_ERRNO) is non-zero, then errno shall be set to
[EDOM]. If the integer expression (math_errhandling &
MATH_ERREXCEPT) is non-zero, then the invalid
floating-point exception shall be raised.
The following sections are informative.
None.
On error, the expressions (math_errhandling & MATH_ERRNO) and
(math_errhandling & MATH_ERREXCEPT) are independent of each other,
but at least one of them must be non-zero.
The errors come from taking the expected floating-point value and
converting it to int, which is an invalid operation in
IEEE Std 754‐1985 (since overflow, infinity, and NaN are not
representable in a type int), so should be a domain error.
There are no known implementations that overflow. For overflow to
happen, {INT_MAX} must be less than LDBL_MAX_EXP*log2(FLT_RADIX)
or {INT_MIN} must be greater than LDBL_MIN_EXP*log2(FLT_RADIX) if
subnormals are not supported, or {INT_MIN} must be greater than
(LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX) if subnormals are
supported.
None.
feclearexcept(3p), fetestexcept(3p), logb(3p), scalbln(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.20,
Treatment of Error Conditions for Mathematical Functions,
float.h(0p), math.h(0p)
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 .
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 .
IEEE/The Open Group 2017 ILOGB(3P)
Pages that refer to this page: math.h(0p), expm1(3p), logb(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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