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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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ilogb(3) Library Functions Manual ilogb(3)
ilogb, ilogbf, ilogbl - get integer exponent of a floating-point
value
Math library (libm, -lm)
#include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
ilogb():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
ilogbf(), ilogbl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
These functions return the exponent part of their argument as a
signed integer. When no error occurs, these functions are
equivalent to the corresponding logb(3) functions, cast to int.
On success, these functions return the exponent of x, as a signed
integer.
If x is zero, then a domain error occurs, and the functions return
FP_ILOGB0.
If x is a NaN, then a domain error occurs, and the functions
return FP_ILOGBNAN.
If x is negative infinity or positive infinity, then a domain
error occurs, and the functions return INT_MAX.
See math_error(7) for information on how to determine whether an
error has occurred when calling these functions.
The following errors can occur:
Domain error: x is 0 or a NaN
An invalid floating-point exception (FE_INVALID) is raised,
and errno is set to EDOM (but see BUGS).
Domain error: x is an infinity
An invalid floating-point exception (FE_INVALID) is raised,
and errno is set to EDOM (but see BUGS).
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ ilogb(), ilogbf(), ilogbl() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
C99, POSIX.1-2001.
Before glibc 2.16, the following bugs existed in the glibc
implementation of these functions:
• The domain error case where x is 0 or a NaN did not cause errno
to be set or (on some architectures) raise a floating-point
exception.
• The domain error case where x is an infinity did not cause
errno to be set or raise a floating-point exception.
log(3), logb(3), significand(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 ilogb(3)
Pages that refer to this page: logb(3), significand(3)
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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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