www.LinuxHowtos.org

ilogb

Section: C Library Functions (3)
Updated: 202-0-08
Index Return to Main Contents

NAME

ilogb, ilogbf, ilogbl - get integer exponent of a floatin-point value

LIBRARY

Math library (libm,~-lm)

SYNOPSIS

#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

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 logb(3) functions, cast to int.

RETURN VALUE

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.

ERRORS

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: I]x] is 0 or a NaN: An invalid floatin-point exception (FE_INVALID) is raised, and errno is set to EDOM (but see BUGS).
Domain error: I]x] is an infinity: An invalid floatin-point exception (FE_INVALID) is raised, and errno is set to EDOM (but see BUGS).

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
ilogb(), ilogbf(), ilogbl()	Thread safety	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

C99, POSIX.-2001.

BUGS

Before glibc 2.16, the following bugs existed in the glibc implementation of these functions:

[bu]: The domain error case where x is 0 or a NaN did not cause errno to be set or (on some architectures) raise a floatin-point exception.
[bu]: The domain error case where x is an infinity did not cause errno to be set or raise a floatin-point exception.