ILOGB(3) Linux Programmer's Manual ILOGB(3)

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

#include <math.h>
int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);

Link with -lm.

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

C99, POSIX.1-2001, POSIX.1-2008.

Before version 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)

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2021-03-22