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
```

```
_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 |

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)