#include <math.h> int ilogb(double x); int ilogbf(float x); int ilogbl(long double x);
These functions shall return the exponent part of their argument x. Formally, the return value is the integral part of $log sub{r}|x|$ 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.
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.
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:
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.
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.
The Base Definitions volume of POSIX.1-2017, Section 4.20, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>
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