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c99 [ flag... ] file... -lm [ library... ] #include <math.h> int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
cc [ flag... ] file... -lm [ library... ] #include <math.h> int ilogb(double x);
int ilogbf(float x);
int ilogbl(long double x);
These functions return the exponent part of their argument x. Formally, the return value is the integral part of log(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>.
Upon successful completion, these functions return the exponent part of x as a signed integer value. They are equivalent to calling the corresponding logb(3M) function and casting the returned value to type int.
If x is 0, the value FP_ILOGB0 is returned. For SUSv3-conforming applications compiled with the c99 compiler driver (see standards(5)), a domain error occurs.
If x is \(+-Inf, the value INT_MAX is returned. For SUSv3-conforming applications compiled with the c99 compiler driver, a domain error occurs.
If x is NaN, the value FP_ILOGBNAN is returned. For SUSv3-conforming applications compiled with the c99 compiler driver, a domain error occurs.
These functions will fail if:
Domain Error
The x argument is zero, NaN, or \(+-Inf. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception is raised.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
feclearexcept(3M), fetestexcept(3M), limits.h(3HEAD), logb(3M), math.h(3HEAD), scalb(3M), attributes(5), standards(5)