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c99 [ flag... ] file... -lm [ library... ] #include <math.h> double logb(double x);
float logbf(float x);
long double logbl(long double x);
cc [ flag... ] file... -lm [ library... ] #include <math.h> double logb(double x);
float logbf(float x);
long double logbl(long double x);
These functions compute the exponent of x, which is the integral part of log(r) |x|, as a signed floating point 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 the <float.h> header.
Upon successful completion, these functions return the exponent of x.
If x is subnormal:
For SUSv3-conforming applications compiled with the c99 compiler driver (see standards(5)), the exponent of x as if x were normalized is returned.
Otherwise, if compiled with the cc compiler driver, -1022, -126, and -16382 are returned for logb(), logbf(), and logbl(), respectively.
If x is \(+-0, a pole error occurs and logb(), logbf(), and logbl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
If x is NaN, a NaN is returned.
If x is \(+-Inf, +Inf is returned.
These functions will fail if:
Pole Error
The value of x is \(+-0. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised. The logb() function sets errno to EDOM if the value of x is 0.
An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.
An application can also set errno to 0 before calling logb(). On return, if errno is non-zero, an error has occurred. The logbf() and logbl() functions do not set errno.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
feclearexcept(3M), fetestexcept(3M), ilogb(3M), math.h(3HEAD), matherr(3M), scalb(3M), attributes(5), standards(5)