Compute the radix-independent exponent
#include <math.h> double logb ( double x ); float logbf ( float x ); long double logbl ( long double x );
Your system requirements will determine how you should work with these libraries:
The logb(), logbf(), and logbl() functions compute the exponent part of x, which is the integral part of:
log_{r} |x|
as a signed floating point value, for nonzero finite x, where r is the radix of the machine's floating point arithmetic.
To check for error situations, use feclearexcept() and fetestexcept(). For example:
The binary exponent of x, a signed integer converted to double-precision floating-point.
If x is: | These functions return: | Errors: |
---|---|---|
x is ±0.0 | -Inf | FE_DIVBYZERO |
±Inf | Inf | — |
NaN | NaN | — |
These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.
#include <stdio.h> #include <inttypes.h> #include <math.h> #include <fenv.h> #include <stdlib.h> int main( void ) { int except_flags; double a, b; a = 0.5; feclearexcept(FE_ALL_EXCEPT); b = logb(a); except_flags = fetestexcept(FE_ALL_EXCEPT); if(except_flags) { /* An error occurred; handle it appropriately. */ } printf("logb(%f) = %f (%f = 2^%f) \n", a, b, a, b); return EXIT_SUCCESS; }
produces the output:
logb(0.500000) = -1.000000 (0.500000 = 2^-1.000000)
Safety: | |
---|---|
Cancellation point | No |
Interrupt handler | Yes |
Signal handler | Yes |
Thread | Yes |