man pages section 3: Extended Library Functions, Volume 2

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Updated: July 2014



logb, logbf, logbl - radix-independent exponent


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 logr |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.

Return Values

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:

Interface Stability

See also

feclearexcept(3M), fetestexcept(3M), ilogb(3M), math.h(3HEAD), matherr(3M), scalb(3M), attributes (5), standards(5)