LOGB(3M) Mathematical Library Functions LOGB(3M)

# NAME

logb, logbf, logbl - radix-independent exponent

# SYNOPSIS

```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);`

# DESCRIPTION

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.

# 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(7)), 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.

# ERRORS

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.

# USAGE

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.

# ATTRIBUTES

See attributes(7) for descriptions of the following attributes:

 ATTRIBUTE TYPE ATTRIBUTE VALUE Interface Stability Standard MT-Level MT-Safe