The **`logbl` function** is defined in the `<tgmath.h>` header file in C. It takes in a single parameter `x` of type `long double`. It then computes the logarithm of $|x|$ using `FLT_RADIX` as the base and returns the result of the `long double` type.

> `FLT_RADIX` has a value of 2 in most platforms. Thus, the function performs similar to log<sub>2</sub> for positive values.

The illustration below shows how the `logbl` function works:

# Declaration

The `logbl` function is defined as follows:

```C
long double logbl(long double x);
```

# Parameters

The `logbl` function takes a single value of type `long double` as a parameter. 

# Return value

The `logbl` function returns the computation of type `long double`.

# Error handling

The `logbl` function returns special values for certain arguments:

| x  | Return Value  |
| - | - |
| 0 | `-INFINITY`  |
|`INFINITY`| `+INFINITY`|
|`NaN`|`NaN`|

# Example

The following code snippet shows how we can use the `logbl` function:





#include <stdio.h> // include header for printf
#include <tgmath.h>  // include header for logbl

int main ()
{
  long double x, result;
  x = 8.0;
  result = logbl(x);
  printf("logbl (%Lf) = %Lf.\n", x, result);
  return 0;
}

The following code snippet shows how error handling in the `logbl` function works:

#include <stdio.h> // include header for printf
#include <tgmath.h>  // include header for logbl

int main ()
{
  long double x, y, result;
  x = 0.0;
  result = logbl(x);
  printf("logbl (%Lf) = %Lf.\n", x, result);

  y = INFINITY;
  result = logbl(y);
  printf("logbl (%Lf) = %Lf.\n", y, result);
  return 0;
}

What is logbl in C?

Computes the logarithm of the absolute value using FLT_RADIX (usually 2), returns a long double.

x	Return Value
0	`-INFINITY`
`INFINITY`	`+INFINITY`
`NaN`	`NaN`