The **`erfl` function** is defined in the `<tgmath.h>` header file in C. It takes in a single parameter: a `long double` value. It then computes the error function of the given argument and returns a value of type `long double`. 

The #key# error function: also known as Gauss error function  #key# in mathematics is defined as follows:

> erf $z$ = $\frac{2}{\sqrt{\pi}}$ $\int_{0}^{arg} e^{-t^2} dt$ 

> The argument to the `erfl` function serves as the upper limit in the integral above.

The error function is often used in probability and statistics. It integrates the normal distribution and gives the probability that a normally distributed random variable Y (with mean 0 and variance ½) falls into the range [−x, x].

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

# Declaration

The `erfl` function is defined as follows:

```C
long double erfl(long double arg);
```
It takes in a single value of type `long double` and computes its error function. It then returns the answer of type `long double`.

# Error handling

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

* If the argument is +-0, +-0 is returned
* If the argument is +-$\infty$, +- $\infty$ is returned
* If the argument is `NAN`, `NAN` is returned

# Examples

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

#include <stdio.h> // Including header file for printf function
#include <math.h>  // Including header file for erf function

int main (){

  long double param, result;
  param = 2.0;
  result = erfl(param);
  printf("erfl (%Lf) = %Lf\n", param, result);
  return 0;
}

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

#include <stdio.h> // Including header file for printf function
#include <math.h>  // Including header file for erf function

int main (){
  long double param = 0.0;
  printf("erfl (%Lf) = %Lf\n", param, erfl(0.0));
  return 0;
}

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What is erfl in C?

Computes the erfl error function for a long double value in the <tgmath.h> header, often used in probability.