The **`erfcl` 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 *complementary error function* of the given argument and returns a type `long double` value. 

The complementary error function in mathematics is defined as follows:

> erfc $z$ = $1- \frac{2}{\sqrt{\pi}}$ $\int_{0}^{arg} e^{-t^2} dt$ 

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

> The argument to the `erfcl` function serves as the lower limit in the integral above.

The error function is most 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 complementary error function, combined with its series expansion, provides approximate expressions for small and large values of x.

> The `erfc` function in mathematics caters to values of all types. There is a unique function for each data type in C.


The figure below shows the plot of the `erfc` function:

The illustration below shows how `erfcl` function works:

# Declaration

The `erfcl` function is defined as follows:

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

# Error handling

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

|  Special Value | Returned   |
| - | - |
| +INFINITY | +0  |
| -INFINITY | +2  |
| `NAN`| `NAN`|


# Example

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


#include <stdio.h> // Including header file for printf function
#include <tgmath.h>  // Including header file for erfcl function

int main (){

  long double param, result;
  param = 1.5;
  result = erfcl(param);
  printf("erfcl (%Lf) = %Lf\n", param, result);
  return 0;
}

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What is erfcl in C?

The erfcl function in C, defined in tgmath.h, computes the complementary error function for a long double value and returns the result as a long double.