**`wcstol()`** is a built-in function defined in the `<wchar.h>` header in C.

Here is the function protype:
```C
long wcstol( const wchar_t * str, wchar_t ** str_end, int base );
```

# Functionality

`wcstol()` is used to interpret a *long integer* value from a *wide character string*. 

# Parameters and return value

The function takes the following as input parameters:

1. `str:` Pointer to a wide character string

2. `str_end:` Double pointer to a wide character which is set to the next valid character after extracting the integer

3. `base:` Base of the extracted integer

`wcstol()` returns the corresponding integer value as interpreted from the string and converts it to the required base.

# Code

#include <wchar.h>
#include <stddef.h>
#include <stdio.h>

int main(){

  // initialize the string
  wchar_t str[] = L"21 Gun Salute!";
  
  // initialize the base
  
  wchar_t* end_str;
  
  // print the long integer value with the end string
  // with base 10
  long value = wcstol(str, &end_str, 10);
  printf("String: %S\n", str);
  printf("Integer value: %d\n", value);
  printf("End String = %S\n", end_str);
  
  return 0;
}

We begin by initializing a `wchar_t` string, `str`, that contains an integer value. To declare a wide-character string, we prefix the string with the letter ‘L’.

Next, we declare an `end_str` pointer. The `end_str` pointer is made to point to the next valid character after extracting the long integer. In the example above, this character was *space*.

>The long integer value returned by `wcstol()` is stored in the `long` variable, `value`

What is wcstol() in C?

wcstol() interprets a long integer from a wide character string, returning the integer as per the specified base.