C Preprocessor Question:

General form of #line preprocessor is #line number "filename"
Here the file name is optional. Filename string replaces the string value of _ _FILE_ _ while the number changes the value of _ _LINE_ _.
The major use of #line is in debugging and rare programming situation.
Following C Source code shows the #line preprocessor in action -
#include <stdio.h>
int main ()
{
printf ("n%d", __LINE__); //Prints 6
#line 100;
printf ("n%d",__LINE__); // Prints 101
printf ("n%d", __FILE__);// Prints original source file name
#line 103 "Super C"
printf ("n%d", __FILE__); //Prints Super C
return 0;
}</stdio.h>