[Segmentation Fault] Oops! The filename points to a constant string

We can compile the following program using the gcc compiler. 

But when running it, we get a segmentation fault.

Why ?

#include <stdio.h>
int main()
{
// The following statement is equal to " constant char* filename = "file1.txt"; "in c++
char* filename = "file1.txt"; // Initialization and after that filename points to a constant string.
printf("\nPlease input for the filename ==> ");
scanf("%s", filename); // Try to modify a constant string. It will cause a segmentation fault !
printf("\nThe file name you have input is %s", filename);
return 0;
}