How can a character array be subtracted from a pointer?

Question

So, I am starting to familiarize with C, and at this point I'm trying to understand pointers. I got the following code from here, but I cannot comprehend, how does one subtract a character array from a pointer.

#include<stdio.h>
#include<string.h>
#include<conio.h>

main() 
{   
char s[30], t[20];   
char *found; 

/* Entering the main string */   
puts("Enter the first string: ");   
gets(s);

/* Entering the string whose position or index to be displayed */   
puts("Enter the string to be searched: ");   
gets(t);

/*Searching string t in string s */   
found=strstr(s,t);   
if(found)
    printf("Second String is found in the First String at %d position.\n",found-s);    
else
    printf("-1");   
getch(); 
}

Isn't the pointer only the address of a given variable/constant? When the subtraction happens the character array automatically assumes that since the operation happens with a pointer is subtracts its address? I am a bit confused here.

Thanks in advance.

Answer 1

Assuming you're wondering about the expression found-s, then what's happening is that you subtract two pointers.

Arrays naturally decay to pointers to their first element. That means plain s is equal to &s[0], which is what's happening here: found-s is equal to found - (&s[0]).

And the subtraction works because found is pointing to an element inside the array s, so the pointers are related (which is a requirement for pointer subtraction). The result is the difference (in elements) between the two pointers.