what is meant by 'Most C system provide for logically infinite floating values'?

Question

Initially I declared variables x and y as type int:

#include<stdio.h>   

     int main(){
        int x, y = 0 ;
        x = 1 / y;
        printf("%d", x);

        return 0;    
    }

Program crashed (for obvious reasons).

Now I declared variables x and y as double:

#include<stdio.h>   

 int main(){
    double x, y = 0 ;
    x = 1 / y;
    printf("%d", x);

    return 0;    
}

But Output: 0. (why?)

Then I changed %d to %f in printf:

#include<stdio.h>   

 int main(){
    double x, y = 0 ;
    x = 1 / y;
    printf("%f", x);

    return 0;    
}

Output: 1.#INF00

I don't understand what is happening here.

Please explain me above cases.

Answer 1

Most systems you're likely to come in contact with use IEEE 754 representation for floating point numbers. That representation has ways to store the values +infinity and -infinity.

While strictly speaking, dividing by 0 is undefined behavior, implementations using IEEE 754 extend the language to allow it for floating point types. In this case, dividing by 0 can be considered infinity, so your implementation allows it, and 1.#INF00 is how MSVC prints the value for infinity.

Also, using the wrong format specifier for printing as in your second example where you use %d to print a double is undefined behavior. Format specifiers must match the datatype of what is passed in.