Question

how are you?

int stack_empty(stack *s){
         return (s == NULL); /* I dont get this part, it returns what if its null? */
}

int main(){
         stack *s;
         if(stack_empty(s)){ /* what it means? like... whats the standard return of a function? */
               printf("its empty");
         }
         return 0;
}

My questions are in the comments of the code. Briefly they are:
-> Whats the standard return of a function?
-> What does return something == NULL means?

*I know what NULL, s or == means… my questions lies on those abreviated expressions.

Answer 1

== is a test for equality.

If s is null then s == null is true the expression has a non zero value, so stack_empty will return non zero (probably 1). If s is not null, then s == null is false so the method will return 0.

The if statement is effectively saying if expression is not 0.