Is it OK to access past the size of a structure via member address, with enough space allocated?

Question

Specifically, is the following code, the line below the marker, OK?

struct S{
    int a;
};

#include 

int main(){
    struct S *p;
    p =          


        

Answer 1

This is undefined behavior, as you are accessing something that is not an array (int a within struct S) as an array, and out of bounds at that.

The correct way to achieve what you want, is to use an array without a size as the last struct member:

#include 

typedef struct S {
    int foo;    //avoid flexible array being the only member
    int a[];
} S;

int main(){
    S *p = malloc(sizeof(*p) + 2*sizeof(int));
    p->a[0] = 0;
    p->a[1] = 42;    //Perfectly legal.
}