Question on some 'borrowed' code

I have a question regarding the following lines of code

Code:

---

#include <math.h>
struct st {
    double x[1];
    double (*func)(double);


};


int main (void) {
    struct st s;
    s.func = sqrt;
    s.x[0] = 1.0;
    s.x[1] = 33.7;  /* bad */
    (void)s.func(s.x[0]);
    return  0;


}

---

How can assigning 33.7 to s.x[1] possibly overwrite the function
pointer? I thought assigning s.x[1]=33.7 was legal because it was within the array boundries.

Quote:

---

Originally Posted by cdalten

I have a question regarding the following lines of code

Code:

---

#include <math.h>
struct st {
    double x[1]; // your making an array of size 1, indexed from 0 to 0
    double (*func)(double);


};


int main (void) {
    struct st s;
    s.func = sqrt;
    s.x[0] = 1.0;
    s.x[1] = 33.7;  /* bad */
    (void)s.func(s.x[0]);
    return  0;


}

---

How can assigning 33.7 to s.x[1] possibly overwrite the function
pointer? I thought assigning s.x[1]=33.7 was legal because it was within the array boundries.

---

Look at the red.

Okay, I see. Once again I'm demonstrating my lack of any real mastery over the C language. Ugghhh...

Quote:

---

Originally Posted by cdalten

Okay, I see. Once again I'm demonstrating my lack of any real mastery over the C language. Ugghhh...

---

Actually that's a very common problem in the C world, so don't feel bad.

I've been programming C for more than 20 years now, and, on occasion, still make mistakes like that. Now granted, it is usually when modifying code that I've previous written and I forget to increase the size of the array, but it is still an array sizing problem.

The hard part is that when you have large amounts of code and they start failing with the most bizarre errors -- that undersized array can sometimes be very hard to find....