Question

I was trying to make an array-based linear list, then I compiled this:

char size = 0;
char testarray[10];

int main() { 
    add('a'); add('b'); add('c');
    add('d'); add('e'); add('f');
    add('g'); add('h'); add('i');
    add('j'); add('k'); add('l');
    add('m'); add('n'); add('o');
    print();
    return 0;
} 

void add(char newchar) {
    testarray[++size] = newchar;
}

void print() {
    char i = 0;
    for (i = 0; i <= size; i++) {
        printf("%c ", testarray[i]);
    }
 }

And compiled it with gcc arraytest.c but the array works just fine. Does that mean arrays are variable-length by default? I thought it was a C99-only feature.

It was compiled under both Gentoo (gcc version 4.5.3 (Gentoo 4.5.3-r2 p1.1, pie-0.4.7) and Ubuntu (gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3).

Oh, and isn’t this a bit dangerous?

Answer 1

So long as you are within your allocated range for your application you can use pointers/address wherever you want. keep going far enough and you either trash your program or hit the edge of your allocated memory and get a protection fault of some sort. Runtime checking is expensive, wouldnt want to have that anyway.

swap your two variables

char testarray[10];
char size = 0;

and see what happens when you run it…

And then do this:

char size = 0;
char testarray[10];
char stuff[10];

before you start adding things to testarray, initialize stuff, then after doing your thing print out the stuff[] array. You should see your overflow. In C it is a good rule to put your non-array variables first in the assignment list and the array or arrays last, you have a better chance at debugging.