C char array overflow, okay practice?

Question

I'm going through the K & R book and the answer to one of the exercises is troubling me.

In the solutions manual, exercise 1-22 declares a char array:

#define MAXCOL 10
char line[MAXCOL];

so my understanding is that in C arrays go from 0 ... n-1. If that's the case then the above declaration should allocate memory for a char array of length 10 starting with 0 and ending with 9. More to the point line[10] is out of bounds according to my understanding? A function in the sample program is eventually passed a integer value pos that is equal to 10 and the following comparison takes place:

int findblnk(int pos) {
while(pos > 0 && line[pos] != ' ')
        --pos;
    if (pos == 0)                   //no blanks in line ?
        return MAXCOL;
    else                        //at least one blank
        return pos+1;               //position after blank
}

If pos is 10 and line[] is only of length 10, then isn't line[pos] out of bounds for the array?

Is it okay to make comparisons this way in C, or could this potentially lead to a segmentation fault? I am sure the solutions manual is right this just really confused me. Also I can post the entire program if necessary. Thanks!

Thanks for the speedy and very helpful responses, I guess it is definitely a bug then. It is called through the following branch:

else if (++pos >= MAXCOL) {
            pos = findblnk(pos);
            printl(pos);
            pos = newpos(pos);
        }

MAXCOL is defined as 10 as stated above. So for this branch findblnk(pos) pos would be passed 10 as a minimum.

Do you think the solution manual for K & R is worth going through or is it known for having buggy code examples?

Answer 1

If pos is indeed 10 then it would be an out of bounds access and accessing an array out of bounds is undefined behavior and therefore anything can happen even a program that appears to work properly at the time, the results are unreliable. The draft C99 standard Annex J.2 undefined behavior contains the follows bullet:

An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).

I don't have a copy of K&R handy but the errata does not list anything for this problem. My best guess is the condition should < instead of >=.

Answer 2

It is never, ever okay to over-run the bounds of an array in C. (Or any language really).

If 10 is really passed to that function, that is certainly a bug. While there are better ways of doing it, that function should at least verify that pos is within the bounds of line before attempting to use it as an index.