Does anyone think there is something wrong in The C Programming Language 2E Page29?

Question

I am currently studying the well-known book for C - The C Programming Language, 2Ed. And when I trying the code in P.29, I think there is something wrong in the getline function:

int getline(char s[], int lim) {
    int c, i;

    for (i=0; i<lim-1 && (c=getchar()) != EOF && c!='\n'; i++)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        i++;
    }

    s[i] = '\0';
    return i;
}

What if when the for loop ended, i == lim-1 and c == '\n'? In this case, I think the array would be out of boundary, since s[lim] would be set to '\0'.

Does anyone think this is wrong? Thanks for your help.

Answer 1

The && operator has "early-out" semantics. This means that if i == lim-1, the rest of the condition is not executed - in particular, c = getchar() will not be called.

This means that in this case, c will have its value from the last iteration of the loop - and since the loop condition includes c != '\n', this value can't be '\n' (or the loop would have exited last time around).

This is true as long as lim is greater than 1, which must be a precondition of the function (because calling the function with lim less than or equal to 1 would cause the uninitialised value of c to be read).

Answer 2

So, let's look at some cases:

If lim == 0:, then this will do undefined behavior. There's two places this will happen:

• We will execute no iterations of the for loop, giving i == 0 and c == undefined.
• We then access c at (c == '\n'). It has no defined value yet, so it's undefined behavior.
• We then cause undefined behavior again by overflowing s with: s[i] = '\0';

What if lim == 1:

• The for loop will not be run, because the condition is not met.
• We will hit undefined behavior just like in lim == 0 because c has no value.
• The last line will work fine.

What if lim == 2, and the input string is "ab":

• The for loop will grab 'a', and place it into s.
• The for loop will exit on the next iteration, with the value of c still being 'a'.
• The if conditional fails.
• The adding of the null character works fine.
• So s == "a\0"

What if lim == 2 and the input string is "a\n" (Which is the case you're worried about):

• The for loop will grab 'a', and place it into s.
• The for loop will exit on the next iteration, with the value of c still being 'a'.
• The if conditional fails.
• The adding of the null character works fine.
• So s == "a\0"