Why does my feof loop run infinitely, even though the file does end?

Question

So I want to count the lines of the file I have as the argument of the function, but I don't know why does this loop run indefinitely.

int count_numbers(FILE *filea) {
    int i;
    while (!feof(filea)) {
        i++;
    }
    fclose(filea);
    i--;
    return i;
}

`feof` doesn't take in anything new. If the file descriptor wasn't already marked as being at the end of file, it won't suddenly be EOF in the loop. — Thomas Jager, Mar 08 '21 at 20:58
See [Why is `while(!feof(fp))` always wrong?](https://stackoverflow.com/questions/5431941) — user3386109, Mar 08 '21 at 21:16
Why hasn't this been closed as a duplicate in under two minutes? [Why is “while ( !feof (file) )” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Marco Bonelli, Mar 08 '21 at 22:11

chqrlie · Answer 1 · 2021-03-09T12:58:44.223

3

Your function does not count the lines at all and it has multiple problems:

i is not initialized
while (!feof(filea)) is always wrong
you do not test if the byte read is a newline
closing the file should be the caller's responsibility

Here is a modified version:

int count_lines(FILE *filea) {
    int c;
    int last = '\n';
    int lines = 0;

    while ((c = getc(filea)) != EOF) {
        if (c == '\n')
            lines++;
        last = c;
    }
    if (last != '\n') {
        // last line does not have a trailing newline, adjust the count
        lines++;
    }
    return lines;
}

edited Mar 09 '21 at 12:58

answered Mar 08 '21 at 21:14

chqrlie

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How does this function use `filea`? – user1717828 Mar 09 '21 at 12:54
@user1717828: oops! answer updated. – chqrlie Mar 09 '21 at 12:59

score 0 · Answer 2 · answered Mar 08 '21 at 20:58

0

Nothing in the loop can possibly change the file's EOF flag. All that's in the loop is i++; which cannot cause the end of the file to be encountered. So the loop will loop an infinite number of times if it loops once.

answered Mar 08 '21 at 20:58

David Schwartz

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Meta-question about your answer: I noticed you started with one-sentence answer and then started updating it to fill out the sentence with more detail. I also notice you have a really high reputation and probably know a thing about getting reputation on SO. I wonder: is the idea to get something in with as early a timestamp as possible to dissuade other answerers from starting a response, or is there another reason to iterate from a stub answer like this? – user1717828 Mar 08 '21 at 21:02
@user1717828: This question is likely to be closed very quickly. David might have tried and succeeded at posting an answer before this... But I am only guessing. – chqrlie Mar 08 '21 at 21:12
@user1717828 I think it's a combination of two things. Part of it is that I always think of something I should have said right after it's too late but I think another is having the painful experience of composing a long, thoughtful answer to a good question only to have the question closed or deleted. Also, a close as a duplicate often doesn't help the OP while an answer that's "mostly" a duplicate does. – David Schwartz Mar 08 '21 at 21:24

Why does my feof loop run infinitely, even though the file does end?

2 Answers2