C loop not breaking

Question

I'm trying to understand why the following program acts in such a weird way. I declare an array of chars, and input chars into it, with a while loop and scanf. However when I input letters, or digits, it runs seemingly forever. If I input a large number, or a string, it stops. Why doesn't it exit the loop after 5 iterations?

// This program runs forever if we input single-digit numbers
#include <stdio.h>

int main() 
{
    char u[5] = {0,};
    for (int i = 0; i<5; i++) {
        scanf(" %s", &u[i]);
    }
    printf("%s\n", u);
}

@SouravGhosh I haven't tested, but I input a digit, and then it asks for another input, so on and so forth for many many times. way more than 5 — blz, Nov 02 '17 at 07:35
First of all, `%s` matches a sequence of non-whitespace characters. This code overflows your array. — , Nov 02 '17 at 07:35
How are you entering your single-digit numbers? White space separated or newline separated? — Ajay Brahmakshatriya, Nov 02 '17 at 07:36
@FelixPalmen could you please explain a little more what I did wrong? — blz, Nov 02 '17 at 07:36
@AjayBrahmakshatriya I press enter after every single digit I input — blz, Nov 02 '17 at 07:37
The last (5th) digit you enter also writes an extra `\0` beyond the array bounds which is Undefined Behaviour. — Ajay Brahmakshatriya, Nov 02 '17 at 07:38
You can't expect multiple characters to fit into a single one? Read a `scanf()` manual. And better, don't use `scanf()` at all. — , Nov 02 '17 at 07:38
@SouravGhosh of course I first went to cplusplus.com, but I couldn't understand all of it, so that's why I've come here for a explanation in English — blz, Nov 02 '17 at 07:39
Search for `man scanf`. Learn about strings and arrays in C. Then read [this FAQ](https://stackoverflow.com/q/35178520/2371524). Then read my [beginners' guide away from `scanf()`](http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html). — , Nov 02 '17 at 07:43

score 4 · Accepted Answer · answered Nov 02 '17 at 07:55

This is broken beyond repair.

scanf("%s", ...) is always a severe bug, because you can't know your input in advance, and %s matches any sequence of non-whitespace characters, of any length -- there's no way to know how large your buffer must be.

%s writes the characters it reads starting at the address you pass as argument. It writes an additional 0 byte as the end mark of a string. So with your code, even if you only enter single characters, the last iteration writes this 0 byte at u[5] which is out of bounds, you overflowed your buffer.

You can't even fix it by just changing %s to %c (which matches a single character) because you don't add a 0 byte to the end of your array, so the array content is not a string and passing it to printf("%s", ...) is again undefined behavior.

Start over, read a good book on C first, and when you're done, best forget about scanf() and use better methods for input like fgets(). See also How to read / parse input in C? The FAQ and my beginners' guide away from scanf().

But why doesn’t it break from the for loop after 5 iterations ? — blz, Nov 02 '17 at 09:54
@blz because undefined behavior is undefined. Anything could happen when you e.g. overflow a buffer. — , Nov 02 '17 at 09:55

C loop not breaking

1 Answers1