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POneed for the last '\0' in fgets
 

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  1. POneed for the last '\0' in fgets
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    <p>I've seen several usage of <code>fgets</code> (for example, <a href="https://stackoverflow.com/a/7672607/1317875">here</a>) that go like this:</p> <pre><code>char buff[7]=""; </code></pre> <p>(...)</p> <pre><code>fgets(buff, sizeof(buff), stdin); </code></pre> <p>The interest being that, if I supply a long input like "aaaaaaaaaaa", <code>fgets</code> will truncate it to "aaaaaa" here, because the 7th character will be used to store <code>'\0'</code>.</p> <p>However, when doing this:</p> <pre><code>int i=0; for (i=0;i&lt;7;i++) { buff[i]='a'; } printf("%s\n",buff); </code></pre> <p>I will always get 7 <code>'a'</code>s, and the program will not crash. But if I try to write 8 <code>'a'</code>s, it will. </p> <p>As I saw it later, the reason for this is that, at least on my system, when I allocate <code>char buff[7]</code> (with or without <code>=""</code>), the 8th byte (counting from 1, not from 0) gets set to 0. From what I guess, things are done like this precisely so that a <code>for</code> loop with 7 writes, followed by a string formatted read, could succeed, whether the last character to be written was <code>'\0'</code> or not, and thus avoiding the need for the programmer to set the last '\0' himself, when writing chars individually.</p> <p>From this, it follows that in the case of</p> <pre><code>fgets(buff, sizeof(buff), stdin); </code></pre> <p>and then providing a too long input, the resulting <code>buff</code>string will automatically have two <code>'\0'</code> characters, one inside the array, and one right after it that was written by the system.</p> <p>I have also observed that doing</p> <pre><code>fgets(buff,(sizeof(buff)+17),stdin); </code></pre> <p>will still work, and output a very long string, without crashing. From what I guessed, this is because <code>fgets</code> will keep writing until <code>sizeof(buff)+17</code>, and the last char to be written will precisely be a <code>'\0'</code>, ensuring that any forthcoming string reading process would terminate properly (although the memory is messed up anyway).</p> <p>But then, what about <code>fgets(buff, (sizeof(buff)+1),stdin);</code>? this would use up all the space that was rightfully allocated in <code>buff</code>, and then write a <code>'\0'</code> right after it, thus overwriting...the <code>'\0'</code> previously written by the system. In other words, yes, <code>fgets</code> would go out of bounds, but it can be proven that when adding only one to the length of the write, the program will never crash.</p> <p>So in the end, here comes the question: why does <code>fgets</code> always terminates its write with a <code>'\0'</code>, when another <code>'\0'</code>, placed by the system right after the array, already exists? why not do like in the one by one <code>for</code>-loop based write, that can access the whole of the array and write anything the programmer wants, without endangering anything?</p> <p>Thank you very much for your answer!</p> <p>EDIT: indeed, there is no proof possible, as long as I do not know whether this 8th <code>'\0'</code> that mysteriously appears upon allocation of buff[7], is part of the C standard or not, specifically for string arrays. If not, then...it's just luck that it works :-)</p>
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    1. COBe careful thinking that because something doesn't crash it means it's actually correct; usually errors like this result in "undefined behavior". Sometimes you'll get a segfault, sometimes you won't. If you have buff[7], there's no guarantee that the 8th byte will be a \0, it could be anything.
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    2. CO"I will always get 7 'a's, and the program will not crash" - That you expect it *should/could* crash at least suggests you understand *undefined behavior*. Regarding your question, because that is how [`fgets`](http://en.cppreference.com/w/c/io/fgets) is required to behave. If you have `char a;` and pass `&a` with some arbitrary size greater than 1 would you *expect* anything *definitive* ? Its a C-api, and like most, either useful or as undefined in its behavior, depending on how *you* call it.
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    3. COI understand that any testing on my single machine will never prove anything. I was just thinking of the string viewed as an array, where you write anything you want without thinking about what the last cell would contain (like you would in an `int[]`), and then thought of as a string, i.e. as a word, with the omnipresent fear of the missing terminating character. Because of that, the standard may have included this 8th '\0' as a hard-set parameter. As I don't know the details of the standard...I was asking the question: is this eighth `'\0'` part of the C standard?
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