what does strlen do if it encounter a space in between char array?

Question

#include<iostream>
#include<string.h>

using namespace std;

int main() {
    char b[] = {'a','b',' ','c'};
    cout << sizeof(b) << endl;
    cout << strlen(b) << endl;
    return 0;
}

Why the above output is 4,6 isnt that 4,4 is the correct answer?

Bathsheba · Answer 1 · 2018-03-14T11:41:08.263

5

The behaviour of your code is undefined: strlen requires that b must contain a NUL character within the bounds of the array.

(Note that ' ' is a space, not NUL. A space is processed no differently by strlen to any other non-NUL character.)

If you had written

char b[5]={'a','b',' ','c'};

then C++ would have set the final element of b to 0 (i.e. NUL) for you.

edited Mar 14 '18 at 11:41

answered Mar 14 '18 at 11:21

Bathsheba

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Alternatively (for not having to specify array size): `char b[] = { 'a', 'b', ' ', 'c', 0 };` – Aconcagua Dec 07 '22 at 13:22

score 1 · Answer 2 · 2018-03-14T11:41:04.907

1

Your code exhibits undefined behavior:

The behavior of strlen is not defined if the argument passed to it is not NUL-terminated.

You seem to confuse the NUL character with the space character.

edited Mar 14 '18 at 11:41

answered Mar 14 '18 at 11:23

1

`hex 20` - Unfounded assumption if one is to be a pedant – StoryTeller - Unslander Monica Mar 14 '18 at 11:24
Feel free to take my wording which I prefer: the argument doesn't need to be NUL-terminated, it just needs to have NUl somewhere in it, – Bathsheba Mar 14 '18 at 11:24
@StoryTeller: Pedantic? You'll have the mainframe dudes after you. – Bathsheba Mar 14 '18 at 11:24
hex 20 in ascii/utf-8 (and a bunch other) indeed. Will edit. – Mar 14 '18 at 11:26
1

@Bathsheba - I'm trying to save Nax here from their wrath :) – StoryTeller - Unslander Monica Mar 14 '18 at 11:26
1

@Nax: Note that ASCII is *very, very* different from UTF-8. – Bathsheba Mar 14 '18 at 11:27
And ASCII is also a *possible* character set. Not *the* character set. No matter how common it is. – StoryTeller - Unslander Monica Mar 14 '18 at 11:28
1

@StoryTeller: My father has already left his house, and has just dumped the clutch in his 911 at 6000 revs. He's off somewhere in a hurry. Nax, run for the hills. – Bathsheba Mar 14 '18 at 11:28
@Bathsheba: Indeed, but the difference is not relevant for the 128 first characters of both charsets. – Mar 14 '18 at 11:28
1

@Nax - It's perfectly valid for a C implementation to use something like [EBCDIC](https://en.wikipedia.org/wiki/EBCDIC) (what Bathsheba was alluding to). I suggest you remove the hex mention, it really is detracting from your answer. – StoryTeller - Unslander Monica Mar 14 '18 at 11:31
@StoryTeller: I know that, I was responding to their comment about the difference between ASCII and UTF-8. The hex mention was supposed to help OP figure out that the space and NUL characters have different values, but it's making us dive into the topic of character sets, which was not OP's question. I will remove it. – Mar 14 '18 at 11:40
Stand down please daddy. – Bathsheba Mar 14 '18 at 11:45

score 1 · Answer 3 · answered Mar 14 '18 at 11:40

what does strlen do if it enounter a space in between char array?

While all the other answers (according to your code sample, legitimately!!!) hint to the undefined behaviour due to the missing null character within your array, I'll be answering your original question:

Nothing special, it will count the space just as any other character (especially, it won't stop processing the string as you seem to have assumed)...

score 0 · Answer 4 · answered Mar 14 '18 at 11:24

0

strlen traverses the array until it encounters a NULL character. Your array does not contain NULL characters (ASCII 0) but it contains a space (ASCII 32).

So strlen continues past your array. It happens to exist a NULL character at position 6 (two characters after the end of the array in memory), so string length is 6. Subsequent executions of this function could return different values, as memory content outside the array is undefined. It may also crash.

answered Mar 14 '18 at 11:24

Mario Cianciolo

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why it happens to exist that null character ,two characters after end of array – T.g Mar 14 '18 at 11:43
@T.g Just by accident. Your array occupies a bunch of bytes in memory. As on the stack, it is quite likely that there are some other (uninitialised!!!) bytes accessible (again: pure accident! - you could have quite well run into an access violation instead), and one of these bytes (the 6th one) "happened" to contain a null value, but it could have contained any other value as well... – Aconcagua Mar 14 '18 at 11:55
@T.g data after the array is unitialized (read: random) – Mario Cianciolo Mar 14 '18 at 12:26

what does strlen do if it encounter a space in between char array?

4 Answers4