Why is this snippet giving 6 as output?

Question

 int main(){    
       char s[1000];    
       s[0]='Q';    
       printf("%d",strlen(s));    
       return 0;    
    }

The above code is giving 6 as output. The output should be 1 in this case.

int main(){    
       char s[1000];     
       printf("%d",strlen(s));    
       return 0;    
}

This code was even giving 6 as output. where it should be 1000 or 0 or some garbage value. I have tried it in " hackerrank ", but it was giving 6 as the length. When the same code was tried in TURBOC it was giving the correct answer. I was unable to find the reason for this output. Can someone please sort it out and give the exact solution to this problem.

did you append `\0` to end of string? – Suraj Rao May 23 '17 at 10:59 — Suraj Rao, May 23 '17 at 10:59
You should look up the docs for strlen(). man strlen. – ThingyWotsit May 23 '17 at 10:59 — ThingyWotsit, May 23 '17 at 10:59
'6' IS a garbage value:) – ThingyWotsit May 23 '17 at 11:00 — ThingyWotsit, May 23 '17 at 11:00

Some programmer dude · Answer 1 · 2017-05-23T11:02:37.787

3

Local non-static variables, like your array, will not be initialized. Its value or contents will be indeterminate.

That means the likelihood of your array containing a string terminator where you want it to be is close to zero.

Passing your array without initialization to a string-function leads to undefined behavior.

You need to explicitly initialize your array, like e.g.

char s[1000] = "";

edited May 23 '17 at 11:02

answered May 23 '17 at 11:01

Some programmer dude

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1

You could also use `char s[1000] = {0};` to fill it with `'\0'`s. – May 23 '17 at 11:02
1

@InternetAussie My initialization is equivalent to yours. – Some programmer dude May 23 '17 at 11:03
1

Oh, true. Oops. Still an alternative :p – May 23 '17 at 11:09

score 2 · Answer 2 · answered May 23 '17 at 11:08

The line:

char s[1000];

declares an array of 1000 chars but it doesn't put anything in it. Because it is a local variable, it is allocated on the stack and, apart from reserving the necessary space to hold 1000 chars, the compiler doesn't generate any code to set their content (to zero or something else).

All 1000 values are garbage. It just happens that the 7^th of them is zero. strlen() stops counting when it reaches it and returns 6 but this value is also garbage. It could be 230790345 or something else as well. It could also produce an infinite loop, a segmentation fault or make you spit fire like a dragon. Anything is possible and permitted by the language in this situation. It is called Undefined Behaviour.

Why is this snippet giving 6 as output?

2 Answers2