C - Comparison of Arrays with == works, why?

Question

I'm a little bit confused. I have the following function:

int comp(char s1[], char s2[]) {
   return s1 == s2;
}

As far as I know this compares only the addresses of the first elements from char array s1 and char array s2.

But strange is if I compare (in Visual Studio) two equal char arrays like

 comp("test","test");

I got 1 (true) instead of 0 (false). But should the addresses not be different and therefore the result should be always 0?

Maybe, compiler optimization.... using same instance of `"test"` ...not sure though — Sourav Ghosh, Jun 17 '15 at 06:57
There is no requirement that two identical string literals refer to two different strings. — user2357112, Jun 17 '15 at 06:58
The addresses compare equal because of [string pooling](http://stackoverflow.com/questions/11399682/c-optimisation-of-string-literals). — Lundin, Jun 17 '15 at 07:01
@SouravGhosh memory allocation will give different pointers and then `==` will give false even for equal strings. — cybersoft, Jun 17 '15 at 07:03

score 7 · Answer 1 · answered Jun 17 '15 at 07:00

I'd say this is the result of a compiler optimisation using the same instance of the string. If you did something like this you'd prove == doesn't work as you suggest:

char s1[10];
char s2[10];
strcpy(s1, "test");
strcpy(s2, "test");
printf("%d\n", comp(s1, s2));

Sourav Kanta · Answer 2 · 2015-06-17T07:05:32.477

4

It is so because same strings are stored as one string in the string pool during compilation . Therefore both points to the same address as there is only one "test" string in the string pool. Memory heap

edited Jun 17 '15 at 07:05

answered Jun 17 '15 at 06:59

Sourav Kanta

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But the idea is right. – cybersoft Jun 17 '15 at 07:10

score 3 · Answer 3 · answered Jun 17 '15 at 06:58

3

String literals are often reused by optimizing compilers, so if you use the same string literals twice, both will be the exactly same string literals. And your function are comparing pointers, and as both string literals are the same then you are comparing the same pointers which of course will give you a "true" value.

answered Jun 17 '15 at 06:58

Some programmer dude

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@haccks Because I had another answer before which was based on me not reading the question :) – Some programmer dude Jun 17 '15 at 07:03
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@haccks NMDV, but maybe because of the ninja edit. :-) – Sourav Ghosh Jun 17 '15 at 07:03
@JoachimPileborg; Oh. But, now its fine. – haccks Jun 17 '15 at 07:04

score 0 · Answer 4 · answered Jun 17 '15 at 07:10

Read about the concept of mutabe and immutable string. String stored in stack when compared returns true but if one string in heap an other in stack it returns false.

Second question is, are you using prpredefined functions to compare 2 string then the functions works as follows

Compare(s1,s2) returns positive zero or negative according as s1 preced, equals or follows s2 in lexicographical ordering basednon unicide character.

Regards.

C - Comparison of Arrays with == works, why?

4 Answers4