Comparing literal empty strings in C

Question

In C, what is the following specified to do?

if ("" == "")
{
    printf("Empty strings are equal\n");
}

I have a compiler on hand that tells me that "" is indeed equal to "". But is this equality guaranteed?

Edit: I understand perfectly well how pointer comparison and string comparison work in C. What I'm asking is what behavior, if any, is specified in the C standard for compile-time constant empty strings. My belief is that the strings are not guaranteed to be equal, but in practice usually will be equal since all const empty strings will be interned to the same address. But I want to know if anyone can provide a definitive reference

It's not guaranteed to have equality. This has been [discussed before](http://stackoverflow.com/questions/4843640/why-is-a-a-in-c) on SO. — Sven Marnach, Apr 28 '11 at 13:36
My belief is that the strings are not *guaranteed* to be equal, but in practice usually will be equal since all const empty strings will be interned to the same address. But I want to know if anyone can provide a definitive reference. — JSBձոգչ, Apr 28 '11 at 13:38
The C Standard says (6.4.5/6): "It is unspecified whether [string literals] are distinct". — pmg, Apr 28 '11 at 13:41
@pmg, that's what I was looking for. Make it an answer and I'll accept it. — JSBձոգչ, Apr 28 '11 at 13:42
Since both strings are compile-time constants, the entire expression seems a bit useless. Is there a real-world case when this really would matter? — unwind, Apr 28 '11 at 13:42
@JSBangs: Consider to add your first comment to your question. — Sven Marnach, Apr 28 '11 at 13:42
For what it's worth, the correct way to compare a string against the empty string is simply `if (!*string)` — R.. GitHub STOP HELPING ICE, Apr 28 '11 at 15:03

score 12 · Accepted Answer · answered Apr 28 '11 at 13:45

12

The C Standard says (6.4.5/6)

It is unspecified whether [string literals] are distinct

answered Apr 28 '11 at 13:45

pmg

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score 5 · Answer 2 · edited May 23 '17 at 11:47

5

Guaranteed? I doubt it. You're not comparing the content of the strings but rather their addresses, which means that you're relying on the compiler to not emit two literal strings that happen to have the same content in the same location. It's likely to work, but not something you should rely on (nor is it clear what it's useful for).

Edit: See also Why is "a" != "a" in C? - it has an answer to basically the same question with nearly a hundred upvotes (and was written by a user whose compiler did it differently).

edited May 23 '17 at 11:47

Community

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answered Apr 28 '11 at 13:36

John Zwinck

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John Zwinck is right, you're betting on the compiler's behavior. You should be using strcmp instead - unless, of course, you're REALLY interested in comparing addresses. – SolarBear Apr 28 '11 at 13:39

score 0 · Answer 3 · answered Apr 28 '11 at 13:41

0

I don't think there is any guarantee that these will have the same address -- I doubt the standard would require such behavior. Why would you need to depend on this being predictable?

answered Apr 28 '11 at 13:41

Glen

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Comparing literal empty strings in C

3 Answers3