Thread: Why is my IDE/compiler allowing me to define a varable twice?

Code:

int b;
int b;

int main(){
    return 0;
}

My understanding is that int b is a definition, not a declaration and identifiers are not allowed to be defined more than once.

• GNU GCC Compiler
• Code Blocks:16.01
• No flags used apart from -Wall (enable all warnings). Not sure which revision of C its following but if I enable the -std=c99 flag then it also doesnt work.

Any ideas?

You're allowed to do that with global variables, although only one is allowed to have an initializer.

Note that you can't do the same with locals that are in the same scope.

The explanation should be in section 6.2 of the standard. I took a quick look but couldn't find it.

Here's a somewhat stranger example:

Code:

#include <stdio.h>

int b;
int b, b, b;

int main() {
    printf("%d\n", b);   // prints 42
    return 0;
}

int b;
int b = 42;
int b;

Do you guys think it has anything to do with "tentative definitions?" I know the standard makes mention of this.

Originally Posted by algorism

You're allowed to do that with global variables, although only one is allowed to have an initializer.

Note that you can't do the same with locals that are in the same scope.

The explanation should be in section 6.2 of the standard. I took a quick look but couldn't find it.

Here's a somewhat stranger example:

Code:

#include <stdio.h>

int b;
int b, b, b;

int main() {
    printf("%d\n", b);   // prints 42
    return 0;
}

int b;
int b = 42;
int b;

That's exactly what it's meant to be, a tentative declaration. See here for more on the distinction between declaration and definition: Declarations - cppreference.com I know it's a C++ site, but it's well referenced from C standards and it's in a C section (like this site).

But itsnt it a "tentative definition" not "tentative declaration"?

Originally Posted by Vespasian_2

But itsnt it a "tentative definition" not "tentative declaration"?

Yes, that's it.

Originally Posted by C99 Standard

6.9.2 External object definitions

Semantics

1 If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier.

2 A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

3 If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.