29

For example:

code1.c / .cpp

int a;

// ... and so on

code2.c / .cpp

int a;

int main(void) {
    return 0;
}

go to compile:

$gcc code1.c code2.c      # this is fine
$

$g++ code1.cpp code2.cpp  # this is dead
/tmp/ccLY66HQ.o:(.bss+0x0): multiple definition of `a'
/tmp/ccnIOmPC.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

Is there any global variable linkage difference between C & C++?

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4
  • 3
    You can avoid the issue in C++ by using either static or (preferably) a nameless namespace, but that's not your question. I'm not familiar enough with C linkage rules (as opposed to C++) to answer that, I'm afraid. Commented Jun 16, 2011 at 11:32
  • 2
    blogs.oracle.com/ali/entry/what_are_tentative_symbols Commented Jun 16, 2011 at 11:42
  • Sorry for leaving keyboard for a while, I still new to here and I began to review my asked question... Commented Jun 16, 2011 at 12:48
  • 1
    possible duplicate of Redefinition allowed in C but not in C++? Commented Mar 3, 2014 at 12:02

4 Answers 4

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22

It's not strictly legal. int a; is a tentative definition in C. You are allowed multiple tentative definitions and at most one non-tentative definition per translation unit of each object with external linkage in C, but only one definition across all translation units in a program.

It is a commonly implemented extension to allow tentative definitions across multiple translation units in C so long as not more than one translation unit contains a non-tentative definition, but it's not strictly standard.

In C++ int a; is just a definition - there's no concept of tentative - and it's still illegal to have multiple definitions of an object across the translation units of a program.

For the C case, you may wish to look at this question.

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2 Comments

It's not an extension; it's part of the standard, just like K&R function syntax (i.e: if a compiler doesn't implement it, then it's not compliant). The ANSI standard requires these things to be supported so that pre-C89 codebases can still be compiled with modern compilers. C++ never made such a requirement as it was never intended to be compatible with C to begin with. If it was, it would be weakly typed and the standard would require compilers to also respect the ANSI C standard.
Where tentative definitions are mentioned in the standard: ISO/IEC 9899:1999 §6.9 External definitions
5

It's illegal in both, but C compilers generally implement an extension. See this answer.

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9 Comments

It's not illegal in C. It's fully legal as per the standard. See ISO/IEC 9899:1999 §6.9 External definitions wherein the C99 standard describes tentative definitions. Not an extension. If your compiler doesn't implement it, and claims it's an "unnecessary extension", then the vendor has unwittingly violated the standard.
@BradenBest The definitions in code1.c and code2.c are both tentative definitions, but both become definitions when the end of the each file is reached. Therefore, the variable a will be defined in two different translation units. This is the situation "J.5.11 Multiple external definitions" refers to. if the files code1.c and code2.c were concatenated and compiled as one it would be fine.
Makes sense, but if we're purely talking about C, then it's still legal since linking (where it will fail) is up to implementation details. A linker could see the same symbol multiple times and arbitrarily decide to throw one of the definitions away. The compiler itself doesn't care.
@BradenBest That is not my understanding of 6.9 paragraph 5: If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof or _Alignof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one
The rule is technically unenforceable because you can just as well compile each module separately and then compile their output together into an executable, where the linker will stop you. To my understanding, once you get into object files, assembly output, and executable/abi-specific stuff, it stops being relevant to C, as the linker doesn't implement C. Character limit hang on.
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3

There are three ways for resolution of problem:

  1. If variable a is the same in both files, you must declare it as extern in all files except one. extern keyword says to linker that this name is located in another files.

  2. You may use static keyword to limit scope of variable to one file. In which it is declared.

  3. Or you may use nameless namespace.

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1 Comment

There's also -zmuldefs, but it's generally considered an evil solution.
1

g++ compiler is more strict then gcc compiler. It also depends on version of gcc, may be higher version of gcc i.e. 4.X onwards it can give same error.

Use extern to avoid

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1 Comment

Both compilers are equally strict. They implement different standards with different rules. Saying C++ is more strict than ANSI C is like saying French is more strict than English: Possibly correct in some circumstances, but at the end of the day you're comparing apples and oranges

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