What is the difference between the global variables in C and C++?

Question

I have tested the following code:

in file a.c/a.cpp

int a; 

in file b.c/b.cpp

int a; int main() { return 0; } 

When I compile the source files with gcc *.c -o test, it succeeds.

But when I compile the source files with g++ *.c -o test, it fails:

ccIJdJPe.o:b.cpp:(.bss+0x0): multiple definition of 'a' ccOSsV4n.o:a.cpp:(.bss+0x0): first defined here collect2.exe: error: ld returned 1 exit status 

I'm really confused about this. Is there any difference between the global variables in C and C++?

Answer 1

Here are the relevant parts of the standard. See my explanation below the standard text:

§6.9.2/2 External object definitions

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.

ISO C99 §6.9/5 External definitions

An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof 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.

With the C version, the 'g' global variables are 'merged' into one, so you will only have one in the end of the day which is declared twice. This is OK due to the time when extern was not needed, or perhaps did not exits. Hence, this is for historical and compatibility reason to build old code. This is a gcc extension for this legacy feature.

It basically makes gcc allocate memory for a variable with the name 'a', so there can be more than one declarations, but only one definition. That is why the code below will not work even with gcc.

This is also called tentative definition. There is no such a thing with C++, and that is while it compiles. C++ has no concept of tentative declaration.

A tentative definition is any external data declaration that has no storage class specifier and no initializer. A tentative definition becomes a full definition if the end of the translation unit is reached and no definition has appeared with an initializer for the identifier. In this situation, the compiler reserves uninitialized space for the object defined.

Note however that the following code will not compile even with gcc because this is tentative definition/declaration anymore with values assigned:

in file "a.c/a.cpp"

int a = 1; 

in file "b.c/b.cpp"

int a = 2; int main() { return 0; } 

Let us go even beyond this with further examples. The following statements show normal definitions and tentative definitions. Note, static would make it a bit difference since that is file scope, and would not be external anymore.

int i1 = 10;         /* definition, external linkage */ static int i2 = 20;  /* definition, internal linkage */ extern int i3 = 30;  /* definition, external linkage */ int i4;              /* tentative definition, external linkage */ static int i5;       /* tentative definition, internal linkage */   int i1;              /* valid tentative definition */ int i2;              /* not legal, linkage disagreement with previous */ int i3;              /* valid tentative definition */ int i4;              /* valid tentative definition */ int i5;              /* not legal, linkage disagreement with previous */ 

Further details can be on the following page:

http://c0x.coding-guidelines.com/6.9.2.html

See also this blog post for further details:

http://ninjalj.blogspot.co.uk/2011/10/tentative-definitions-in-c.html