Different compilation results not using extern in C vs in C++

Question

When I declare a global variable in two different source files and only define it in one of the source files, I get different results compiling for C++ than for C. See the f

Answer 1

I believe you are observing an extension to C known as "common symbols", implemented by most, but not all, Unix-lineage C compilers, originally (IIUC) for compatibility with FORTRAN. The extension generalizes the "tentative definitions" rule described in StoryTeller's answer to multiple translation units. All external object definitions with the same name and no initializer,

int foo; // at file scope

are collapsed into one, even if they appear in more than one TU, and if there exists an external definition with an initializer for that name,

int foo = 1; // different TU, also file scope

then all of the external definitions with no initializers are treated as external declarations. C++ compilers do not implement this extension, because (oversimplifying) nobody wanted to figure out what it should do in the presence of templates. For GCC and Clang, you can disable the extension with -fno-common, but other Unix C compilers may not have any way to turn it off.