What does mean for a name or type to have a certain language linkage?

Question

According to (c) ANSI ISO/IEC 14882:2003, page 127:

Linkage specifications nest. When linkage specifications nest, the innermost one determines the langu

Answer 1

As we all know in C/C++ code translation is composed of two principal phases: compilation and linking. When compiler generates object files it passes information to linker specifying in which object files given function is called or referenced. In C it is just like that, function has a name and matching definition.

// file1.c
void foo(void) {}

And after compilation file1.obj stores the code and information about the definition of the foo symbol.

But when C++ comes in the symbol names become more complicated. A function may be overloaded or be a member of a class. But the linker does not want to know it. To preserve simplicity and re-usability of older linkers it needs a single name whether foo is:

void foo(void) {}
void foo(int) {}
void ClassA::foo(void) {}

But it cannot be called just foo anymore so here comes name mangling. And we may get from the compiler some variations like foo_void, foo_int, foo_void_classa. And finally the linker is happy as all those look to it like simple symbols.

When we want to call the foo function compiled with the C compiler in C++ code we must tell the compiler that we want foo to be C style foo and not foo_void as C++ compiler might assume. It is done using:

extern "C" void foo();

Now the compiler knows that foo is compiled using C compiler and will pass information to the linker that this code calls foo. The linker will match it with the foo definition in file1.obj. So it's all I think.

Some other directives like cdecl or stdcall are Windows specific and tell how parameters in function calls are being passed. Yes, for C and C++ it is cdecl. But Windows API functions use stdcall - Pascal convention (simplicity and historically Microsoft once provided Windows dev environment in Pascal).