Thread: How to : Using C and C++ headers

The header itself SHOULD have extern "C" in them, but I guess if they are really low-level headers, maybe no one has updated them. I would post a bug-report to the relevant maintainer (probably glibc).

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Mats

It tells the compiler that these function declarations are "C" rather than "C++". This is because certain concepts are every so slightly different in C and C++.

For example, in C:

Code:

int foo();

means that there is a function foo that returns an integer, and it takes an arbitrary number of arguments.

In C++ the same would mean "it takes NO arguments".

Another example is the C++ keyword "new" that is not a keyword in C. So you could have:

Code:

extern "C" void foo(int new, int old);

But without extern "C" the this would cause a compile error.

--
Mats

This will cause a compile error with extern "C", too. The linkage specification only affects linkage.

OK, here's the real reason: C names traditionally follow a very simple "name mangling" (that's the process of turning the names you use in the programming language into those used at the final linking level): just prepend an underscore. E.g. this C function:

Code:

void foo(void)
{
}

is called _foo at the binary level.

This C function:

Code:

int foo(int i1, int i2, float f, struct bar *pb)
{
  return i1 * i2 + (int)(f * pb->whoa);
}

is also called _foo at the binary level. You can't have both in the same program. Overloading is not allowed in C.

In C++, a different way had to be found. It uses a much more complicated mangling scheme that encodes the parameter types into the name. E.g. the first foo could become "_foo$v", and the second could become "_foo$iif$$bar$" or something like that. Now the functions can be distinguished.

The problem is obvious. Compile the first foo with a C compiler. Then use a C++ compiler to compile code that uses it. The C compiler generates the name "_foo", but the C++ compiler will parse the declaration and make a reference to "_foo$v". The linker comes along and can't find "_foo$v" and throws an error.

The extern "C" tells the C++ compiler to use C mangling rules.

Actually something like:

Code:

#if __cplusplus
extern "C" {
#endif
...
#if __cplusplus
}
#endif

Otherwise it won't compile with a non-C++ compiler.

--
Mats