C / C ++：在我的库中使用extern的正确位置吗？

Question

I am making a shared library and its functions remained undefined during linking when built (and linking with a test program), and now I have learned name mangling and why it would cause something like this.

I assume in the libfoo.h header I would put:

#ifdef __cplusplus
  extern "C" {
#endif
int foobar();
#ifdef __cplusplus
  }
#endif

And would the externs for the prototypes be all I need to apply them to, or would I need to wrap them around the function declarations in libfoo.c as well so my C++ program can read them?

Update for comment below:

The linking error is this:

g++ prototypes.cxx -L/usr/lib/  -lfoo
/tmp/ccKQmen4.o: In function `main':
prototypes.cxx:(.text+0x19): undefined reference to `foobar()'
collect2: ld returned 1 exit status

Answer 1

You don't have to do anything with the source, as long as you compile it with a C compiler.

That's what you tell the C++ compiler in the header - use C calling and nameing conventions.

Answer 2

The C++ FAQ has a whole section devoted to mixing C and C++ http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.4

Basically the only place to put extern "C" is in those header files included from C and C++. If you don't have control over those, follow the previous answer (32.3).