3

Does the standard say anything about limitations on functions used with extern "C"? In a questions of mine that makes no sense to anyone else. It appears that when i have a complex class gcc breaks (runtime oddities/errors) if i call a non default constructor (anything with a parameter i believe) and msvc seems to break if it isn't a POD (however i could still use my constructors and it ran fine).

I suspect all the undefined behavior happens because a C library is calling my C++ which returns a type that is exactly the size of a pointer but has operator overloading and constructors. I don't know why it happens and gcc certainly doesn't give me any warnings.

Does extern "C" force limitations on my code? All i know is exceptions can't go through them which is fine as i don't use them in this code. What are other limitations?

-edit- Related: Reproducible: Why does passing this object in C break my code?

Community
  • 1
  • 1

2 Answers2

6

You can't have extern 'c' that isn't compatible with 'c' - that means no default parameters and no overloaded functions

Martin Beckett
  • 94,801
  • 28
  • 188
  • 263
  • Hmm, thats good for thought. What about the return type? Can it be a C++ class with templates? gcc doesnt compile (msvc does). But gcc doesnt seem to like contructors being called in extern C (yet msvc doesnt care). +1 for your info. –  Mar 09 '12 at 05:23
3

I believe the answer to this question gives a good overview of the requirements on extern "c" linkage. The issue I believe you are running into is the last item:

Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved.

Since your function is returning a class it will be implementation-defined depending on the definition of that class. This explains why it works in one compiler but not another.

The C++ faq has a few relevant sections on this as well.

uesp
  • 6,194
  • 20
  • 15