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Warning for incompatible functions declared 'extern "C"'?

+1 vote
581 views

I have an oversight in my code where I'm declaring & defining a function
with C-linkage, though it's not possible.

Example snippet:

#ifdef __cplusplus
extern "C"
{
#endif

// ... some functions which are compatible with C linkage

// Intended to be a helper function not exposed from library
std::string GetEngineVersion()
{
 // ...
}

#ifdef __cplusplus
}
#endif

Obviously the GetEngineVersion function cannot have C linkage because it returns a C++ class.

My question is: does GCC have a warning for this scenario? Specifically, can it warn when something is declared extern "C" that's incompatible with C linkage?

I compile with the following warning flags and I didn't get a warning:

-Wall -Wunused-parameter -Wextra -Weffc++ -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder -Wold-style-cast -Woverloaded-virtual
-Werror

Incidentally, when I compile the same code in VS2013 I get this warning:

warning C4190: 'GetEngineVersion' has C-linkage specified, but returns UDT 'std::basic_string' which is incompatible with C
posted Jul 23, 2014 by Honey

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1 Answer

+1 vote

That is valid C++ code. C linkage doesn't mean the function has to actually be usable from C.

You can also declare functions with C linkage that take reference parameters or throw exceptions.

My question is: does GCC have a warning for this scenario? Specifically, can it warn when something is declared extern "C" that's incompatible with C linkage?

No, there is no such warning as far as I know.

Such a warning doesn't seem unreasonable, since usually (not not always) you want functions with C language linkage to be callable from C.

answer Jul 24, 2014 by Luv Kumar
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