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Warning with extern "C" blocks

+2 votes
82 views

I tried using -Wzero-as-null-pointer-constant in a C++ program, but I got warnings in some C headers, even though they are wrapped in extern "C" blocks.

Since there is no nullptr in C, wouldn't it make more sense to disable the warning within those blocks? Is there some option that I'm missing?

posted Oct 13, 2014 by anonymous

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1 Answer

+1 vote

Although it's usually used in headers that need to be valid in both C and C++, extern "C" does not mean you can't use C++ features.

extern "C" {
 void* ptr = nullptr;
}
answer Oct 13, 2014 by Gurminder
Similar Questions
+1 vote

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
+7 votes

In lot of C++ code in my company I see extern C code something like

extern "C"
{
    int sum(int x, int y)
    {
        return x+y;
    }
}

Please explain the significance of this?

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