Using “extern C” on non function call related decl

问题:

I know questions regarding extern "C" have been asked before but I am getting mixed signals and would like it if someone could point me to what the best practice is in the scenario below. I have written a driver for Linux and have several struct defined as well as some _IO, _IOR, and _IOW definitions for ioctl(...) calls. None of my structures contain any functions, below is an example struct, enum and ioctl that I use:

#ifdef __cplusplus
extern "C" {
#endif
enum Alignment
{
  Left = 0,
  Right = 1,
  Middle = 3
};

struct Data
{
  int Size;
  void* Address;
};

#define foo  _IOR(DRV_ID, 1, struct Data*);
#ifdef __cplusplus
}
#endif

My question is, do I need to add an extern "C" to this header? The header is defined in my driver which is written in C and used by user programs written in C++. I figure because there are no member functions or specific library function calls I do not need extern "C". Also, is it safe to change my enum to below without extern "C":

#ifdef __cplusplus >= 201103L
enum class Alignment
#else
enum Alignment
#endif
{
  Left = 0,
  Right = 1,
  Middle = 3
};

Edit:

My header is already wrapped in extern "C". I'm trying to understand if this is needed for items that do not call functions and name mangling is an issue.

回答1:

in Standard C++ , extern "C" only affects: function types, function names and variable names. Not to enum definitions.

Your compiler might do something beyond what the standard says, you'd have to consult its documentation.

The conditional enum class is a bad idea. You might end up with the enum having a different size in C than in C++, leading to problems with interoperability. This isn't covered by either standard, but for interoperability it'd be best to make sure the exact same code is used for both languages, other than extern "C".

回答2:

There is a conflict here:

your code goes like this:

#ifdef __cplusplus >= 201103L
enum class Alignment
#else
enum Alignment

but extern "c" should be put in the exact macro:

#ifdef __cplusplus
    extern "C" 
 #endif
 enum Alignment

the outcome, if you append both macros:

extern "C" enum class Alignment

but this is illegal, since C language doesn't knows such as declaration.

therefore, I guess that if you want to use extern "C", to be able to use the header from c, not only c++, you should gave up of the enum class and use old fashion enum type. if you decide to use the header only from c++, the extern "C" is not needed, and you can use enum class.

回答3:

Name mangling is not important for data types. What you get from your library is pure binary data and they don't care how you name them. More important for your types is to have correct alignment and order.