Using C++ struct in C

Question

I have a C++ struct with methods inside:

struct S
{
   int a;
   int b;

   void foo(void)
   {
       ... 
   };
}

I have a userprogram, written in C. Is it possible to get a pointer to a S-struct and access the member aand b?

The S-pointer points a C++ object. I want to access to this memory from C. — Razer
– Razer, Commented Oct 21, 2012 at 17:41

mauve · Accepted Answer · 2012-10-21 17:45:09Z

10

You can access the members of a struct written in C++ from a C-program given you ensure that the C++ additions to the struct syntax are removed:

// header
struct S {
  int a, b;

#ifdef __cplusplus
  void foo(); 
#endif
};

// c-file:
#include "header.h"

void something(struct S* s)
{
  printf("%d, %d", s->a, s->b);
}

The memory layout of structs and classes in C++ is compatible with C for the C-parts of the language. As soon as you add a vtable (by adding virtual functions) to your struct it will no longer be compatible and you must use some other technique.

edited Oct 21, 2012 at 17:45

user405725

answered Oct 21, 2012 at 17:42

mauve

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4 Comments

user405725 Over a year ago

I guess it is worth mentioning that the trick will stop working as soon as you add a virtual table into the picture. Or start using inheritance...

AProgrammer Over a year ago

vtables or just private part. IMO, that's the kind of solution which will start by working and then stop at the most inconvenient moment.

Tomek Over a year ago

Having members with different access specifiers (private/public/protected) may also change layout of the struct.

bames53 Over a year ago

What you want is a standard layout class. The requirements are defined in §9/7. For example inheritance is allowed under certain strict limitations.

PiotrNycz · Accepted Answer · 2012-10-21 17:44:13Z

3

If these methods are not virtual then it is OK. You can even have common header for C/C++ with using of __cplusplus macro:

struct S
{
   int a;
   int b;

#ifdef __cplusplus
   void foo(void)
   {
       ... 
   }
#endif /* end section for C++ only */
};

Remember that name of this struct in C is struct S not just S.

answered Oct 21, 2012 at 17:44

PiotrNycz

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1 Comment

Marco Luglio Over a year ago

Upvoted for "Remember that name of this struct in C is struct S not just S."

AProgrammer · Accepted Answer · 2012-10-21 17:49:25Z

2

How do you get an S if your program is written in C? My guess is that a most precise description is that your program is written in a mix of C and C++ and you want to access some members of a C++ struct in the C part.

Solution 1: modify your C part so that it is in the common subset of C and C++, compile the result as C++ and now you may gradually use whatever C++ feature you want. That's what lot of projects did in the past. The most well know recent one being GCC.

Solution 2: provide an extern "C" interface to S and use it in your C part.

#ifdef __cplusplus
extern "C" {
#endif
    struct S;
    int getA(S*);
    int getB(S*);
#ifdef __cplusplus
}
#endif

The part which provides the implementation of getA and getB must be compiled as C++, but will be callable from C.

edited Oct 21, 2012 at 17:49

answered Oct 21, 2012 at 17:42

AProgrammer

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6 Comments

Aniket Inge Over a year ago

This is a more correct way than the rest. C++ will do some name-mangling to struct's name. This will make it inaccessible IIRC

Razer Over a year ago

Well, the problem is more complicated. It's OS-programming. The kernel is implemented in C++. My userspace has only C runtime. I have some shared structs between file system and userspace, but these include C++ specific code.

AProgrammer Over a year ago

@Razer, The fact that the C++ part comes from the OS doesn't change anything. You may have to maintain your extern "C" wrappers independently from the provider of the C++ interface but that could be the case as well in other circumstances. Or do I miss something in your setup?

Razer Over a year ago

To describe in more precisely: Let's take opendir: linux.die.net/man/3/opendir This returns a DIR* and exactly this DIR is implemented in C++ in the filesystem.

Razer Over a year ago

But, what about this thought: DIR* doesn't need to be accessed directly in C. Therefore I can define DIR* as simple unsigned int pointer. For simpler structs, with no virtual methods, the solution from @mauve works.

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