Build C application by C++ compiler

Question

I have an application, which is written on C. A new functionality I need to add in C++. I build C application with GCC. To add new functionality:

To build an existing application with G++.
Add new functionality.

When I tried to do (1.), I met following problem: In file dev_table.c defined array:

const type_name devices[] = {...};

in file stm.c I export this array by:

extern const type_name devices[];

And when I assign

stm->dev = devices;

linker fail.

If I build the same code by GCC, everything is OK and working well, but with G++ it fails.

If I remove const from both places(dev_table.c and stm.c), it is also working well, so the problem is resolved, but I do not understand what is the reason.

What is your command line when you build with `g++`, and what is the output? — Mathieu, May 02 '16 at 11:55
You cannot assign arrays in C++. IDK if you can in C but you can't in C++. If you need that I suggest you look into using a [std::array](http://en.cppreference.com/w/cpp/container/array) — NathanOliver, May 02 '16 at 11:56
Compile C source with `gcc`, C++ source with `g++` and link with `g++`. — jdarthenay, May 02 '16 at 11:57
In C headers use `#ifdef __cplusplus` directives to add `extern "C"` blocks only for C++ complier. — jdarthenay, May 02 '16 at 12:00
at deceleration and extern places keep it a pointer not array, at the time of assign array will decay to pointer. If the array is fixed size it might work but in case of unknown array size it won't work. — sanjay, May 02 '16 at 12:02
NathanOliver: If I cannot assign arrays, why it works, when I remove `const`. — Apter Rostislav, May 02 '16 at 12:11
You should keep using a C compiler to build the C code. You can write new code in C++ in other files and link the object files together. — M.M, May 02 '16 at 12:50
**M.M** thank you. I except what you sugest and I will do this, but I want to underestand the issue described above. — Apter Rostislav, May 02 '16 at 12:55
@ApterRostislav What Nathan Oliver meant was that having `const int arr[] = {...};` you cant do `const int foo[] = arr;` in C++. But you can use pointers with no problem. — pandaman1234, May 02 '16 at 13:03
That is exactly the problem. In my case, `stm->dev` is pointer to `const type_name`. So it should be ok if I do `stm->dev = devices;`, but: when I link this code with GCC - OK, when I link with G++ - linker fails. — Apter Rostislav, May 02 '16 at 13:19

score 2 · Answer 1 · edited May 23 '17 at 12:31

I suggest you take a look at this post

Um, since const s are implicitly static, you need an extern even on the a_global_var definition (in file.c). Without this, anything that includes file.h will not link because it is looking for a const int a_global_var with external linkage.

Or

You can use them together (extern and const). But you need to be consistent on your use of const because when C++ does name decoration, const is included in the type information that is used to decorate the symbol names. so extern const int i will refer to a different variable than extern int i Unless you use extern "C" {}. C name decoration doesn't pay attention to const.

Build C application by C++ compiler

1 Answers1