Hello everyone,


I have learned about the concept of extern "C". It is used to compile a C++ file into C binary format. Am I correct?

But I can not quite understand the concept and its special benefits until learn from a simple proof of concept sample and run the sample by myself. However, I can not find out such a simple proof of concept sample.

Could anyone help to find one a simple proof of concept sample or write one for me?


thanks in advance,
George

The scripting language Lua requires you to extern "C" its header files. I did a quick look up on why that is and here's what I found:


Hope that clears things up a little.

Name mangling means that the compiler adds some prefixes and/or suffixes to function names so you can have multiple functions with the same name but different parameters and return type. The 'extern "C"' disables this.

You can test it yourself this way: write some C++ code and compile it to assembler (-S switch in gcc). Then add extern "C" and compile again. Then compare the output files. They should be identical except function names.

Thanks Harmaa,


So, extern "C" is used in C++ file only, is that correct? And it is used to provide C++ functions to C modules? Am I correct?


regards,
George

Thanks R00ts,


It is mentioned that Lua is in ANSI C, but it is also mentioned that it is correct C++ code. I can not imagine why it could be both C code and C++ code. Could you help please?

What namespace pollution will be if Lua place extern "C" directly into its file?

In my previous understanding, I think extern "C" is used in C++ to make C++ functions be able to be called in C. But from your reply, it seems that things are adverse, i.e. your points are that it is used to make C function to be able to be called in C++. How strange it is!


have a good weekend,
George

It is needed in both cases. Assume we have a function prototype from a library written in C:
Without extern "C" gcc seems to name that as "_Z3fooPc", and uses that name in the resulting binary code. But in the library, the function is named simply "foo", so the linker doesn't find it.

Think what happens if you have the "foo" above written in C++ and a different function named "_Z3fooPc" written in C.

C++ was designed that way. In principle, all valid C code in also valid C++.

Thanks Harmaa,


I do not understand what do you mean "But in the library, the function is named simply "foo", so the linker doesn't find it.". I think you mean some other modules ask the linker to find the function foo, when we perform link operation. But what modules? Could you provide more information please?

Let us suppose a simple case. Suppose I am using the same compiler and compile two C modules (foo.c and goo.c) into different object files (foo.o and goo.o). Suppose in module foo.c, I define foo function and in module goo.c, I define goo function. We will invoke foo function in goo function.

As you mentioned above, in foo.o file, the method name may change (to "_Z3fooPc", and not simple foo). But when writing source codes, I do not know what name foo will be "_Z3fooPc" in foo.o, so in source codes of goo function, I invoke "foo", since the function name in foo.o is not the same as foo, will there be any troubles when linking foo.o and goo.o together? Are there any issues for goo function to find out foo function?


have a good weekend,
George