error: expected unqualified-id on extern "C"

Question

I have a cpp code in which I want to call a c function. Both compile well to .o files, but when the clang++ is executing for compilation, I receive the following error:

file.cpp:74:12: error: expected unqualified-id
    extern "C"
           ^

The code in the cpp file is the following:

void parseExtern(QString str)
{
#ifdef __cplusplus
    extern "C"
    {
#endif
        function_in_C(str);
#ifdef __cplusplus
    }
#endif

}

How can I avoid the error ? I can't compile the c file with clang++, I really need to use extern. Thanks.

@Mat: That's an _answer_!! Yes it's a short one but, well, it's an easy question :P — Lightness Races in Orbit, May 25 '15 at 14:19
So what's the best way for me to call my C function from CPP ? Regarding QString, I can easily convert to char *. — Laurent Crivello, May 25 '15 at 14:20
@LaurentCrivello: Declare the function with `extern "C"`. Call it just like any other function. — Mike Seymour, May 25 '15 at 14:24
@LaurentCrivello - I think ***[This answer](http://stackoverflow.com/a/16851023/645128)*** addresses your comment question. — ryyker, May 25 '15 at 14:25

score 12 · Accepted Answer · answered May 25 '15 at 14:26

The extern "C" linkage specification is something you attach to a function declaration. You don't put it at the call site.

In your case, you'd put the following in a header file:

#ifdef __cplusplus
    extern "C"
    {
#endif
        void function_in_C(char const *); /* insert correct prototype */
        /* add other C function prototypes here if needed */
#ifdef __cplusplus
    }
#endif

Then in your C++ code, you just call it like any other function. No extra decoration required.

char const * data = ...;
function_in_C(data);

This error can also happen if you accidentally put the extern "C" declaration inside a class declaration. Solution: move it outside the class. — Firstrock, Oct 04 '22 at 15:22

error: expected unqualified-id on extern "C"

1 Answers1

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