Unsure of how to properly use extern "C"

I have a child class

Code:

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class IOLabel : public IOField
{
        private:
                        int len;
                        char format[11];
        public:
                        IOLabel(int row, int col, int len);
                        IOLabel(const char *str, int row, int col);               
                        ~IOLabel(){};  //Deallocating non members?
                        void display();
                        int edit(){
                                display();
                                return 0;
                        }
                        IOLabel &setFormat(const char *format){
                                strcpy(this->format,format);
                                return *this;
                        }
                        IOLabel &operator=(const char *data){
                                strncpy((char*)this->data,data,len); //Correct? Data null terminated
                        }
                        IOLabel &operator=(int number){ sprintf((char*)data,format,number);}
                        IOLabel &operator=(double number){ sprintf((char*)data,format, number); }
};

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In my definition for one method I need to call a function written in C in the file cio.h. My professor said to use

Code:

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extern "C" #include "cio.h"

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but I am unsure how

Code:

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void IOLabel::display(){
        extern "C"
        #include "cio.h"
        cio_display((char*)data,Row(),Col(),len);
}

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Tried that but it didnt work

Code:

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error C2598: linkage specification must be at global scope

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extern goes before your definition, like so extern "C" void IOLabel::display()

Quote:

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Originally Posted by durban

extern goes before your definition, like so extern "C" void IOLabel::display()

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Code:

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scrio.obj : error LNK2019: unresolved external symbol "protected: int __thiscall IOField::Row(void)" (?Row@IOField@@IAEHXZ) referenced in function "public: virtual void __thiscall IOLabel::display(void)" (?display@IOLabel@@UAEXXZ)
scrio.obj : error LNK2019: unresolved external symbol "protected: int __thiscall IOField::Col(void)" (?Col@IOField@@IAEHXZ) referenced in function "public: virtual void __thiscall IOLabel::display(void)" (?display@IOLabel@@UAEXXZ)

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Woops I forgot the definitions for row and col right

With another function in another class I get this error now

Code:

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extern "C" void IOLineEdit::display(){

        #include "cio.h"
        cio_display((char*)data,Row(),Col(),flen);
       
}

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Code:

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error C3861: 'cio_display': identifier not found

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When it obviously worked with the parent class's display method...

Code:

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extern "C" void IOLabel::display(){

#include "cio.h"
        cio_display((char*)data,Row(),Col(),len);
}

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Aha, I didnt know I had to declare it at the top of the program

Code:

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extern "C"{ #include "cio.h"}

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and then insert it at the begining of each function definition

Code:

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extern "C" int whatever(int a){}

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:)