I’m currently working on a project that includes multiple source files and header files. I have a weird problem that occurs when I declare a function in a header file.

The problem is as following: In a source file named kernel.cpp I implement a few functions in the following manner:

#include "kernel.h"

void printf(uint8_t* str)
{

    static uint16_t* VideoMemory = (uint16_t*) 0xb8000; 
    static uint8_t x = 0, y = 0;
    for (int32_t i = 0; str[i] != ''; ++i)
    {
        switch(str[i])
        {
            case 'n': // New line
                y++;
                x = 0;
                VideoMemory[80*y + x] = (VideoMemory[80*y + x]&0xFF00) | (int32_t)">";
                break;
            default:
                VideoMemory[80*y + x] = (VideoMemory[80*y + x]&0xFF00) | str[i];
                x++;
                break;
        }
        if (x >= 80)
        {
            y++; // Next line
            x = 0;
        }
        if (y >= 25)
        {
            for (y = 0; y < 25; y++)
                for (x = 0; x < 80; x++)
                    VideoMemory[80*y + x] = (VideoMemory[80*y + x]&0xFF00) | ' '; // Clear screen
            x = 0;
            y = 0;
        }
    }
}




extern "C" void fill_keyboard_buffer(uint8_t letter)
{
    static int32_t index = 0;
    command_buffer[index] = letter;
    printf(&letter);
    keyboard_buffer[index] = letter;
    if (index < 127)
    {
        index++;
    }
    if (letter == 'n')
    {
        index = 0;
    }
}

In addition to the kernel.cpp file I have another file named kernel.h in which I declare these functions. The header file is as following:

#ifndef __KERNEL_H
#define __KERNEL_H
#include "types.h"
#define KEYBOARD_BUFFER_SIZE 128
extern "C" void fill_keyboard_buffer(uint8_t letter);
void printf(uint8_t* ptr);
#endif

for clarification, the types.h header file is declared in the following fashion:

#ifndef __TYPES_H
#define __TYPES_H
     typedef char int8_t;
     typedef unsigned char uint8_t; // signed byte, unsigned byte

     typedef short int16_t;
     typedef unsigned short uint16_t; // signed 2 bytes, unsigned 2 bytes

     typedef int int32_t;
     typedef unsigned int uint32_t; // signed 4 bytes, unsigned 4 bytes

     typedef long long int int64_t;
     typedef unsigned long long int uint64_t; // signed 8 bytes, unsigned 8 bytes

#endif

For some reason, when I compile the kernel file I get the weird error ” error: expected constructor, destructor, or type conversion before ‘extern’ ” that happens in the line ” extern “C” void fill_keyboard_buffer(uint8_t letter); ” in the header file.

any idea?

I tried a few solution like defining the function in the header file in the following fashion:

#ifdef __cplusplus
extern "C" {
#endif

void fill_keyboard_buffer(uint8_t letter);

#ifdef __cplusplus
}
#endif

Also, this problem happens if I completely remove the function declaration as well, but this time with the printf function.