Why does global variables in a header file cause link error?

Question

I always get the following error, even if I have put include guard in header file.

duplicate symbol _Bittorrent in:
    /Users/tracking/Library/Developer/Xco         


        

Answer 1

Lines like

const char    *Bittorrent     =   "BitTorrent protocol";
const char    eight_byte[8]   =   {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };

define theses global variables no matter if that codes is in header file or ine a .c directly (#include is just textual insertion of the header's contents). Instead you should have the definitions in exaclty one source file and change the header to provide an extern declaration instead:

extern const char *Bittorrent;
extern const char *eight_byte;

Then all your sources using thes variables can be compiled but the linker will the variables once only.