c - "Multiple definition", "first defined here" errors -

i have 3 projects: server, client , commons. making header & source pairs in commons doesn't cause problems , can access functions freely both server , client.

however, reason making additional source/header files within server or client project causes multiple definition of (...) , first defined here errors.

example:

commands.h (in root dir of client project)

#ifndef commands_h_ #define commands_h_  #include "commands.c"  void f123();  #endif /* commands_h_ */ 

commands.c (in root dir of client project)

void f123(){  } 

main.c (in root dir of client project)

#include "commands.h" int main(int argc, char** argv){  } 

errors:

make: *** [client] error 1      client first defined here              client multiple definition of `f123'   commands.c 

cleaning, rebuilding index, rebuilding projects doesn't help. neither restarting computer.

the problem here including commands.c in commands.h before function prototype. therefore, c pre-processor inserts content of commands.c commands.h before function prototype. commands.c contains function definition. result, function definition ends before function declaration causing error.

the content of commands.h after pre-processor phase looks this:

#ifndef commands_h_ #define commands_h_  // function definition void f123(){  }  // function declaration void f123();  #endif /* commands_h_ */ 

this error because can't declare function after definition in c. if swapped #include "commands.c" , function declaration error shouldn't happen because, now, function prototype comes before function declaration.

however, including .c file bad practice , should avoided. better solution problem include commands.h in commands.c , link compiled version of command main file. example:

commands.h

ifndef commands_h_ #define commands_h_  void f123(); // function declaration  #endif 

commands.c

#include "commands.h"  void f123(){} // function definition