Multiple definition first defined here errors

Question

I have 3 projects  Server  Client and Commons  Making header  amp  source pairs in Commons doesn t cause any problems and I can access the functions freely from both Server and Client   However  for some reason making additional source header files within Server or Client project always causes multiple definition of       and first defined here errors   Example   commands h  in root dir of the Client project    ifndef COMMANDS H   define COMMANDS H    include  commands c   void f123      endif    COMMANDS H       commands c  in root dir of the Client project   void f123         main c  in root dir of the 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 does restarting the computer

User · Accepted Answer

The problem here is that you are including commands.c in commands.h before the function prototype. Therefore, the C pre-processor inserts the content of commands.c into commands.h before the function prototype. commands.c contains the function definition. As a result, the function definition ends up before than the function declaration causing the error.

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

#ifndef COMMANDS_H_
#define COMMANDS_H_

// function definition
void f123(){

}

// function declaration
void f123();

#endif /* COMMANDS_H_ */

This is an error because you can't declare a function after its definition in C. If you swapped #include "commands.c" and the function declaration the error shouldn't happen because, now, the function prototype comes before the function declaration.

However, including a .c file is a bad practice and should be avoided. A better solution for this problem would be to include commands.h in commands.c and link the compiled version of command to the main file. For example:

commands.h

#ifndef COMMANDS_H_
#define COMMANDS_H_

void f123(); // function declaration

#endif

commands.c

#include "commands.h"

void f123(){} // function definition

User · Answer

I had a similar issue when not using inline for my global function that was included in two places

User · Answer

I am adding this A because I got caught with a bizarre version of this which really had me scratching my head for about a hour until I spotted the root cause   My load was failing because of multiple repeats of this format   lt path gt  linit o   rodata1 libs 0x50   multiple definition of  lua lib BASE   lt path gt  linit o   rodata1 libs 0x50   first defined here   I turned out to be a bug in my Makefile magic where I had a list of C files and using vpath etc   so the compiles would pick them up from the correct directory in hierarchy   However one C file was repeated in the list  at the end of one line and the start of the next so the gcc load generated by the make had the  o file twice on the command line   Durrrrh   The multiple definitions were from multiple occurances of the same file   The linker ignored duplicates apart from static initialisers

User · Answer

Maybe you included the  c file in makefile multiple times

User · Answer

You should not include commands c in your header file  In general  you should not include  c files  Rather  commands c should include commands h  As defined here  the C preprocessor is inserting the contents of commands c into commands h where the include is  You end up with two definitions of f123 in commands h   commands h   ifndef COMMANDS H   define COMMANDS H   void f123      endif   commands c   include  commands h   void f123            code

[c] "Multiple definition", "first defined here" errors

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