How to prevent multiple definitions in C

Question

I m a C newbie and I was just trying to write a console application with Code  Blocks  Here s the  simplified  code  main c    include  lt stdio h gt   include  lt stdlib h gt   include  test c     include not necessary for error in Code  Blocks  int main           t   test       calling of method also not necessary     return 0      test c   void test        When I try to build this program  it gives the following errors     path  test c 1 multiple definition of    test   obj Debug main o  path  test c 1 first defined here    There is no way that I m multiply defining test  although I don t know where the underscore is coming from  and it seems highly unlikely that the definition is somehow included twice  This is all the code there is   I ve ruled out that this error is due to some naming conflict with other functions or files being called test or test c  Note that the multiple and the first definition are on the same line in the same file   Does anyone know what is causing this and what I can do about it  Thanks

User · Accepted Answer

You actually compile the source code of test.c twice:

The first time when compiling test.c itself,
The second time when compiling main.c which includes all the test.c source.

What you need in your main.c in order to use the test() function is a simple declaration, not its definition. This is achieved by including a test.h header file which contains something like:

void test(void);

This informs the compiler that such a function with input parameters and return type exists. What this function does ( everything inside { and } ) is left in your test.c file.

In main.c, replace #include "test.c" by #include "test.h".

A last point: with your programs being more complex, you will be faced to situations when header files may be included several times. To prevent this, header sources are sometimes enclosed by specific macro definitions, like:

#ifndef TEST_H_INCLUDED
#define TEST_H_INCLUDED

void test(void);

#endif

User · Answer

Including the implementation file  test c  causes it to be prepended to your main c and complied there and then again separately  So  the function test has two definitions -- one in the object code of main c and once in that of test c  which gives you a ODR violation  You need to create a header file containing the declaration of test and include it in main c      test h     ifndef TEST H  define TEST H void test       declaration     endif    TEST H

User · Answer

If you re using Visual Studio you could also do   pragma once  at the top of the headerfile to achieve the same thing as the   ifndef     -wrapping  Some other compilers probably support it as well       However  don t do this  D Stick with the  ifndef-wrapping to achieve cross-compiler compatibility  I just wanted to let you know that you could also do  pragma once  since you ll probably meet this statement quite a bit when reading other peoples code   Good luck with it

User · Answer

You shouldn t include other source files    c  in  c files  I think you want to have a header   h  file with the DECLARATION of test function  and have it s DEFINITION in a separate  c file   The error is caused by multiple definitions of the test function  one in test c and other in main c

User · Answer

I had similar problem and i solved it following way   Solve as follows    Function prototype declarations and global variable should be in test h file and you can not initialize global variable in header file   Function definition and use of global variable in test c file   if you initialize global variables in header it will have following error     multiple definition of    test     obj Debug main o path test c 1 first defined here    Just declarations of global variables in Header file no initialization should work    Hope it helps   Cheers

User · Answer

The underscore is put there by the compiler and used by the linker  The basic path is   main c test h --- gt   compiler  --- gt  main o --                                         test c --- gt   compiler  --- gt  test o -- -- gt   linker  --- gt  main exe   So  your main program should include the header file for the test module which should consist only of declarations  such as the function prototype   void test void     This lets the compiler know that it exists when main c is being compiled but the actual code is in test c  then test o   It s the linking phase that joins together the two modules   By including test c into main c  you re defining the test   function in main o  Presumably  you re then linking main o and test o  both of which contain the function test

User · Answer

If you have added test c to your Code  Blocks project  the definition will be seen twice - once via the  include and once by the linker  You need to   remove the  include  quot test c quot  create a file test h which contains the declaration  void test    include the file test h in main c

[c] How to prevent multiple definitions in C?

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