[c] How to prevent multiple definitions in C?

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 <stdio.h>
#include <stdlib.h>
#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!

The underscore is put there by the compiler and used by the linker. The basic path is:

main.c
test.h ---> [compiler] ---> main.o --+
                                     |
test.c ---> [compiler] ---> test.o --+--> [linker] ---> 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().

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 */

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)

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

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 "test.c"
• create a file test.h which contains the declaration: void test();

• include the file test.h in main.c

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