访问库中可执行文件中定义的全局变量时重定义错误

Question

I have static library file( lib_XXX.a ) with global variable defined in it. I am trying to access the global variable in my executable( exe_XXX.o ). Linker error is coming. Any help would be thankful.

Languae : c
OS : Ubuntu gcc compiler

Sample as follows

exe_xxx.o module has 2 files resource.h and main.c

resource.h code as follows:

#ifndef RESOURCE_H
#define RESOURCE_H

#define APL

extern const StructTest g_AplObjDef;

const StructTest g_AplObjDef = {
abc, def, ghi,
....
};

#endif //APL

main.c code as follows:

#include "resource.h"
....
....
....

lib_xxx.a has another main.c in it. Its sample code as follows:

#include "resource.h"

int main()
{
#if defined(APL)
     fun1(g_AplObjDef);
#endif
}

I suspect the reason is because resource.h included in both the main.c files. I couldn't way to get rid of this. Can anyone help?

Error details: /lib_XXX.a(lib_XXX_a-main.o):(.data.rel.ro.local+0x40): `g_AplObjDef' が重複して定義されています /exe_xxx-main.o:(.data.rel.ro.local+0x260): ここで最初に定義されています

Above error is in Japanese.. 1st line says "Duplicate is defined". 2nd line says "Here it is defined"

Answer 1

This part:

const StructTest g_AplObjDef = {
    abc, def, ghi,
    ....
};

is a definition, and should not be in a header. Move it to a .c file.

The reason for this is that header files are textually inserted, so if a header has a definition, and is included from multiple translation units, the symbol will be defined multiple times, which is an error.

Answer 2

Here you are defining a variable in the header

 const StructTest g_AplObjDef = 

You should only declare, which you did the line before. This definition should go into a code file, accessing it will be possible by the knowledge provded by the declaration in the header. But the definition in the header will be done in each code file which includes it, which causes the redundant definition mentioned in the error message.

Moving the definition (as you now have it in the header, including the {...} into the libs code file should help.

Note that having two main() will probably get you into trouble, I only focus this answer on the double definition of the variable.