Programming Forums > Application Development > C++

carlgreen · Feb 26th, 2005, 5:26 PM

Hi,

I have been having trouble with defining global variables in a header file and including this in other files. It seems that every way I do it so that I can compile the code, the linker gives me a multiple definition error.

Here is a simplified version if anyone can help me out:

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/* main.c */

#include "include.h"
#include "a.h"

int main(int argc, char *argv[]) {
  variable = 1;
  return 0;
}

(Toggle Plain Text)

/* include.h */

#ifndef INCLUDE_H
#define INCLUDE_H

int variable = 0;

#endif

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/* a.h */

#include "include.h"

int getVariable();

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/* a.c */

#include "a.h"

int getVariable() {
  return variable;
}

This is the output from the compiler:

gcc.exe -c main.c -o main.o -I"E:/Dev-Cpp/include"
gcc.exe -c a.c -o a.o -I"E:/Dev-Cpp/include"
gcc.exe main.o a.o -o "c.exe" -L"E:/Dev-Cpp/lib"
a.o(.data+0x0):a.c: multiple definition of `variable'
main.o(.data+0x0):main.c: first defined here

If anyone can tell me what I am doing wrong I would greatly appreciate it.

TerraNerd · Feb 26th, 2005, 6:03 PM

I dont know any C or C++, but from one of the errors, (a.o(.data+0x0):a.c: multiple definition of `variable') i can see whats going on. Your assigning 'variable' two different values at once.

When your telling 'main.c' to include 'incude.h' variable is being defined twice, because the documents 'merge,' if you will.

(Toggle Plain Text)

/* main.c */

#include "include.h"
#include "a.h"

int main(int argc, char *argv[]) {
  variable = 1;
  return 0;
}

(Toggle Plain Text)

/* include.h */

#ifndef INCLUDE_H
#define INCLUDE_H

int variable = 0;

#endif

The integer 'variable' is being assigned 0 as well as 1.

ZenMasterJG · Feb 26th, 2005, 6:35 PM

there are 3 ways you can fix that:
a) change the names of your variables so they dont repeat
b) put the variable in a class
c) define a namespace for your variable

namespace example:

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/* include.h */

#ifndef INCLUDE_H
#define INCLUDE_H
namespace myInclude {
   int variable = 0;
}
#endif

and then you'd access variable as
myInclude::variable

carlgreen · Feb 26th, 2005, 7:02 PM

TerraNerd, your suggestion worked for this example. I changed the declaration to int variable; and it works. I would have thought it would just get set to 0, and then to 1.

but

My original problem was in a c++ program. So this example now works when I use gcc, but if I try it with g++ I still get pretty much the same error:

g++ -o main.exe main.o a.o
a.o(.bss+0x0):a.c: multiple definition of `variable'
main.o(.bss+0x0):main.c: first defined here

ZenMasterJG,

I don't think (a) will work, because I want this to be one global variable. I tried (c), but it seems that the section:

(Toggle Plain Text)

namespace myInclude {
   int variable = 0;
}

is being carried out twice. My understanding was that the ifndef would stop that, but I guess that must only work at the compiling stage?

I guess I should really be doing it like (b) and setting the values in the classes that I want it.

Feb 26th, 2005, 5:26 PM	#1
carlgreen Newbie Join Date: Feb 2005 Location: new zealand Posts: 2 Rep Power: 0	multiple definition of variables in include files Hi, I have been having trouble with defining global variables in a header file and including this in other files. It seems that every way I do it so that I can compile the code, the linker gives me a multiple definition error. Here is a simplified version if anyone can help me out: (Toggle Plain Text) /* main.c / #include "include.h" #include "a.h" int main(int argc, char argv[]) { variable = 1; return 0; } /* main.c / #include "include.h" #include "a.h" int main(int argc, char argv[]) { variable = 1; return 0; } (Toggle Plain Text) /* include.h / #ifndef INCLUDE_H #define INCLUDE_H int variable = 0; #endif / include.h / #ifndef INCLUDE_H #define INCLUDE_H int variable = 0; #endif (Toggle Plain Text) / a.h / #include "include.h" int getVariable(); / a.h / #include "include.h" int getVariable(); (Toggle Plain Text) / a.c / #include "a.h" int getVariable() { return variable; } / a.c */ #include "a.h" int getVariable() { return variable; } This is the output from the compiler: gcc.exe -c main.c -o main.o -I"E:/Dev-Cpp/include" gcc.exe -c a.c -o a.o -I"E:/Dev-Cpp/include" gcc.exe main.o a.o -o "c.exe" -L"E:/Dev-Cpp/lib" a.o(.data+0x0):a.c: multiple definition of `variable' main.o(.data+0x0):main.c: first defined here If anyone can tell me what I am doing wrong I would greatly appreciate it.

Feb 26th, 2005, 6:03 PM	#2
TerraNerd Programmer Join Date: Jul 2004 Location: Vermont, USA Posts: 65 Rep Power: 5	I dont know any C or C++, but from one of the errors, (a.o(.data+0x0):a.c: multiple definition of `variable') i can see whats going on. Your assigning 'variable' two different values at once. When your telling 'main.c' to include 'incude.h' variable is being defined twice, because the documents 'merge,' if you will. (Toggle Plain Text) /* main.c / #include "include.h" #include "a.h" int main(int argc, char argv[]) { variable = 1; return 0; } /* main.c / #include "include.h" #include "a.h" int main(int argc, char argv[]) { variable = 1; return 0; } (Toggle Plain Text) /* include.h / #ifndef INCLUDE_H #define INCLUDE_H int variable = 0;* #endif /* include.h */ #ifndef INCLUDE_H #define INCLUDE_H int variable = 0; #endif The integer 'variable' is being assigned 0 as well as 1. __________________ [See a gallery of my Graphic art Here.][Visit my website Here]

Feb 26th, 2005, 6:35 PM	#3
ZenMasterJG Hobbyist Programmer Join Date: Nov 2004 Location: Boston, MA Posts: 148 Rep Power: 4	there are 3 ways you can fix that: a) change the names of your variables so they dont repeat b) put the variable in a class c) define a namespace for your variable namespace example: (Toggle Plain Text) /* include.h / #ifndef INCLUDE_H #define INCLUDE_H namespace myInclude { int variable = 0; } #endif / include.h */ #ifndef INCLUDE_H #define INCLUDE_H namespace myInclude { int variable = 0; } #endif and then you'd access variable as myInclude::variable

Feb 26th, 2005, 7:02 PM	#4
carlgreen Newbie Join Date: Feb 2005 Location: new zealand Posts: 2 Rep Power: 0	TerraNerd, your suggestion worked for this example. I changed the declaration to int variable; and it works. I would have thought it would just get set to 0, and then to 1. but My original problem was in a c++ program. So this example now works when I use gcc, but if I try it with g++ I still get pretty much the same error: g++ -o main.exe main.o a.o a.o(.bss+0x0):a.c: multiple definition of `variable' main.o(.bss+0x0):main.c: first defined here ZenMasterJG, I don't think (a) will work, because I want this to be one global variable. I tried (c), but it seems that the section: (Toggle Plain Text) namespace myInclude { int variable = 0; } namespace myInclude { int variable = 0; } is being carried out twice. My understanding was that the ifndef would stop that, but I guess that must only work at the compiling stage? I guess I should really be doing it like (b) and setting the values in the classes that I want it.

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Programming Forums > Application Development > C++

multiple definition of variables in include files