6

I'm writing little student project and stuck with the problem that I have a few global variables and need to use it in a few source files, but I get the error undefined reference to variable_name. Let's create three source files for example:

tst1.h:

extern int global_a; void Init();

tst1.cpp:

#include "tst1.h" void Init(){ global_a = 1; }

tst2.cpp:

#include "tst1.h" int main(){ Init(); }

When I compile and link, that's what I get:

$ g++ -c tst1.cpp $ g++ -c tst2.cpp $ g++ tst2.o tst1.o tst1.o: In function `Init()': tst1.cpp:(.text+0x6): undefined reference to `global_a' collect2: error: ld returned 1 exit status

If I remove the extern statement, then I get the other problem, let me show:

$ g++ -c tst1.cpp $ g++ -c tst2.cpp $ g++ tst2.o tst1.o tst1.o:(.bss+0x0): multiple definition of `global_a' tst2.o:(.bss+0x0): first defined here collect2: error: ld returned 1 exit status

I really need some variables to be global, for example my little project works with assembly code, and have a variables like string rax = "%rax %eax %ax %ah %al"; which should be referenced through different source files.

So, how to properly initialize the global variables?

Improve this question
3
  • 2
    The best solution for this is to not use globals. For a hint on how you can avoid them: dependency injection. Commented Dec 23, 2013 at 19:17
  • @Tim, I know, that global variables is bad style, but in some situations it is the right way. For example if my project works with assembly, then many files shall know the names of CPU registers, and the variable string rax = "%rax %eax %ax %ah %al"; should be referenced through different source files. Commented Dec 23, 2013 at 19:27
  • In general, static class members should be preferred for this. Commented Dec 23, 2013 at 19:35

3 Answers 3

Reset to default
7

You only declared the variable but not defined it. This record

extern int global_a;

is a declaration not a definition. To define it you could in any module to write

int global_a;

Or it would be better to define function init the following way

int Init { /* some code */; return 1; }

and in main module before function main to write

int global_a = Init();
Improve this answer
Sign up to request clarification or add additional context in comments.

6 Comments

As I've commented below, the compiler says tst1.cpp:2:5: error: redefinition of ‘int global_a.
I am sorry I have not understood what you commented. I showed a relevant code. Are you unable simply to copy and paste it?
I am simply wrote in tst1.cpp, in global scope int global_a;. The compiler gives the error.
You shall keep the declaration in the header. I wrote what you need. What is the problem?
Oh, sorry, i'm an idiot :) I through some experiments removed the extern statement. That's right.
|
2

tst1.cpp should read instead:

#include "tst1.h" int global_a = 1; void Init(){ }

You can also write the initializer line as:

int global_a(1);

Or in C++11:

int global_a{1};

A global should only be defined (i.e. written without the extern prefix) in one source file, and not in a header file.

Improve this answer

5 Comments

No, that's what I get: tst1.cpp:2:5: error: redefinition of ‘int global_a’ int global_a = 33; ^ In file included from tst1.cpp:1:0: tst1.h:1:5: error: ‘int global_a’ previously declared here int global_a; ^
Sorry, for the formatting. Shortly it says tst1.cpp:2:5: error: redefinition of ‘int global_a’
I was assuming your tst1.h had extern int global_a;, as you wrote in the question, not just int global_a;.
that's right, but there was two right answer in the same time. I choose one, which autor brought me to the answer. Sorry, there is no way to choose two right answer, although I wanted and tried.
That's fine. It's just annoying when the code someone posts and the code they're trying to build aren't the same.
0

you need to to add

#ifndef TST1_H #define TST1_H ..... #endif

to tst1.h. it included twice in tst2.cpp

Improve this answer

1 Comment

No, it's not included twice, it's compiled twice (in different translation units). An include guard doesn't solve this.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.