#include "main.h"
main()
{
assign();
printf("Value: %d",var);
}
#include <stdio.h>
int var;
void assign();
#include "main.h"
void assign()
{
var=5;
}
Problem with c/c++ linking under MinGW
Author
Programmers,
As always I must be missing something or some detail slips my attention. Here is the scenario, 3 files:
main.c
main.h
test.c
Quick, simple and pointles but this is just to ilustrate what I'm getting at.
To compile the files I use MinGW:
gcc -c -o main.o main.cpp
gcc -c -o test.o test.cpp
gcc -o test.exe main.o test.o
and all above works perfectly.
The problem appears when I change filenames to:
main.cpp
main.h
test.h
and try to compile the program with the same commands. When I do that I keep getting error:
multiple definition of `var'
and information that it was first defined in main.cpp
I understand the header was compiled more than once so I tried surrounding the main.h file with:
#ifndef MAIN_H
#define MAIN_H
.
.
.
#endif
but that doesn't help.
What am I doing worng?
Many thanks in advance
If I'm understanding what you're trying to say, and not what you've actually said, the problem is that C and C++ have different rules about global variable declarations and linkage, which is giving the difference in behavior that you're seeing. To properly declare a global variable in a header, you need to declare it as extern in the header file. Then in one source file, you need to define it at namespace scope (not inside a function or class). So in your main.h, you would have extern int var; and then in either main.cpp or text.cpp put int var;. For more details see this article.
Header files should not contain function or variable definitions.
In your example, "int var;" is a definition. What you want to do is make the fact that there is an int called var available. To do this use the keyword "extern".
This should compile:
One thing though, I would have named main.h test.h as main.h contains the function and variable declarations for test.c which isn't intuitive.
In your example, "int var;" is a definition. What you want to do is make the fact that there is an int called var available. To do this use the keyword "extern".
This should compile:
// main.c#include "main.h"int main(){ assign(); printf("Value: %d",var);}// main.h#include <stdio.h>extern int var;void assign();// test.c#include "main.h"int var = 0;void assign(){ var=5;}One thing though, I would have named main.h test.h as main.h contains the function and variable declarations for test.c which isn't intuitive.
Author
Thank you for your help.
I guess I need some reading on this. I've been mostly using pure C so far and things seemed to be working fine.
I guess I need some reading on this. I've been mostly using pure C so far and things seemed to be working fine.
This topic is closed to new replies.
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