CODEWITHSUNDEEP
carraysheader-files
bob.sacamento
bob.sacamento

Reputation: 6651

need a workaround for a "multiple definition" error

A toy code illustrating my problem is as follows:

stuff.h:

#ifndef STUFF
#define STUFF

int a;
int testarr[]={1,2,3};

#endif

fcn.h:

#include "stuff.h"
int b[]={5,6,7};
void fcn();

main.h:

#include "stuff.h"
#include <stdio.h>

fcn.c:

#include "main.h"

void fcn() {
   printf("Hello\n");
}

main.c:

#include "main.h"
#include "fcn.h"

int main() {
   fcn();
   printf("HI\n");
}

An attempt to compile fails with:

/g/pe_19976/fcn_2.o:(.data+0x40): multiple definition of `testarr'
/g/pe_19976/main_1.o:(.data+0x40): first defined here

After doing some reading, I realize that defining the array testarr in the header file is a problem. But the thing is, in my real code, several files need access to testarr and it needs to have the same assigned values everywhere. I guess I could put it in main.h (?) but even if that would work, in my real code it logically belongs in stuff.h. How do I solve this conundrum?

BTW, based on something else I found, I tried defining testarr as extern but got the same problem.

Upvotes: 3

Views: 75

Answers (2)

Vlad from Moscow
Vlad from Moscow

Reputation: 310960

It is not clear why you are using so many global variables. The array

int testarr[]={1,2,3};

is defined as many times as there are compilation units (in your example there are at least two compilation units) that include the corresponding header.

Declare the array in a header like

extern int testarr[3];

and define it in a cpp module.

int testarr[]={1,2,3};

The same is valid for other global variables that have external linkage.

As for this remark

BTW, based on something else I found, I tried defining testarr as extern but got the same problem.

Then the array with the specifier extern shall not be initialized in a header. Otherwise it is a definition of the array.

Upvotes: 0

dbush
dbush

Reputation: 223739

When you put a variable definition into a header file, any .c file that includes it will have a copy of that variable. When you then attempt to link them, you get a multiple definition error.

Your header files should contain only a declaration of the variable. This is done using the extern keyword, and with no initializer.

Then in exactly one .c file, you put the definition along with an optional initializer.

For example:

main.c:

#include "main.h"
#include "fcn.h"

int a;
int testarr[]={1,2,3};

int main() {
   fcn();
   printf("HI\n");
}

stuff.h:

#ifndef STUFF
#define STUFF

extern int a;
extern int testarr[];

#endif

fcn.h:

#include "stuff.h"
extern int b[];
void fcn();

fcn.c:

#include "main.h"

int b[]={5,6,7};

void fcn() {
   printf("Hello\n");
}

Upvotes: 3

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