Question

I have structure declaration in a header
globalStruct.h

#ifndef _GLOBALSTRUCT_H
#define _GLOBALSTRUCT_H
typedef struct 
    {
        
        Int32   frameID;
        Int32   slotIndx;
        Int32   symNumber;

    }   recControlList;     
        recControlList  *recControlListPtr;

#endif

fun.h is

include "globalStruct.h"

void fun( recControlList *recControlListPtr);

Inside File1.c I do the following

#include "globalStruct.h"
#include "fun.h"

recControlList  temp;
recControlListPtr=&temp;

fun( recControlListPtr); //Function prototype is declared in some fun.h file.

fun.c looks like

#include "globalStruct.h"

void fun( recControlList *recControlListPtr)

I get linker error: _recControlListPtr defined multiple times.
one in File1.c and other fun.c

I don’t understand what is wrong here? globalStruct needs extern?

Thanks

Answer 1

Include guards won’t help you here. They will stop a single compilation unit from getting two copies of the data if it includes the header file twice but you have two totally separate compilation units here. The compiler will put a recControlListPtr in both of the object files.

Then, when you link those object files together, the linker will complain because there are two instances of that symbol.

What you need to do is ensure that only one instance exists. In your header file, replace:

recControlList  *recControlListPtr;

with:

extern recControlList  *recControlListPtr;

Then, in one of your C files, put:

recControlList  *recControlListPtr;

---

Bottom line: try to avoid defining anything in header files. Declaring things (stating that they exist, like prototypes, externs, typedefs, structs and so on) is okay, defining (allocating space for variables, giving the bodies of functions ans so on) is generally not.