extern keyword and structures

Hi folks,

I understand the use of extern for different type objects, but I'm having trouble using extern on a global structure from one file in another file.

file1.c declares and defines:

struct FREQ{
int a;
char str_freq[MAX_FREQ_STRING_SIZE];
}freq = {0," "};

struct FREQ *p_freq = &freq;


file2.h has:

extern struct FREQ freq;
extern struct FREQ *p_freq;

file2.c has:

freq.a = 1;

If I comment out the line in file2.c above, everything compiles fine.

Without file2.c/.h in the project, I can use this structure in any function in file1.c fine, but if I compile with file2.c/.h in the project, I get a compiler error: unknown size of struct or union .


If I bring the whole structure declaration to file2.h and declare it as extern as follows:

extern struct FREQ{
int a;
char str_freq[MAX_FREQ_STRING_SIZE];
}freq;

I get a compiler error: redefinition tag of struct FREQ
even though it's declared as extern.

I have no problem with external int, char...etc.., but this external structure declaration is giving me fits.
I'm missing something here, can anyone help me?

Thanks alot in advance.

Quote:

---

extern struct FREQ{
int a;
char str_freq[MAX_FREQ_STRING_SIZE];
}freq;

---

This is invalid. You don't declare this this way. What you need is a header file with the definition of the structure in it:

MYHEADER.H
struct FREQ {
int a;
char str_freq[MAX_FREQ_STRING_SIZE];
};


Now then, in file1, you have:

struct FREQ someVar;

And in file2, to access it, you use:

extern struct FREQ someVar;

Then, for both of them, you use the header defining the structure type.

Quzah.

Thanks,

Yes you are so right. The thing that kept getting me was that I could declare other types as extern such as:

extern int a;
extern char b;

The difference is that the compiler can see the type in this extern declaration.


Thanks again.:)