extern struct array error: array type has incomplete element type

Question

I have 3 files: main.c , def.c , def.h . Both .c files include def.h . All the files are in the same directory. My compiler is gcc version 4.9.2 .

In def.h :

struct _info {
    int a;
};

In def.c :

#include "def.h"
struct _info info[] = {};

And in main.c :

#include "def.h"
extern struct _info info[];

When I build def.c as an object file and then build with main.c like:

gcc -c def.c
gcc main.c def.o

And I got an error message: array type has incomplete element type

---

If I use typedef to define struct _info as INFO like:

typedef struct _info INFO;

And replace struct _info with INFO in .c files. Compile ok then.

But why and what does typedef do?

Answer 1

Thanks for everybody's help. This question end up with a misspelling in main.c . Something like:

extern struct _infoo info[];

When typedef replace them, all work fine definitely.