how to extern structure?

[dyu9999]

Hi,

I have a following error and would appreciate any help.

========================================================

* in foo.c I have

typedef struct _my_data_t {
int a;
int b;
} my_data_t;

* in foo.h, I have

typede struct _my_data_t my_data_t;

* in foo2.c, I have

#include "foo.h"

int value = some_func(((my_data_t *)x)->a);

gcc complains "dereferencing pointer to incomplete type"

What didI do wrong here ? Is it correct to to definition in foo.h ?
Thanks

 

[bd]

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Hi,

I have a following error and would appreciate any help.

========================================================

* in foo.c I have

typedef struct _my_data_t {
int a;
int b;
} my_data_t;

Put that in foo.h, and include foo.h into foo.c if needed.

* in foo.h, I have

typede struct _my_data_t my_data_t;

* in foo2.c, I have

#include "foo.h"

int value = some_func(((my_data_t *)x)->a);

gcc complains "dereferencing pointer to incomplete type"

What didI do wrong here ? Is it correct to to definition in foo.h ?
Thanks

The compiler must see the struct definition when each file is being
compiled, before the first dereference or sizeof on it. Note however you
can declare pointers to structs without definitions - 'incomplete types',
as in:
struct somestruct *ptr;

You cannot dereference this without a definition for struct somestruct. This
technique is often used for 'opaque' pointers used to hide details of a
module's functioning from the rest of a program.
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