alternative to defining struct in header file for 'error dereferencing pointer to incomplete type' in c

Question

I am aware that there are other questions about this which usually point to the standard solution of having both the struct and typedef struct declared and defined in the header file. However, I am looking at someone's else code which hasin the header:

struct A;
typedef struct A A_t;

Then the struct A is defined in *.c file together with the implementation of other functions. The implementation of A includes many defines, etc that are as well included in *.c not in the header.

Then in main.c the header is included and I have declared and defined new functions making use of (null) pointers to A_t which gives error dereferencing pointer to incomplete type when trying to access to a member of the struct.

Is there an alternative to declare and define the struct and typedef into the header for this case?

EDIT: Just to clarify that the original code foresees that the user creates some functions which are then passed to the main routines. For instance the user is supposed to create a function to connect to a socket, close a socket connection, transmit and receive data. Since I need an identifier/filedescriptor for the socket I have added an int to A 's definition which is what I can't dereference. I hope this clarifies the context.

Answer 1

You have two basic options:

1. Define the structure and all its members in a header file to be included in all the .c files that use this structure.
2. Leave the definition of the structure in a specific .c file which will also include the definitions of all the functions that access its members and those functions could be declared in a header file to be included in other .c files that need to manipulate the structure. Of course, you will still be able to use pointers to this structure in other .c files but you will not be able to access its members. This method may be viewed as is one of C 's ways of providing sort of encapsulation or information hiding .

Answer 2

If someone has coded it like this and you assume that someone is a decent enough programmer, he might have done this on purpose.

In a proper C API design, you might have the need to expose a pointer to an internal data structure in order to be able to store context over function calls and expect your library user to receive that pointer from you, store it and hand it back with the next call into your library.

In this case, the most simple answer is: You are not expected to mess with this pointer, and you are not expected to allocate one of those structures yourselves nor dereference a pointer to it. X11 is a famous example of doing that.

If you actually need to be able to dereference such a pointer or access structure members, you need access to the full structure definition in some header file. Your choice where you want to put the definition, you can have more than one (ie there is no such thing as "the" header file) header.

Following your edit, I would propose you do it like so:

Apparently, the creator of your library has taken a lot of care to not expose the structure outside his module - I would leave it like that.

You need to add an int to the structure and have done so, so you obviously have access to the .c source. Do that, put it into the C file and leave the structure definition there. Add a setInt(struct A*, int) and a getInt(struct A*) function that allows you to set and retrieve the int from such an opaque pointer (getter and setter functions). Expose those 2 functions in the header of the C file. This leaves the original intention of the information hiding intact but still allows you to extend the structure.