"parameter has incomplete type" warning

Question

I have this in a C file:

struct T
{
    int foo;
};

the C file has an include to an h file with those lines:

typedef struct T T;
void listInsertFirst(T data, int key, LinkedList* ListToInsertTo);

the function listInsertFirst is the one I'm getting the warning on. How can I fix it?

Answer 1

As we've found out in the comments, the problem was that the definition of struct T occurred after the definition of T in the header. You really have things backwards here. The header should be defining all the types and function prototypes and your C files should be using them.

What you want to be doing instead is change the signature of your insert function to receive a pointer to your data and the size of the data. Then you can allocate some memory for the data, copy it and store it. You don't need a specific type, just declare it a void *.

void listInsertFirst(void *data, size_t data_size, int key, LinkedList* ListToInsertTo);

Then the caller would do something like this:

struct T { int foo; };
struct T x = { ... };
int someKey = ...;
LinkedList *someList = ...;
listInsertFirst(&x, sizeof x, someKey, someList);

Answer 2

When you include the header file, the compiler knows that T is a structure of unknown size and that listInsertFirst wants one as its first argument. But the compiler cannot arrange a call to listInsertFirst as it doesn't know how many bytes to push on the stack for the T data parameter, the size of T is only known inside the file where listInsertFirst is defined.

The best solution would be to change listInsertFirst to take a T* as its first argument so your header file would say this:

extern void listInsertFirst(T *data, int key, LinkedList* ListToInsertTo);

Then you get an opaque pointer for your T data type and, since all pointers are the same size (in the modern world at least), the compiler will know how to build the stack when calling listInsertFirst.