I have the folthiwing header file:

/* bin_sort_and_search.h */

#ifndef BIN_SORT_AND_SEARCH_H_INCLUDED
#define BIN_SORT_AND_SEARCH_H_INCLUDED

void quick_sort_hoare(FILE *file);

int find_entry_with_binary_search(
    FILE *file, const char *file_name, const char *entry_name
);

#endif

If its body bin_sort_and_search.c starts with

/* bin_sort_and_search.c */

#include "bin_sort_and_search.h"
#include <stdio.h>
/* ... */

I get

error: unknown type name ‘FILE’

If I start it with

/* bin_sort_and_search.c */

#include <stdio.h>
#include "bin_sort_and_search.h"
/* ... */

or include stdio.h in my header file, everything will be fine.

I completely understand that this is due to the need for the type definition to precede any use of the type.

But doesn’t it contradict the concept of incomplete types?

I pre-defining a function header with an as-yet-unknown type, but does that really matter – the variable’s address is always the same size, so it can be found later.

Why doesn’t this work in practice?