Question

There’s many instances of code similar to this in the codebase I’m working in these days. This is a thin C wrapper aroundthe OS API.

// From the OS
HANDLE CreateObject();
void CloseHandle(HANDLE);

typedef struct tagFOO {} FOO;

FOO* Foo_New()
{
    return (FOO*)CreateObject();
}

void Foo_Delete(FOO* foo)
{
    if(foo != NULL)
    {
        CloseHandle((HANDLE)foo);
    }
}

void Foo_Bar(FOO* foo)
{
    if(foo != NULL)
    {
        HANDLE h = (HANDLE)foo;
        // Do something interesting with h
    }
}

This seems to work and I want to avoid touching it if I can, but is this well defined? It seems very fishy to me

Answer 1

The C99 standard says:

6.7.2.1 Structure and union specifiers

Syntax

struct-or-union-specifier:
       struct-or-union identifieropt { struct-declaration-list }
       struct-or-union identifier
struct-or-union:
       struct
       union
struct-declaration-list:
       struct-declaration
       struct-declaration-list struct-declaration

Officially, that precludes your empty structure definition (and so does §6.5.2.1 in the C89 standard); there should be at least one member in the structure. However, you have to push GCC fairly hard to get it to complain; I used -std=c99 -pedantic and only with the -pedantic did it warn ‘struct has no members’.

In my view, you would be better off (more strictly portable) with:

typedef struct tagFOO FOO;

Or, even using:

typedef struct FOO FOO;