Re: [PATCH cb/pedantic-build-for-developers] lazyload.h: fix warnings about mismatching function pointer types

From: Carlo Arenas <carenas@gmail.com>
Date: Sep 22, 2021, 20:16 UTC
Message-ID: <CAPUEspguHsx+BtViT5tO3eyZSJf7mJ0of-K3vAMqB0-ju4fpsg@mail.gmail.com>
In-Reply-To: <4381472f-a9db-b8a7-a395-81c3935309ae@kdbg.org>

On Wed, Sep 22, 2021 at 12:56 PM Johannes Sixt <j6t@kdbg.org> wrote:

Show 15 quoted lines

>
> Here, GCC warns about every use of the INIT_PROC_ADDR macro, for example:
>
> In file included from compat/mingw.c:8:
> compat/mingw.c: In function 'mingw_strftime':
> compat/win32/lazyload.h:38:12: warning: assignment to
>    'size_t (*)(char *, size_t,  const char *, const struct tm *)'
>    {aka 'long long unsigned int (*)(char *, long long unsigned int,
>       const char *, const struct tm *)'} from incompatible pointer type
>    'FARPROC' {aka 'long long int (*)()'} [-Wincompatible-pointer-types]
>    38 |  (function = get_proc_addr(&proc_addr_##function))
>       |            ^
> compat/mingw.c:1014:6: note: in expansion of macro 'INIT_PROC_ADDR'
>  1014 |  if (INIT_PROC_ADDR(strftime))
>       |      ^~~~~~~~~~~~~~

did you have CFLAGS adding -Wincompatible-pointer-types explicitly?

This is the reason why the code that got merged to master had -Wno for this case.

> (message wrapper for convenience). Insert a cast to keep the compiler
> happy. A cast is fine in these cases because they are generic function
> pointer values that have been looked up in a DLL.

I have a more complete "fix" which I got stuck testing GGG[1]; you are likely going to also hit -Wcast-function-type otherwise.

>
> Signed-off-by: Johannes Sixt <j6t@kdbg.org>
> ---
>  How can this have worked ever without a warning?

POSIX have a specific exception that allows (void *) for this, it is incorrect though in platforms where pointers to code and data might be different (ex DOS with its segmented model)

Carlo

[1] https://github.com/git/git/pull/1094