Re: [PATCH for 7.2] Fix broken configure with -Wunused-parameter

[Peter Maydell]

On Wed, 2 Nov 2022 at 20:24, Stefan Weil via <qemu-devel@nongnu.org> wrote:
>
> The configure script fails because it tries to compile small C programs
> with a main function which is declared with arguments argc and argv
> although those arguments are unused.
>
> Running `configure -extra-cflags=-Wunused-parameter` triggers the problem.
> configure for a native build does abort but shows the error in config.log.
> A cross build configure for Windows with Debian stable aborts with an
> error.
>
> Avoiding unused arguments fixes this.
>
> Signed-off-by: Stefan Weil <sw@weilnetz.de>
> ---
>
> See https://gitlab.com/qemu-project/qemu/-/issues/1295.
>
> I noticed the problem because I often compile with -Wextra.
>
> Stefan
>
>  configure | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/configure b/configure
> index 4275f5419f..1106c04fea 100755
> --- a/configure
> +++ b/configure
> @@ -1258,6 +1258,7 @@ if test "$stack_protector" != "no"; then
>    cat > $TMPC << EOF
>  int main(int argc, char *argv[])
>  {
> +    (void)argc;

I'm not a huge fan of this syntax, and it doesn't match the way
we deal with "argument is unused" elsewhere in the codebase
(where we either don't care about it or else use the GCC 'unused'
attribute hidden behind the glib G_GNUC_UNUSED macro).

I am surprised that this didn't get caught by the check in
do_compiler_werror(), which is supposed to report "this
configure test passed without -Werror but failed with
-Werror, so configure is probably buggy.". That's what's
supposed to catch "your compiler warns on stuff our doesn't
in the test case programs".

If you're building with --disable-werror then configure
should be OK anyway. This is probably a good idea if you want
to build with extra warning arguments in --extra-cflags.
If it doesn't work right even with --disable-werror that's
also something we should investigate.

>      char arr[64], *p = arr, *c = argv[0];
>      while (*c) {
>          *p++ = *c++;
> @@ -1607,7 +1608,7 @@ fi
>
>  if test "$safe_stack" = "yes"; then
>  cat > $TMPC << EOF
> -int main(int argc, char *argv[])
> +int main(void)
>  {
>  #if ! __has_feature(safe_stack)
>  #error SafeStack Disabled
> @@ -1629,7 +1630,7 @@ EOF
>    fi
>  else
>  cat > $TMPC << EOF
> -int main(int argc, char *argv[])
> +int main(void)
>  {
>  #if defined(__has_feature)
>  #if __has_feature(safe_stack)
> @@ -1675,7 +1676,7 @@ static const int Z = 1;
>  #define TAUT(X) ((X) == Z)
>  #define PAREN(X, Y) (X == Y)
>  #define ID(X) (X)
> -int main(int argc, char *argv[])
> +int main(void)
>  {
>      int x = 0, y = 0;
>      x = ID(x);

No objection to the cases where we can pass "void", that's
a neater way to write the test anyway.

thanks
-- PMM