[v3] Force the C standard to gnu99

[Qemu-devel] [PATCH v3 2/2] configure: Force the C standard to gnu99

Posted by Thomas Huth 6 years, 10 months ago

Different versions of GCC and Clang use different versions of the C standard.
This repeatedly caused problems already, e.g. with duplicated typedefs:

 https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

or with for-loop variable initializers:

 https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html

To avoid these problems, we should enforce the C language version to the
same level for all compilers. Since our minimum compiler versions is
GCC v4.8, our best option is "gnu99" for C code right now ("gnu17" is not
available there yet, and "gnu11" is marked as "experimental"), and "gnu++98"
for the few C++ code that we have in the repository.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v3: Compile C++ code with -std=gnu++98

 configure | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/configure b/configure
index b9f34af..448dbc8 100755
--- a/configure
+++ b/configure
@@ -107,6 +107,9 @@ update_cxxflags() {
             -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
             -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
                 ;;
+            -std=gnu99)
+                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }"-std=gnu++98"
+                ;;
             *)
                 QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
                 ;;
@@ -585,7 +588,7 @@ ARFLAGS="${ARFLAGS-rv}"
 # left shift of signed integers is well defined and has the expected
 # 2s-complement style results. (Both clang and gcc agree that it
 # provides these semantics.)
-QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
+QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv -std=gnu99 $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
 QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH v3 2/2] configure: Force the C standard to gnu99

Posted by Philippe Mathieu-Daudé 6 years, 10 months ago

On 1/9/19 5:39 PM, Thomas Huth wrote:
> Different versions of GCC and Clang use different versions of the C standard.
> This repeatedly caused problems already, e.g. with duplicated typedefs:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> or with for-loop variable initializers:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> 
> To avoid these problems, we should enforce the C language version to the
> same level for all compilers. Since our minimum compiler versions is
> GCC v4.8, our best option is "gnu99" for C code right now ("gnu17" is not
> available there yet, and "gnu11" is marked as "experimental"), and "gnu++98"
> for the few C++ code that we have in the repository.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>

Raising from a 30 years old stantard to a 20 years old one is a great
improvement (I used to be a defender of the C89, but I evolved and now
admit C99 has helpful features).

Using '-std=gnu++98' for g++ v4.8 looks like a good compromise to the
issues Daniel mentioned (still 'experimental').

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> ---
>  v3: Compile C++ code with -std=gnu++98
> 
>  configure | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index b9f34af..448dbc8 100755
> --- a/configure
> +++ b/configure
> @@ -107,6 +107,9 @@ update_cxxflags() {
>              -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
>              -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
>                  ;;
> +            -std=gnu99)
> +                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }"-std=gnu++98"
> +                ;;
>              *)
>                  QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
>                  ;;
> @@ -585,7 +588,7 @@ ARFLAGS="${ARFLAGS-rv}"
>  # left shift of signed integers is well defined and has the expected
>  # 2s-complement style results. (Both clang and gcc agree that it
>  # provides these semantics.)
> -QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
> +QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv -std=gnu99 $QEMU_CFLAGS"
>  QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
>  QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
>  QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
>

Re: [Qemu-devel] [PATCH v3 2/2] configure: Force the C standard to gnu99

Posted by Paolo Bonzini 6 years, 10 months ago

On 09/01/19 19:10, Philippe Mathieu-Daudé wrote:
> Using '-std=gnu++98' for g++ v4.8 looks like a good compromise to the
> issues Daniel mentioned (still 'experimental').
> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

C++11 has many new features that almost make it an entirely different
language (the main being rvalue references, and type inference with auto
and decltype).  If it works for GCC 4.8, I would prefer using that
instead of g++98.

Paolo

Re: [Qemu-devel] [PATCH v3 2/2] configure: Force the C standard to gnu99

Posted by Thomas Huth 6 years, 10 months ago

On 2019-01-09 22:26, Paolo Bonzini wrote:
> On 09/01/19 19:10, Philippe Mathieu-Daudé wrote:
>> Using '-std=gnu++98' for g++ v4.8 looks like a good compromise to the
>> issues Daniel mentioned (still 'experimental').
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> C++11 has many new features that almost make it an entirely different
> language (the main being rvalue references, and type inference with auto
> and decltype).  If it works for GCC 4.8, I would prefer using that
> instead of g++98.

I think we can revisit that setting when we really want to introduce
code that needs it. Currently it does not seem to be necessary, so that
does not justify to use an "experimental" feature of GCC 4.8, I guess.

 Thomas

Re: [Qemu-devel] [PATCH v3 2/2] configure: Force the C standard to gnu99

Posted by Richard Henderson 6 years, 10 months ago

On 1/10/19 3:39 AM, Thomas Huth wrote:
> Different versions of GCC and Clang use different versions of the C standard.
> This repeatedly caused problems already, e.g. with duplicated typedefs:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> or with for-loop variable initializers:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
> 
> To avoid these problems, we should enforce the C language version to the
> same level for all compilers. Since our minimum compiler versions is
> GCC v4.8, our best option is "gnu99" for C code right now ("gnu17" is not
> available there yet, and "gnu11" is marked as "experimental"), and "gnu++98"
> for the few C++ code that we have in the repository.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  v3: Compile C++ code with -std=gnu++98

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~