Merge branch 'alignof' into 'master'

use _Alignof to avoid UB in ALIGNOF

See merge request xorg/lib/libxcb!42

configure.ac

@@ -40,6 +40,11 @@ fi
 PKG_CHECK_MODULES(CHECK, [check >= 0.9.6], [HAVE_CHECK=yes], [HAVE_CHECK=no])
 AM_CONDITIONAL(HAVE_CHECK, test x$HAVE_CHECK = xyes)
 
+AX_CHECK_COMPILE_FLAG([-std=c11], [], [
+  echo "C compiler cannot compile C11 code"
+  exit -1
+])
+
 XSLTPROC=no
 HTML_CHECK_RESULT=no
 if test x"$HAVE_CHECK" = xyes; then

m4/ax_check_compile_flag.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS

src/c_client.py

@@ -288,7 +288,6 @@ def c_open(self):
     _c('#include "%s.h"', _ns.header)
 
     _c('')
-    _c('#define ALIGNOF(type) offsetof(struct { char dummy; type member; }, member)')
 
     if _ns.is_ext:
         for (n, h) in self.direct_imports:
@@ -1266,7 +1265,7 @@ def _c_serialize_helper_fields(context, self,
             count += 1
 
         code_lines.append(
-            '%s    xcb_align_to = ALIGNOF(%s);'
+            '%s    xcb_align_to = _Alignof(%s);'
              % (space,
                  'char'
                   if field.c_field_type == 'void' or field.type.is_switch