configure: Check if __attribute__((packed)) works as expected

This is really hard to detect if not, and is not unlikely. If -mms-bitfields
is given, the attribute does not work. Even worse, that switch is by default
on with GCC/MinGW 4.7+ for Windows targets.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52991

diff --git a/configure.ac b/configure.ac
index ca5c636683fa609586c119a7498abdd736376980..4e714237cf56a298f8be5cee9d6d754e2dd804e1 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -755,6 +755,17 @@ AC_COMPILE_IFELSE(
 AC_SUBST(OPENSSL_LIB, [-l$openssl_lib])
 AM_CONDITIONAL(USE_WINDOWS, [test "x$windows" = xtrue])
 
+AC_MSG_CHECKING([for working __attribute__((packed))])
+AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM([], [[
+               struct test { char a; short b; } __attribute__((packed));
+               char x[sizeof(struct test) == sizeof(char) + sizeof(short) ? 1 : -1];
+               return 0;
+       ]])],
+       [AC_MSG_RESULT([yes])],
+       [AC_MSG_RESULT([no]); AC_MSG_ERROR([__attribute__((packed)) does not work])]
+)
+
 if test x$printf_hooks = xvstr; then
        AC_CHECK_LIB([vstr],[main],[LIBS="$LIBS"],[AC_MSG_ERROR([Vstr string library not found])],[])
        AC_DEFINE([USE_VSTR], [], [use Vstr string library for printf hooks])