Re: [libvirt] [PATCH] Fix build with GCC 8 new warnings

GCC 8 added a -Wcast-function-type warning to -Wextra by default. This complains if you try to explicitly cast one function signature to another function signature with incompatible args. This is something we do many times in libvirt especially with event callbacks. The hack to silence the warning requires casting via a void(*)(void) signature before casting to the desired type. This gross, so we just disable this new warning. GCC 8 also became more fussy about detecting switch fallthroughs. First it doesn't like it if you have a fallthrough attribute that is not before a case statement. e.g. FOO: BAR: WIZZ: ATTRIBUTE_FALLTHROUGH; Is unacceptable as there's no final case statement, so while FOO & BAR are falling through, WIZZ is not falling through. IOW, GCC wants us to write FOO: BAR: ATTRIBUTE_FALLTHROUGH; WIZZ: Second, it will report risk of fallthrough even if you have a case statement for every single enum value, but only if the switch is nested inside another switch and the outer case statement has no final break. This is is arguably valid because despite the fact that we have cast from "int" to the enum typedef, nothing guarantees that the variable we're switching on only contains values that have corresponding switch labels. e.g. int domstate = 87539319; switch ((virDomainState)domstate) { ... } will not match enum value, but also not raise any kind of compiler warning. So it is right to complain about risk of fallthrough if no default: is present. Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; --- m4/virt-compile-warnings.m4 | 2 ++ src/qemu/qemu_domain.c | 14 +++++++------- src/qemu/qemu_domain_address.c | 11 +++++++++++ 3 files changed, 20 insertions(+), 7 deletions(-) diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4 index b9c974842..4b35a6f6b 100644 --- a/m4/virt-compile-warnings.m4 +++ b/m4/virt-compile-warnings.m4 @@ -177,6 +177,8 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[ # with gl_MANYWARN_COMPLEMENT # So we have -W enabled, and then have to explicitly turn off... wantwarn="$wantwarn -Wno-sign-compare" + # We do "bad" function casts all the time for event callbacks + wantwarn="$wantwarn -Wno-cast-function-type" # GNULIB expects this to be part of -Wc++-compat, but we turn # that one off, so we need to manually enable this again diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c index 178ec24ae..560436f8e 100644 --- a/src/qemu/qemu_domain.c +++ b/src/qemu/qemu_domain.c @@ -175,9 +175,9 @@ qemuDomainAsyncJobPhaseToString(qemuDomainAsyncJob job, case QEMU_ASYNC_JOB_NONE: case QEMU_ASYNC_JOB_LAST: ATTRIBUTE_FALLTHROUGH; + default: + return "none"; } - - return "none"; } int