On Tue, Dec 07, 2021 at 08:13:43PM -0800, Junio C Hamano wrote: > Jeff King <peff@xxxxxxxx> writes: > > >> error: ISO C99 doesn't support unnamed structs/unions [-Werror=pedantic] > > > > Hmm. It's interesting that the regular DEVELOPER=1 doesn't catch this. > > It's because we don't specify -std there, and newer gcc defaults to > > gnu17 (unnamed unions appeared in c11, I think). I wonder if it would be > > helpful to teach config.mak.dev to pass -std=c99. > > FWIW, I use -std=gnu99 as our Makefile suggests. I think the patch below would have detected this both locally for Han-Wen, but also in C. -- >8 -- Subject: [PATCH] config.mak.dev: specify -std=gnu99 for gcc/clang The point of DEVELOPER=1 is to turn up the warnings so we can catch portability or correctness mistakes at the compiler level. But since modern compilers tend to default to modern standards like gnu17, we might miss warnings about older standards, even though we expect Git to build with compilers that use them. So it's helpful for developer builds to set the -std argument to our lowest-common denominator. Traditionally this was c89, but since we're moving to assuming c99 in 7bc341e21b (git-compat-util: add a test balloon for C99 support, 2021-12-01) that seems like a good spot to land. And as explained in that commit, we want "gnu99" because we still want to take advantage of some extensions when they're available. The new argument kicks in only for clang and gcc (which we know to support "-std=" and "gnu" standards). And only for compiler versions which default to a newer standard. That will avoid accidentally silencing any build problems that non-developers would run into on older compilers that default to c89. My digging found that the default switched to gnu11 in gcc 5.1.0. Clang's documentation is less clear, but has done so since at least clang-7. So that's what I put in the conditional here. It's OK to err on the side of not-enabling this for older compilers. Most developers (as well as CI) are using much more recent versions, so any warnings will eventually surface. A concrete example is anonymous unions, which became legal in c11. Without this patch, "gcc -pedantic" will not complain about them, but will if we add in "-std=gnu99". Signed-off-by: Jeff King <peff@xxxxxxxx> --- config.mak.dev | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/config.mak.dev b/config.mak.dev index 7673fed114..d4afac6b51 100644 --- a/config.mak.dev +++ b/config.mak.dev @@ -19,6 +19,11 @@ endif endif endif endif + +ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),) +DEVELOPER_CFLAGS += -std=gnu99 +endif + DEVELOPER_CFLAGS += -Wdeclaration-after-statement DEVELOPER_CFLAGS += -Wformat-security DEVELOPER_CFLAGS += -Wold-style-definition -- 2.34.1.503.gbf394de69d
