From: Phillip Wood Date: Tue, 16 Nov 2021 19:44:21 GMT Subject: Re: [PATCH v2 1/1] git-compat-util: add a test balloon for C99 support Message-ID: In-Reply-To: On 16/11/2021 12:19, Jeff King wrote: > On Tue, Nov 16, 2021 at 02:12:41AM +0000, brian m. carlson wrote: > >> The C99 standard was released in January 1999, now 22 years ago. It >> provides a variety of useful features, including variadic arguments for >> macros, declarations after statements, variable length arrays, and a >> wide variety of other useful features, many of which we already use. > > I like the idea of being able to assume C99. And I know this list is > just "here are some things we could do". But I'd like to express caution > over variable length arrays. We've already had problems with alloca() > causing stack exhaustion, and VLAs are basically the same thing. And the > worst part is there's no way to recover; you just get a segfault. I agree with this, also C11 and later make variable length array support an optional compiler feature which is another reason not to rely on them. Best Wishes Phillip >> Let's add a test balloon to git-compat-util.h to see if anyone is using >> an older compiler. We'll add a comment telling people how to enable >> this functionality on GCC and Clang, even though modern versions of both >> will automatically do the right thing, and ask people still experiencing >> a problem to report that to us on the list. >> >> Note that C89 compilers don't provide the __STDC_VERSION__ macro, so we >> use a well-known hack of using "- 0". On compilers with this macro, it >> doesn't change the value, and on C89 compilers, the macro will be >> replaced with nothing, and our value will be 0. > > This part makes sense to me. The macro check will complain if any > compiler isn't C99. But this hunk seems like it is going to cause > headaches: > >> diff --git a/Makefile b/Makefile >> index 12be39ac49..893d533d22 100644 >> --- a/Makefile >> +++ b/Makefile >> @@ -1204,7 +1204,7 @@ endif >> # Set CFLAGS, LDFLAGS and other *FLAGS variables. These might be >> # tweaked by config.* below as well as the command-line, both of >> # which'll override these defaults. >> -CFLAGS = -g -O2 -Wall >> +CFLAGS = -g -O2 -Wall -std=gnu99 >> LDFLAGS = >> CC_LD_DYNPATH = -Wl,-rpath, >> BASIC_CFLAGS = -I. > > Do most compilers understand -std=gnu99? It seems like we're breaking > the out-of-the-box build for everything that isn't gcc or clang. > > I understand that older versions of gcc (prior to 5.1.0, from my > digging) default to gnu89, and so they would be broken _without_ this. > So it is a tradeoff one way or the other. But somehow this seems > backwards to me. We should assume that modern compilers support C99 out > of the box, and put the burden on older ones to trigger C99 support in > whatever non-portable way they need. > > I also checked clang, and it looks like it has defaulted to gnu11 since > clang-7 (I'm not sure about clang-6; its documentation wasn't clear). > > -Peff >