[RFC 0/4] meson: Enable -Wundef

A few days ago I have posted a patch[1] that addresses an issue introduced when a meson check was dropped but some uses of the corresponding WITH_ macro were not removed at the same time. That got me thinking about what we can do to prevent such scenarios from happening again in the future. I have come up with something that I think would be effective, but since applying the approach throughout the entire codebase would require a non-trivial amount of work, I figured I'd ask for feedback before embarking on it. The idea is that there are two types of macros we can use for conditional compilation: external ones, coming from the OS or other libraries, and internal ones, which are the result of meson tests. The external ones (e.g. SIOCSIFFLAGS, __APPLE__) are usually only defined if they apply, so it is correct to check for their presence with #ifdef. Using #if will also work, as undefined macros evaluate to zero, but it's not good practice to use them that way. If -Wundef has been passed to the compiler, those incorrect uses will be reported (only on platforms where they are not defined, of course). The internal ones (e.g. WITH_QEMU, WITH_STRUCT_IFREQ) are similar, but in this case we control their definition. This means that using means that the feature is not available on the machine we're building on, but it could also mean that we've removed the meson check and forgot to update all users of the macro. In this case, -Wundef would work 100% reliably to detect the issue: if the meson check doesn't exist, neither will the macro, regardless of what platform we're building on. So the approach I'm suggesting is to use a syntax-check rule to ensure that internal macros are only ever checked with #if instead of Of course this requires a full sweep to fix all cases in which we're not already doing things according to the proposal. Should be fairly easy, if annoying. A couple of examples are included here for demonstration purposes. The bigger impact is going to be on the build system. Right now we generally only define WITH_ macros if the check passed, but that will have to change and the result is going to be quite a bit of additional meson code I'm afraid. Thoughts? [1] https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/message/S... Andrea Bolognani (4): configmake: Check for WIN32 correctly meson: Always define WITH_*_DECL macros syntax-check: Ensure WITH_ macros are used correctly meson: Enable -Wundef build-aux/syntax-check.mk | 5 +++++ configmake.h.in | 2 +- meson.build | 3 +++ tests/virmockstathelpers.c | 28 ++++++++++++++-------------- 4 files changed, 23 insertions(+), 15 deletions(-) -- 2.43.2