Makefile: override CFLAGS and CPPFLAGS

[ffontaine]

Makefile specifies some CFLAGS and CPPFLAGS values, but it may be needed to pass additional custom flags on the make command line (e.g. when cross-compiling). To make this possible, we switch from a plain "+=" operator to the "override ... +=" operator.

Don't override the first occurence of CFLAGS to allow the user to finely tune optimisation flags and to avoid hardcoding -Werror which will raise unexpected build failures with old or new toolchains.

Signed-off-by: Fabrice Fontaine [email protected]

[mutability]

Hi! Do you have a concrete example of how you're trying to invoke make with overridden CFLAGS/CPPFLAGS?

[ffontaine]

Sure, here is an example of the issue:

make CPPFLAGS="-I/usr/local/include"
Building with:
  Version string:  unknown
  DSP mix:         x86
  RTLSDR support:  no
  BladeRF support: no
  HackRF support:  no
  LimeSDR support: no
cc -I/usr/local/include -std=c11 -O3 -g -Wall -Wmissing-declarations -Werror -W -D_DEFAULT_SOURCE -fno-common -c dump1090.c -o dump1090.o
In file included from dump1090.h:84,
                 from dump1090.c:50:
dsp/generated/starch.h:4:10: fatal error: dsp-types.h: Aucun fichier ou dossier de ce type
    4 | #include "dsp-types.h"
      |          ^~~~~~~~~~~~~

Overriding CFLAGS and CPPFLAGS are especially useful when cross-compiling (e.g. with buildroot)

[mutability]

Ok. What I might do instead is move the internally-generated options into a separate var and have only the stuff that really is overridable be in CFLAGS/CPPFLAGS.

[ffontaine]

Indeed, that's another and probably better option.

[mutability]

Does b645f7d cover what you need?

[ffontaine]

At first sight, -Werror should be moved from DUMP1090_CFLAGS to CFLAGS. Indeed, hardcoding -Werror will raise unexexpected build failures with old or new toolchains.

[mutability]

The issue is that I really want -Werror in there by default, and the Debian toolchain is going to be providing CFLAGS that does not contain -Werror. What's the specific case you see where there are warnings generated?

(also, you could pass -Wno-error=whatever in CFLAGS)

[ffontaine]

Then, I would suggest to add -Werror in its own variable (e.g. DUMP1090_WERROR) so I could disable it in buildroot.
From my experience, adding -Werror can break the build at each major gcc release, for example when gcc 10 decided to default to -fno-common (#65). More information on this topic can be found here: https://embeddedartistry.com/blog/2017/05/22/werror-is-not-your-friend/

[mutability]

I understand your desire to have it "just build", but I really would prefer that you have to explicitly do something to say "hey, I know better than the toolchain". The -fno-common issue is actually a good example of this - ignoring that warning would have hidden a longstanding bug.

Does -Wno-error=specific-warning-that-I-have-examined in CFLAGS not work for you?

[mutability]

Actually I revisited the -fno-common issue and I think that was an outright (link) error, not a warning promoted to error by -Werror anyway