How do I pass CC compiler flags

[voidtrance]

Hi,

I am working on Python project that includes an extension module, which is compiled using meson-python. I am using the following Makefile target to compile the project:

PYTHON ?= $(shell which python3)
VENV_PYTHON := $(VENV)/bin/python3
DEBUG_OPTS :=

ifeq ($(DEBUG),1)
	DEBUG_OPTS=--config-settings=setup-args="-Dbuildtype=debug"
endif

all:
	$(PYTHON) -m pip install --no-build-isolation \
		--editable . $(DEBUG_OPTS)

I want to be able to pass -D<define> command line options to the compiler. However, I can't figure out how to do that. I tried using --config-settings=compile-args but that only passes arguments to ninja, not the CC compiler.

Thanks for the help.

[eli-schwartz]

It is handled using the traditional environment variable $CFLAGS.

[voidtrance]

That doesn't seem to work. Changing my makefile to:

DEBUG_OPTS :=
MESON_DEBUG_OPTS :=

ifeq ($(DEBUG),1)
	DEBUG_OPTS=CFLAGS=-DVORTEX_DEBUG
	MESON_DEBUG_OPTS=--config-settings=setup-args="-Dbuildtype=debug"
endif

all:
	$(DEBUG_OPTS) $(PYTHON) -m pip install --no-build-isolation \
		--editable . $(MESON_DEBUG_OPTS)
	@if [ ! -L compile_commands.json ]; then \
		ln -s build/cp$(word 1,$(PYTHON_VERSION_NUMS))$(word 2,$(PYTHON_VERSION_NUMS))/compile_commands.json \
			compile_commands.json; \
	fi

and then running make DEBUG=1 produces the following compile command (yes, there is an intentional error because I don't know how to tell meson to show all compile commands):

      + /usr/bin/ninja
      [1/5] Generating src/core/auto-events.h with a custom command
      [2/5] Generating src/core/logging.h with a custom command
      [3/5] Generating src/core/objects/auto-types.h with a custom command
      [4/5] Compiling C object src/core/objects/stepper.so.p/stepper.c.o
      FAILED: src/core/objects/stepper.so.p/stepper.c.o
      cc -Isrc/core/objects/stepper.so.p -Isrc/core/objects -I../../src/core/objects -Isrc/core -I../../src/core -Isrc/lib -I../../src/lib -Isrc/core/kinematics -I../../src/core/kinematics -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O0 -g -fPIC -MD -MQ src/core/objects/stepper.so.p/stepper.c.o -MF src/core/objects/stepper.so.p/stepper.c.o.d -o src/core/objects/stepper.so.p/stepper.c.o -c ../../src/core/objects/stepper.c
      ../../src/core/objects/stepper.c:223:13: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘++’ token
        223 | static int a++;
            |             ^~
      ninja: build stopped: subcommand failed.
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

As you can see, the -DVORTEX_DEBUG flag is not present on the command line.

[dnicolodi]

Enviroment variables are read only when the project is configured and ignored during compilation. You need to reconfigure the project for a new value of $CFLAGS to have effect. If you use an editable build, you can execute something like this:

meson setup --reconfigure build/$(python -c 'import sys; print("py"+"".join(map(str, sys.version_info[:2])))')