Dilemer using target-specific or pattern-specific variables

[Chen Jun （陈军）]

I'm using GNU make 3.81 .

I want to filter out -DNDEBUG compiler options(cflags) when compiling my.obj, so the makefile:

# Makefile start.
all: my.obj his.obj
	@echo "linking $^"

cflags = -c -DNDEBUG -O2

%.obj:
	@echo "Compile $@: $(cflags)"

%.c:
	@echo "$@" > $@

### special, target specific variable assignment >>>
my.obj: cflags := $(filter-out -DNDEBUG,$(cflags))
### special <<<
# Makefile end.

The output is:

Compile my.obj: -c -O2
Compile his.obj: -c -DNDEBUG -O2
linking my.obj his.obj

The output was in my expectation so far.

I have to use ``cflags := '' in the last statement instead of ``cflags = '', because the latter would cause a "references itself" error for cflags. HOWEVER, I immediately realized that using := above is an immediate expasion , that is, cflags expanded when that line is read by GNU make, instead of when my.obj is being made.

To illustrate this, see the following makefile:

# Makefile start.
all: my.obj his.obj
	@echo "linking $^"

cflags = -c -DNDEBUG -O2

%.obj:
	@echo "Compile $@: $(cflags)"

%.c:
	@echo "$@" > $@

### special >>>
LateDef += -D LATEDEF1

my.obj: cflags := $(LateDef) $(filter-out -DNDEBUG,$(cflags))

LateDef += -D LATEDEF2
### special <<<

# Makefile end.

The output now is:

Compile my.obj: -D LATEDEF1 -c -O2
Compile his.obj: -c -DNDEBUG -O2
linking my.obj his.obj

You see that ``-D LATEDEF2'' is not appended for my.obj – which is what I don't expect.

So, I think it is reasonble that expansion of any target/pattern specific variable assignment should be deferred when the corresponding target is being made.

By the way, if make provides a ``-='' operation that filters out a word, it would still be better.

Any solutions to this problem? Hoping to hear your response.

chenjun.vcf
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