Alex Rivera | Logout

Append to GNU 'make' variables via the command line

Asked 2010-01-24T23:33:50.200
83

I am using a GNU Make Makefile to build a C project with several targets (all, clean, and a few project specific targets). In the process of debugging, I would like to append some flags to a single build without permanently editing the Makefile (e.g., add debugging symbols or set a preprocessor flag).

In the past, I have done that as follows (using the debugging symbols example):

make target CFLAGS+=-g

Unfortunately, this is not appending to the CFLAGS variable, but instead, clearing it and stopping it from compiling. Is there a clean way of doing this without defining some sort of dummy variable appended to the end of CFLAGS and LDFLAGS?

c makefile gnu-make
Edit
Report

1 Answer

9

Just a note, as I got confused - let this be file testmake:

$(eval $(info A: CFLAGS here is $(CFLAGS)))

override CFLAGS += -B

$(eval $(info B: CFLAGS here is $(CFLAGS)))

CFLAGS += -C

$(eval $(info C: CFLAGS here is $(CFLAGS)))

override CFLAGS += -D

$(eval $(info D: CFLAGS here is $(CFLAGS)))

CFLAGS += -E

$(eval $(info E: CFLAGS here is $(CFLAGS)))

Then: 

$ make -f testmake
A: CFLAGS here is 
B: CFLAGS here is -B
C: CFLAGS here is -B
D: CFLAGS here is -B -D
E: CFLAGS here is -B -D
make: *** No targets.  Stop.
$ make -f testmake CFLAGS+=-g
A: CFLAGS here is -g
B: CFLAGS here is -g -B
C: CFLAGS here is -g -B
D: CFLAGS here is -g -B -D
E: CFLAGS here is -g -B -D
make: *** No targets.  Stop.

With the override directives deleted from the testmake file:

$ make -f testmake
A: CFLAGS here is 
B: CFLAGS here is -B
C: CFLAGS here is -B -C
D: CFLAGS here is -B -C -D
E: CFLAGS here is -B -C -D -E
make: *** No targets.  Stop.
$ make -f testmake CFLAGS+=-g
A: CFLAGS here is -g
B: CFLAGS here is -g
C: CFLAGS here is -g
D: CFLAGS here is -g
E: CFLAGS here is -g
make: *** No targets.  Stop.

So,

  • if a variable used override once, it can only be appended with another statement with override (the normal assignments will be ignored);
  • when there's been no override at all; trying to append (as in +=) from the command line overwrites every instance of that variable.
answered 2013-04-17T04:06:12.597

Your Answer