在Makefile中更改变量的值无效

Question

I am new to Makefile and I have written a simple Makefile to compile my C++ program.

TSP_OBU target checks if two libraries (lib_wave.a and libsaej2735.a) exists in the parent folder. If yes, the default value of 'LIBWAVE_PATH' and 'LIBSAEJ2735_PATH' variables are changed (in target LIBWAVE_check and LIBSAEJ2735_check). But apparently changing the 'LIBWAVE_PATH' and 'LIBSAEJ2735_PATH' variables do not have any effect and the default value (defined at the top of Makefile) is always used.

LIBWAVE_PATH = /home/ubuntu/VENTOS/source/libs/lib_wave.a
LIBSAEJ2735_PATH = /home/ubuntu/VENTOS/source/libs/libsaej2735.a

all: TSP_OBU

# link
TSP_OBU: TSP_OBU.o gps.o wave.o LIBWAVE_check LIBSAEJ2735_check
    g++ -std=c++11 -g -o TSP_OBU out/TSP_OBU.o out/gps.o out/wave.o -lboost_filesystem -lboost_system -lgps -lm -pthread $(LIBWAVE_PATH) $(LIBSAEJ2735_PATH)

# compile TSP_OBU
TSP_OBU.o: src/TSP_OBU.cc
    g++ -std=c++11 -Wall -g -O2 -c -o out/TSP_OBU.o src/TSP_OBU.cc

# compile gps
gps.o: src/gps.cc
    g++ -std=c++11 -Wall -g -O2 -c -o out/gps.o src/gps.cc

# compile wave
wave.o: src/wave.cc
    g++ -std=c++11 -Wall -g -O2 -c -o out/wave.o src/wave.cc

# check if 'lib_wave.a' exists
LIBWAVE_check:
ifeq (,$(wildcard ../lib_wave.a))
    ifeq (,$(wildcard $(LIBWAVE_PATH)))
        $(error 'lib_wave.a' does not exist in $(LIBWAVE_PATH))
    endif
else
    LIBWAVE_PATH=../lib_wave.a
endif

# check if 'libsaej2735.a' exists
LIBSAEJ2735_check:
ifeq (,$(wildcard ../libsaej2735.a))
    ifeq (,$(wildcard $(LIBSAEJ2735_PATH)))
        $(error 'libsaej2735.a' does not exist in $(LIBSAEJ2735_PATH))
    endif
else
    LIBSAEJ2735_PATH=../libsaej2735.a
endif

# clean
clean:
    rm -rf TSP_OBU
    rm -rf out/TSP_OBU.o
    rm -rf out/gps.o
    rm -rf out/wave.o

Answer 1

Consider a simpler version of this rule:

LIBWAVE_check:
    LIBWAVE_PATH=../lib_wave.a

Each command in a rule executes in its own sub-shell. The command LIBWAVE_PATH=... modifies the shell variable, which then expires when the command terminates, so the change has no effect.

To modify the Make variables in a way that the TSP_OBU rule can use, you have two options. You could modify them within the command that uses them:

TSP_OBU: ...
    LIBWAVE_PATH=../lib_wave.a; LIBSAEJ2735_PATH=../libsaej2735.a; g++ ...

Then for the conditionals you could either use shell conditionals within the command, or Make conditionals surrounding the command, a pain either way.

The other option (which I recommend) is to remove the "check" targets and put all of this logic outside the rules:

LIBWAVE_PATH = /home/ubuntu/VENTOS/source/libs/lib_wave.a
ifeq (,$(wildcard ../lib_wave.a))
  ifeq (,$(wildcard $(LIBWAVE_PATH)))
    $(error 'lib_wave.a' does not exist in $(LIBWAVE_PATH))
  endif
else
  LIBWAVE_PATH=../lib_wave.a
endif

...and likewise for LIBSAEJ2735_PATH .