I have the following structure for a project and I am just starting to introduce a Makefile to build the software:
├── Makefile
├── README.md
├── cg
│ └── cg.c
└── utilities
├── utilities.c
└── utilities.h
I am trying to put object files in a directory called obj yet I can't seem to get it working.
My makefile looks like:
CC=mpicc
CFLAGS=-O3 -std=c99
LIBS=
MKDIR_P = mkdir -p
make_build_dir:
@mkdir -p obj/
utilities.o: utilities/utilities.c
$(CC) $(CFLAGS) -o ./obj/$@ -c $<
cg.o: cg/cg.c
$(CC) $(CFLAGS) -o ./obj/$@ -c $<
.PHONY: make_build_dir
cg.exe: make_build_dir utilities.o cg.o
$(CC) $(CFLAGS) -o $@ $<
clean:
rm -fr obj
rm cg.exe
Yet this generates the following error:
a@a:b/b ‹master*›$ make cg.exe
mpicc -O3 -std=c99 -o ./obj/utilities.o -c utilities/utilities.c
mpicc -O3 -std=c99 -o ./obj/cg.o -c cg/cg.c
cg/cg.c:133:3: warning: implicit declaration of function 'decompose' is invalid in C99
[-Wimplicit-function-declaration]
decompose(num_chunks, chunks_per_rank,me, &settings);
^
1 warning generated.
mpicc -O3 -std=c99 -o cg.exe make_build_dir
clang: error: no such file or directory: 'make_build_dir'
make: *** [cg.exe] Error 1
How can I get it to generate the object files in the obj directory and then an executable in the top-level directory?
This linking part of the makefile
cg.exe: make_build_dir utilities.o cg.o
$(CC) $(CFLAGS) -o $@ $<
has two issues. First, $< refers to the first prerequesite of the target cg.exe, and that is make_build_dir. Declaring it as .PHONY doesn't help here, it's simply passed to $(CC). Second, utilities.o cg.o both don't exist at this location. You can change the rule to
cg.exe: obj/utilities.o obj/cg.o
$(CC) $(CFLAGS) -o $@ $^
Note the automatic variable $^ which refers to all prerequisites. Additionally, the object file targets should be
obj/cg.o: cg/cg.c
$(CC) $(CFLAGS) -o $@ -c $<
(identical for utilities.o).
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