Multiple hops in Makefile - c++

Question

I have a C/C++ mixed project and for each source code, I want to generate an assembly file (.S) and then an object file (.o) from the assembly. I thought the following Makefile should work:
all: a.o b.o main.o
g++ $^ -o main
%.o: %.S
gcc -o $# $<
%.S: %.c
gcc -S -o $# $<
%.S: %.cc
g++ -S -o $# $<
clean:
rm -rf *.o main
Ideally, in order to generate X.o, X.S needs to be generated by using one of the rules depending on whether X.c or X.cpp is available.
However, make apparently doesn't work the way I imagined. It resorts to the default rule for X.o and neither %.S: %.c nor %.S: %.cpp was applied... make -n gives
cc -c -o a.o a.c
g++ -c -o b.o b.cc
g++ -c -o main.o main.cc
g++ a.o b.o main.o -o main
The following example code can reproduce this:
// a.c
int a() { return 0;}
// b.cc
int b() { return 0;}
// main.cc
extern "C" int a();
extern int b();
int main() { a(); b(); return 0; }
I must have some misunderstanding of how a target is resolved. Any idea?

Answer 1

Use .SUFFIXES: at the top of your Makefile to flush predefined suffixes. Working example:
$tail -n +1 a.c b.cc main.cc Makefile; make clean; make; ./main
==> a.c <==
#include <stdio.h>
void a(void) {
printf("a\n");
}
==> b.cc <==
#include <stdio.h>
void b(void) {
printf("b\n");
}
==> main.cc <==
extern "C" {
void a(void);
}
void b(void);
int main(void) {
a();
b();
}
==> Makefile <==
.SUFFIXES:
all: main.o a.o b.o
g++ $^ -o main # all
%.o: %.S
gcc -c -o $# $< #o
%.S: %.c
gcc -S -o $# $< #Sc
%.S: %.cc
g++ -S -o $# $< #Scc
clean:
rm -rf *.o main
rm -rf *.o main
g++ -S -o main.S main.cc #Scc
gcc -c -o main.o main.S #o
gcc -S -o a.S a.c #Sc
gcc -c -o a.o a.S #o
g++ -S -o b.S b.cc #Scc
gcc -c -o b.o b.S #o
g++ main.o a.o b.o -o main # all
rm b.S main.S a.S
a
b

Answer 2

You need to cancel the implicit rules as they are a better match, add the following lines
%.o: %.cc
%.o: %.c
Note that your rule for all is broken as it doesn't create a file called all, make already knows how to assemble object files from assembly, and it also already knows how to link object files into a program if one of the object files matches the target
assembly := a.s b.s main.s
objects := $(assembly:.s=.o)
.SECONDARY: $(assembly)
main: CC := $(CXX)
main: $(objects)
%.s: %.c
$(CC) $(CFLAGS) $(CPPFLAGS) -S -o $# $<
%.s: %.cc
$(CXX) $(CXXFLAGS) $(CPPFLAGS) -S -o $# $<
%.o: %.cc
%.o: %.c
clean: ; $(RM) $(objects) $(assembly) main