Makefile 变量未按预期工作

Question

I am trying to compile a program using this makefile:

GCCPARAMS = -m32 -fno-use-cxa-atexit -nostdlib -fno-builtin -fno-rtti -fno-exceptions -fno-leading-underscore
ASPARAMS = --32

objects = src/loader.o src/kernel.o

%.o: src/%.cpp
    gcc $(GCCPARAMS) -c -o $@ $<

%.o: src/%.s
    echo $(ASPARAMS)
    as $(ASPARAMS) -o $@ $<

kernel.bin: linker.ld $(objects)
    ld $(LDPARAMS) -T $< -o $@ $(objects)

install: kernel.bin
    sudo cp $< boot/mykernel.bin

The problem comes when doing make which prints the following:

as   -o src/loader.o src/loader.s
src/loader.s: Assembler messages:
src/loader.s: Warning: end of file not at end of a line; newline inserted
src/loader.s:18: Error: operand type mismatch for `push'
src/loader.s:19: Error: operand type mismatch for `push'
make: *** [<builtin>: src/loader.o] Error 1

As you can se the variable is not properly set, instead, make puts a space where the variable should be. That causes the assembler compiler to think it's a 64 bit architecture and giving some errors.

Any idea of what is going on? I am doing this using WSL2 by the way.

Answer 1

The reason you see this is because your recipe rules are wrong.

You have these object files:

objects = src/loader.o src/kernel.o

and you have this rule:

%.o: src/%.s
        echo $(ASPARAMS)
        as $(ASPARAMS) -o $@ $<

When trying to build src/loader.o what does the stem of the pattern %.o match? It will match src/loader of course. So then what will the prerequisite be? It will be src/%.s with the stem expanded, so it will be src/src/loader.s . I presume that this doesn't exist.

As a result of this, this pattern rule doesn't match. So what does make do? It looks for a different pattern rule, and it turns out that GNU make provides a default pattern rule to build a .s file, which uses a pattern %.o: %.s and that pattern DOES match, so that's used.

But that built-in rule uses the standard make variables, which are AS and ASFLAGS . You haven't set ASFLAGS , so no flags are used.

You can tell that it's not using your rule because your rule has an echo... in it and make does not print that command out. Therefore, it's not running your rule.