How to properly clean in a Makefile?

For small C/C++ 开发者_运维知识库projects I am used to write my clean target as follow:

clean:
    $(RM) *.o *~

For slightly bigger projects, when sources are dispatched in subdirectories, say src1 and src2, I would write

clean:
    $(RM) src1/*.o src1/*~ src2/*.o src2/*~

With even more subdirectories, it quickly becomes messy… So I realized I could use the find command like this:

clean:
    find . -name "*.o" -exec $(RM) {} \;
    find . -name "*~"  -exec $(RM) {} \;

However I saw that people often use find in conjunction with xargs instead of using -exec and I wonder why since it seems to work fine…

What do you use and why?

I know that for big projects, or for better compatibility, I should probably use cmake or autotools but I like the simplicity of a Makefile for small projects.

The version of 'find' I'm used to (GNU find) also supports a '-delete' flag. I might do something like:

clean:
    find . -name '*.o' -delete

because it's effective, simple and clean.

If I have multiple patterns to delete, I normally repeat the command:

clean:
    rm -f a.out
    find . -name '*.o' -delete
    find . -name '*~' -delete

Because exec creates one sub-process per file being deleted. xargs, on the other hand, batches them up.

It's effectively the difference between:

rm src1/a.o
rm src1/b.o
rm src2/c.o

and:

rm src1/a.o src1/b.o src2/c.o

It's probably not going to matter that much unless you have a lot of files but it's worth knowing for that use case.

But I don't use either of those methods. I make each directory responsible for building itself (and it's subordinate directories), by having a Makefile in there. Then, from the top level make, I have something like:

all:
    for i in module1 module2 \
    do \
        cd $i \
        $(MAKE) all \
        cd .. \
    done

Same deal for the clean target. I don't assume that cleaning is the same action for every single subdirectory.

By doing that, the relevant actions are localised. If I have a directory where .o files are actually third party objects for me to link with (so I don't have the source), I make sure that clean won't delete them in that relevant makefile. The solution you have would hose them, causing much angst and gnashing of teeth :-)