How can I capture GNUMake differences between two directories

I have a tricky issue with gmake, when I build from the parent directory, something is different and the make does not build all the .o(s) it needs and fails, but if I cd to the directory and do a make it builds them fine.

How can I get GNUmake to tell me the difference between these two runs? There must be some make v开发者_运维知识库ariables set in the parent that break the child, but I need help figuring out how to track them down.

If running make from the parent directory fails to build foo.o, then try make foo.o. If that fails then try running make -n foo.o in both directories (to print the commands instead of executing them) to see what it's doing differently. If it succeeds, then it's not even trying to build foo.o when run from the parent directory; make -n may shed some light, and as a last resort make -d will give you a torrent of information about the decision process, why it's doing what it's doing.

Here's a handy trick to see the value of variables. Put this rule in your makefile:

show_%:
    @echo $@ is $($*)

Now you can run make show_FOO and it will tell you the value of the variable FOO.

Finally, are you sure you know where you build your .o files? Make is very good at using things there to build files here, but not the other way around, so it can lose track of intermediate files if you're not careful.