GNU Make

Important Special Targets

.DELETE_ON_ERROR

• Generally, if make fails to build a target, and the target corresponds to a file, you want that file removed.
  • Why might this happen? Ctrl-c being pressed during a build. Multiple statements in a build recipe. Memory allocation failure during build. Any number of things.
• By adding .DELETE_ON_ERROR to the Makefile, this becomes the default behavior.
  • kill -9'ing make will prevent this from happening! :/
• gcc and some other tools will clean up on error, for some cases. Use .DELETE_ON_ERROR to be safe.

.PHONY:

• GNU Make manual section 4.6, "Phony Targets"
• clean, all, test targets ought almost always be .PHONY

.DEFAULT:

• GNU Make manual section 10.6, "Defining Last Resort Rules"

Make and subversion

• After running make in a pristine tree, the output of svn status ought be empty. Use svn:ignore properties to ignore your generated files.
  • Most importantly, svn status should not show any entries in the 'M'odified state. This indicates either generated files being checked into source control, or the build process modifying source in-place (the same thing, really)
  • If svn:ignore is properly used, the following rule suffices as a project-independent clean target, assuming the presence of xmlstarlet:

    svn --xml --no-ignore status | xmlstarlet sel -t -m //entry -i "wc-status[@item='ignored']" -v @path -n | xargs rm -rf

bash interactions

• The shell to use can be specified via $(SHELL). By default, /bin/sh is used
  • Unlike most variables, it cannot be inherited from the environment (except on Windows, where it is)
  • It is not exported by default to sub-Makes, but it can be
• Arguments to the shell can be specified via .SHELLFLAGS
  • By default, it's -c, or -ec in POSIX mode
• Remember that errors in the body of a shell loop don't set the overall return value:

target:
        while true ; do false ; done

This recipe cannot be successfully completed! You can use

target:
        set -e ; while true ; do false ; done

or

target:
        while true ; do false || exit ; done

The former is probably nicer, since you can then use ; in the place of &&, but it deviates from expected gmake semantics. In general, you don't want to be doing this kind of thing anyway; it's useful for cases like running unit testing on a set of inputs, where outputs won't be generated.