gnu make "Removing intermediate files"

Question

I have the following rules

define compile_c
$(ECHO) "CC $<"
$(Q)$(CC) $(CFLAGS) -c -MD -o $@ $<
@# The following fixes the dependency file.
@# See http://make.paulandlesley.org/autodep.html for details.
@# Regex adjusted from the above to play better with Windows paths, etc.
@$(CP) $(@:.o=.d) $(@:.o=.P); \
  $(SED) -e 's/#.*//' -e 's/^.*:  *//' -e 's/ *\\$$//' \
      -e '/^$$/ d' -e 's/$$/ :/' < $(@:.o=.d) >> $(@:.o=.P); \
  $(RM) -f $(@:.o=.d)
endef

vpath %.c . $(TOP)
$(BUILD)/%.o: %.c $(BUILD)/%.pp
    $(call compile_c)

vpath %.c . $(TOP)

$(BUILD)/%.pp: %.c
    $(ECHO) "PreProcess $<"
    $(Q)$(CC) $(CFLAGS) -E -Wp,-C,-dD,-dI -o $@ $<

When the build finishes, GNU make says
Removing intermediate files... and deletes all the .pp files which I do NOT want.

Why is it doing this?
How do I stop it?

Answer 1

If you search for "gnu make intermediate files" you'll immediately find the answer as to why it's happening, in the GNU make manual section Chains of Implicit Rules.

It also tells you how to avoid it: a file cannot be intermediate if it is mentioned in the makefile as a target or prerequisite.

So, just list your .pp files as a prerequisite of some rule, somewhere. It doesn't have to be a rule that's ever invoked. You don't give enough of your makefile here for us to provide a complete answer, but it would be something like:

all_pps: $(ALL_OBJECTS:.o=.pp)

assuming you had a variable ALL_OBJECTS containing all your .o files.

Answer 2

Since you're using GNU Make, you can make the following adjustment to your Makefile:

.PRECIOUS: $(BUILD)/%.pp  # ADD THIS LINE
$(BUILD)/%.pp: %.c
    $(ECHO) "PreProcess $<"
    $(Q)$(CC) $(CFLAGS) -E -Wp,-C,-dD,-dI -o $@ $<

The documentation has this to say about .PRECIOUS directives:

The targets which .PRECIOUS depends on are given the following special treatment: if make is killed or interrupted during the execution of their recipes, the target is not deleted.

[...]

Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done.

[...]

You can also list the target pattern of an implicit rule (such as ‘%.o’) as a prerequisite file of the special target .PRECIOUS to preserve intermediate files created by rules whose target patterns match that file's name.

This has the benefit of not creating an unwanted additional rule. It's also clearer what you're trying to do: keep the precious intermediate files that might be expensive to recreate.

Answer 3

I think the best solution is to use the .SECONDARY special target. Just add this line:

.SECONDARY:

Quoting the manual:

.SECONDARY with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate).

Why is this better than making the targets prerequisites of a throw-away target? That's more clutter, and has to be done explicitly for every set of files that might be generated with pattern rules.

Why is this better than .PRECIOUS? That causes files to be retained even if their recipe fails when using .DELETE_ON_ERROR. The latter is important to avoid failing recipes leaving behind bad outputs that are then treated as current by subsequent make invocations. IMO, you always want .DELETE_ON_ERROR, but .PRECIOUS breaks it.

Answer 4

Here is a detail that finally got PRECIOUS working for me. The pattern that you give to PRECIOUS has to be exactly the pattern that is being used in the rule that creates the intermediate file. I want to save all files prefixed by moc_. At first I used .PRECIOUS: moc_% to no avail. Then I tried .PRECIOUS: moc_%.cpp and this did the trick.