How to redirect output to a specific file into a sed command?

Question

Some context : I'm trying to build a generic Makefile, with auto-dependency generation (using GCC as described here, non recursive, which could be used from any level of my projects. For that purpose, the dependency files generated need to be of the following form :

$(DIR)file.o: $(DIR)file.c $(DIR)file.h

$(DIR)file.h

That way, whatever the level of the make invocation, make can properly locate the files.

In order to do so, I have the following implicit rule :

DEPFLAGS = -MT $@ -MP -MMD -MF $(word 2,$^).tmp
COMPILE = $(CC) $(DEPFLAGS) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<
POSTCOMPILE = sed -E 's|([0-9a-z_.:]*/)?([0-9a-z_.:]+)|$$(DIR)\2|g' $(word 2,$^).tmp > $(word 2,$^)

%.o: %.c %.dep
    $(COMPILE)
    $(POSTCOMPILE)
    $(RM) $(word 2,$^).tmp
    touch $@

The sed invocation replace any directory part before the filename by $(DIR)

The problem is that since the dependency file is generated after the object file, it causes the Makefile to relink every time. And so i have to add a touch command to be sure that the object file will be considered up-to-date. Which is not very elegant. So I have this question : instead of using a temporary file, is there a way to directly redirect the gcc output (which is directed into the .tmp file) into the sed command ?

In others words, is there a way to do the equivalent of the pipe redirection, but for an other stream/file descriptor than stdout ?

Answer 1

If you are in a full fledged linux environment, you can indeed skip the creation of the .tmp file and pipe the dependecies directly into sed.

# This is needed, because the default shell does not support
# process substitution
SHELL = /bin/bash

SED_COMMAND = sed -E 's|([0-9a-z_.:]*/)?([0-9a-z_.:]+)|$$(DIR)\2|g' > $(word 2,$^)
DEPFLAGS = -MT $@ -MP -MMD -MF >($(SED_COMMAND))
COMPILE = $(CC) $(DEPFLAGS) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<

%.o: %.c %.dep
        $(COMPILE)
        touch $@

However, this still needs the touch command at the end, for the reason correctly mentioned in a comment by @MadScientist above:

there's no way to be 100% sure that the sed command would finish writing its file before the compiler finished writing the object file: they are running in parallel so you can't guarantee which one will finish first.