Join two lists of strings in a makefile

Question

In my makefile there is a variable that contains all source files:

SOURCES := $(shell find $(SRCDIR) -type f -name *.$(SRCEXT))

I now want to create a DEPFILES that contains the same files as SOURCES with two main differences:

1. Each file ends with ".d" instead of ".c"
2. Each file contains a "." before its basename, such that the resulting file is hidden.

I came up with this expression, which works well:

DEPFILES := $(foreach s,$(SOURCES:.$(SRCEXT)=.$(DEPSEXT)),$(shell echo $(s) | sed -e 's/^\(.*\/\)/\1./')))

Initially, I split SOURCES into the following lists:

PRE := $(dir $(SOURCES))
SUF := $(addprefix ".", $(notdir $(SOURCES)))

Question: How do you join those two lists in a makefile, such that the results equals DEPFILES? In other words: How do you pairwisely concat the strings from both lists?

Answer 1

Here is a simpler approach using makefile only and no shell - if I understood your requirement ok:

SRCS += bob/src1.c fred/src2.c src3.c
DEPS=$(join $(addsuffix ., $(dir $(SRCS))), $(notdir $(SRCS:.c=.d)))

# Some debug:
$(info 1. $(SRCS))
$(info 2. $(SRCS:.c=.d))
$(info 3. $(notdir $(SRCS:.c=.d)))
$(info 4. $(dir $(SRCS)))

.PHONY: all
all:
    @echo SRCS: $(SRCS)
    @echo DEPS: $(DEPS)

I broke the output down into steps so you can see what each part does - saves me to explain it!

outputs:

1. bob/src1.c fred/src2.c src3.c
2. bob/src1.d fred/src2.d src3.d
3. src1.d src2.d src3.d
4. bob/ fred/ ./
SRCS: bob/src1.c fred/src2.c src3.c
DEPS: bob/.src1.d fred/.src2.d ./.src3.d