How to write make files with auto dependancy generator - c++

Question

In expansion to my previous question how to write a makefile for structured file system:
The file structure:
.
├── Headers
│   ├── function.h
│   └── test.h
├── makefile
├── README.md
└── Sources
├── function.c
├── main.c
└── test.c
I'm trying to write a makefile that reads the #include<...> on any given source file and compile as required.
Originally I used to have a make file that looked like this:
INC_DIR = Headers
SRC_DIR = Sources
OBJ_DIR = Objects
#CXXFLAGS = -c -Wall -I. -IHeaders
CXXFLAGS = -c -Wall -I. -IHeaders
CC = gcc
SRCS = $(SRC_DIR)/*.c
OBJS = $(OBJ_DIR)/*.o
#The wildcard and patsubt commads will come handy
DEPS = $(INC_DIR)/*.h
#need to use an automatic dependency generator
output: $(OBJ_DIR)/main.o $(OBJ_DIR)/function.o
$(CC) $^ -o $#
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c $(DEPS)
$(CC) $(CXXFLAGS) $< -o $#
run: output
./output
clean:
rm $(OBJ_DIR)/*.o
rm output
-#echo "Clean completed"
That meant that for every source file that output was dependent on i had to add that object to this line.
output: $(OBJ_DIR)/main.o $(OBJ_DIR)/function.o $(OBJ_DIR)/test.o
$(CC) $^ -o $#
And is any other source file was dependent on one or more sources additional rules had to be added.
To solve this:
here is what I have gathered from Auto-Dependency Generation and 4.14 Generating Prerequisites Automatically
As mentioned by the community members, I have a mixed understanding of dependency generation and how to make use of the files generated.
DEP_DIR = .d
$(shell mkdir -p $(DEP_DIR) >/dev/null)
DEPFLAGS = -MT $# -MMD -MP -MF $(DEP_DIR)/$*.Td
INC_DIR = Headers
SRC_DIR = Sources
OBJ_DIR = Objects
$(shell mkdir -p $(OBJ_DIR) >/dev/null)
CXXFLAGS = -c -Wall -I. -IHeaders
CC = gcc
SRCS = $(SRC_DIR)/*.c
OBJS = $(OBJ_DIR)/*.o
#The wildcard and patsubt commads will come handy
#DEPS = $(INC_DIR)/*.h
MAKEDEPEND = $(CC) $(CXXFLAGS) $< \
| sed -n 's/^\# *[0-9][0-9]* *"\([^"]*\)".*/$*.o: \1/p' \
| sort | uniq > $*.Td
COMPILE.c = $(CC) $(DEPFLAGS) $(CXXFLAGS)
#need to use an automatic dependency generator
#%.d: %.c
# #set -e; rm -f $#; \
# $(CC) -MP -MD $(CXXFLAGS) $< > $#.$$$$; \
# sed 's,\($*\)\.o[ :]*,\1.o $# : ,g' < $#.$$$$ > $#; \
# rm -f $#.$$$$
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c $(DEP_DIR)/%.d
#$(MAKEDEPEND); \
cp $*.Td $*.d; \
sed -e 's/#.*//' -e 's/^[^:]*: *//' -e 's/ *\\$$//' \
-e '/^$$/ d' -e 's/$$/ :/' < $*.Td >> $*.d; \
rm -f $*.Td
#$(COMPILE.c) -o $# $<
$(CC) $(CXXFLAGS) $(DEPFLAGS) $< -o $#
-include $(SRCS:.c=.d)
$(DEP_DIR)/%.d: ;
.PRECIOUS: $(DEP_DIR)/%.d
output: $(OBJS)
$(CC) $^ -o $#
run: output
./output
clean:
rm -r $(OBJ_DIR)
rm -r $(DEP_DIR)
rm output
-#echo "Clean completed"
The error when make is executed is:
$ make
gcc -c -Wall -I. -IHeaders -MT Objects/*.o -MMD -MP -MF .d/*.Td Sources/function.c -o Objects/*.o
gcc Objects/*.o -o output
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
makefile:48: recipe for target 'output' failed
make: *** [output] Error 1
So, I'm hoping to achieve the auto dependency detection and compilation. I think the error is in the way that the dependencies are detected and the way they are generated for a given source file.

Answer 1

It looks to me like you're trying to combine multiple different ways of generating dependency info, and that can't work. The advanced post talks about multiple ways to solve the problem: you need to pick one not try to use them all together.
If you want to use the advanced method then use it as described in the "TL;DR" section at the top, or else in the "Combining Compilation and Dependency Generation" section at the bottom. If you're using GCC there's no need for MAKEDEPEND, sed, etc.
The advanced post says that your rule should look like this:
COMPILE.c = $(CC) $(DEPFLAGS) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c
POSTCOMPILE = mv -f $(DEPDIR)/$*.Td $(DEPDIR)/$*.d
%.o : %.c
%.o : %.c $(DEPDIR)/%.d
$(COMPILE.c) $(OUTPUT_OPTION) $<
$(POSTCOMPILE)
That's it.