This is my first Makefile, and I am can't figure out some of the syntax used. The questions are marked below.
$(BUILD_DIR)/%.o: %.c $(BUILD_DIR)
$(CC) -c $(CFLAGS) -Wa,-a,-ad,-alms=$(BUILD_DIR)/$(notdir $(<:.c=.lst)) $< -o $#
What is the usage of "$(BUILD_DIR)" in the dependency?
What is the meaning of "$(BUILD_DIR)/$(notdir $(<:.c=.lst)) $< -o $#" in the role?
As with most computer languages the syntax of make cannot be clear if you don't know it. If you are using GNU make the GNU make manual is your friend. In the following explanations I will assume that BUILD_DIR = build and that one of the source files is bar/foo.c.
$(BUILD_DIR) in the list of prerequisites (dependencies) tells make that the build directory (in which object files are supposed to go) must exist before the recipe is executed; logical. There must be another rule somewhere to create the directory if it does not exist yet. Something like:
$(BUILD_DIR):
mkdir -p $#
But unless you forgot to copy an important character, this dependency is terribly sub-optimal. As the last modification time of a directory changes each time its content changes (files or sub-directories added or removed), it will force the re-compilation of all source files every time the directory changes, which is not what you want. A better dependency would be order-only:
$(BUILD_DIR)/%.o: %.c | $(BUILD_DIR)
that tells make to consider only the existence of $(BUILD_DIR), not its last modification time, when deciding to re-build or not.
$(BUILD_DIR)/$(notdir $(<:.c=.lst)) $< -o $# is just a combination of make automatic variables and functions.
$< and $# expand as the first prerequisite (bar/foo.c) and the target (build/bar/foo.o) respectively.
$(<:.c=.lst) replaces .c by .lst in $<: bar/foo.lst.
$(notdir $(<:.c=.lst)) removes the directory part: foo.lst.
All in all, for a bar/foo.c source file, and with BUILD_DIR = build, the pattern rule would be equivalent to:
build/bar/foo.o: bar/foo.c | build
$(CC) -c $(CFLAGS) -Wa,-a,-ad,-alms=build/foo.lst bar/foo.c -o build/bar/foo.o
Note that there are two different situations to consider:
All your source files are in the same directory as the Makefile (no bar/foo.c, just foo.c). Then you can simplify your recipe:
$(CC) -c $(CFLAGS) -Wa,-a,-ad,-alms=$(BUILD_DIR)/$(<:.c=.lst) $< -o $#
because the $(notdir...) is useless.
Your source files can be in sub-directories (bar/foo.c). Then you need the $(notdir...) in your recipe. But be warned that if you have two source files with the same base name (bar/foo.c and baz/foo.c) you will have a name conflict for $(BUILD_DIR)/foo.lst and your Makefile will not work as expected. Moreover, the order-only prerequisite of the rule should be equivalent to build/bar (or build/baz), not just build. And there should be a rule to create it if needed. If it is your case I suggest to change your pattern rule for:
$(BUILD_DIR)/%.o: %.c
mkdir -p $(dir $#)
$(CC) -c $(CFLAGS) -Wa,-a,-ad,-alms=$(BUILD_DIR)/$(notdir $(<:.c=.lst)) $< -o $#
There are other solutions (secondary expansion...) but there are a bit too complicated for this already too long answer.
Related
I am using a Makefile including a rule to create a dependency file. The compiler is GCC.
%.d: %.c
mkdir -p $(#D)
$(CC) $(CFLAGS) $(CPPFLAGS) -M $< | \
sed 's,\($(notdir $*)\.o\) *:,$(dir $#)\1 $#: ,' > $#.tmp
mv $#.tmp $#
I am quite new in Makefile technique and I find it a little bit difficult to understand this rule composed of a mixture of several options.
Can somebody give a simple explanation how this rule works?
Thanks in advance.
%.d: %.c
All files ending in .d can be made using this rule, as long as there is a .c file with the same directory and stem relative to the current directory or one of the vpaths.
mkdir -p $(#D)
Create the topmost and all intermediate directories for $(#D), which is an automatic make variable that expands to the directory part of the current target.
$(CC) $(CFLAGS) $(CPPFLAGS) -M $< | \
Invoke the C compiler on the first prerequisite (whatever.c) and tell it to output a list of make dependencies to the standard output. Pipe this output to
sed 's,\($(notdir $*)\.o\) *:,$(dir $#)\1 $#: ,' > $#.tmp
sed. The rules in the output will have the same path as the source files, but whoever wrote this rule wants the objects files in a different directory so we need sed to substitute the paths.
Sed captures any rules with targets ending with .o that match $(notdir $*). $* is another automatic variable that expands to the pattern that matches % in the current rule, and notdir will strip any directory parts so you're left with the filename stem with no extension.
Sed then prepends $(dir $#) to the object file target, which is the same thing as $(#D) we saw above, and adds the dependency file itself ($#) as a target of the same prerequisites. This output is redirected to a file with the name of the current target + .tmp.
mv $#.tmp $#
Moves the previous file to the real target of the rule.
Side note: if you don't mind the dependency files being generated in the same directory as the object files then these recipes are outdated, you can achieve the same thing with something like:
sources := src/somefile1.c src/somefile2.c
objects := $(sources:src/%.c=obj/%.o)
deps := $(objects:.o=.d)
CFLAGS := -MMD -MP
.PHONY: all
all $(objects)
$(objects): obj/%.o: src/%.c
$(COMPILE.c) $(OUTPUT_OPTION) $<
-include $(deps)
With the following makefile snippet:
main: main.o f1.o f2.o
$(CC) $(CFLAGS) -o program main.o f1.o f2.o
main.o: main.cc
$(CC) $(CFLAGS) -c main.cc
f1.o: f1.cc
$(CC) $(CFLAGS) -c f1.cc
f2.o: f2.cc
$(CC) $(CFLAGS) -c f2.cc
If I just change one file, only that file get recompiled when I rerun make, as desired. However, I'm having a hard time generalizing this without having to list each file individually. When I try something like:
$(OBJECTS): $(SOURCES)
$(CC) $(CFLAGS) -o $# -c $(patsubst %.o,%.cc,$#)
It builds each object file individually, but each object file depends on ALL my sources, so a change in any one file causing a full recompile. What's a good way to accomplish this?
Basically,
you do have to list each .o file's dependencies individually.
For example, each .o is likely to depend on a different bunch of headers.
Taking your f1.o, you need something like:
f1.o: include/i.h
f1.o: another.h dir/and-another.h
f1.o: f1.cc
$(CC) $(CFLAGS) -c f1.cc
(you can have as many dependency lines for a target as you like).
Maintaining that list is a nightmare.
Broken dependency lists render your Makefile worse than useless—you might as well use a batch file.
All is not lost!
If you are tidy,
you can get the compiler to do it automatically,
and pretty much for free.
Makes your Makefile tidier to boot.
Win Win.
As Ismail Badawi commented, pattern rules provide a nice solution. They are a type of implicit rule for make. Basically, implicit rules are automatic recipes based off the file extension. For example, make knows how to convert .c files into .o files implicitly. By default make will run the following recipe for .c files (see the rule catalogue):
$(CC) $(CPPFLAGS) $(CFLAGS) -c
You can modify the process either by setting the variables CC, CPPFLAGS, and CFLAGS, or by defining a pattern rule:
%.o: %.c
$(CC) $(CFLAGS) -c $<
The "$<" above matches the name of the first prerequisite, which will be the .c file in this example. See Beta's comment and automatic variables.
My Makefile has been recompiling all my source files even if they haven't changed. I gave it a look with make -d and found stuff like this:
...
Prerequisite `setup' of target `bin/exception/Exceptions.o' does not exist.
Must remake target `bin/exception/Exceptions.o'.
...
Please note that this is what I get after bin/exception/Exceptions.o has already been compiled.
Now what on Earth does it mean by the prerequisite not existing? I've quite clearly declared it, and marked it as PHONY. This example should produce the same results for you (You should just be able to run this - I've made it produce all the necessary files for you)
OBJ_DIR=bin
SRC_DIR=src
OBJS=$(OBJ_DIR)/exception/Exception.o
.PHONY: all
all: $(OBJ_DIR)/app
.PHONY: setup
setup:
mkdir -p $(sort $(dir $(OBJS)))
# To make the source file for you
$(SRC_DIR)/%.cpp:
#mkdir -p $(dir $#)
touch $#
$(OBJ_DIR)/app: setup $(OBJS)
#echo CXX -o $#
#touch $#
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp setup
#echo CXX -c -o $#
#cp $< $#
If I take out the requirement for setup, then my program will not recompile unchanged sources as expected.
Why does make think it needs to rebuild?
[Phony targets] should not be [...] prerequisite[s] of a real target file.
That's from the manual section on Phony targets.
What you are seeing is part of why. make always considers a phony target as out-of-date and needing to be rebuilt. As a result (and since the file doesn't exist) that also applies to anything that depends on it.
There are two simple solutions to this problem.
The first is to make setup a Force Target instead of a .PHONY target which will allow it to operate normally (it might want to also be an Empty Target too but as long as it is touched at least once it should work).
The second is to make setup an order-only prerequisite:
$(OBJ_DIR)/app: $(OBJS) | setup
and
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp | setup
You could also just make sure that the normal output target recipes create the output directory before trying to use it.
$(OBJ_DIR)/app: $(OBJS)
#mkdir -p $(#D)
#echo CXX -o $#
#touch $#
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp
#mkdir -p $(#D)
#echo CXX -c -o $#
#cp $< $#
I've ha makefile with following entries. Will the first rule depend on the secon rule ? So that it builds all the .o files from second files ?
all:$(PROG)
$(PROG): *.o
$(LD) -o $(PROG) -c $< $(LFLAGS)
%.o : %.c
$(CC) $(CFLAGS) -o $# -c $<
To be specific if i invoke 'make all' will it invoke the second rule if no *.o files were found ?
All other Variables have usual meaning .
No, that will not work. When you run your makefile for the first time, are there any .o files? No. So the expression *.o will expand to nothing.
Of course, your recipe for $(PROG) doesn't actually use any of the object files anyway, as written.
You can do something like this (although personally I prefer to simply list the files out by hand; it's not very common to create all new files so it's not much effort, and it's safer than just trying to grab every file in the directory):
SOURCES := $(wildcard *.c)
OBJECTS := $(SOURCES:%.c=%.o)
$(PROG): $(OBJECTS)
If I have the following rule in a makefile:
$(OBJ)/%.o: $(SRC)/%.c
$(CC) -c -o $# $< $(CFLAGS)
Every file matching the prefix ./obj/ and sufix .o will have its stem passed to %, so I can provide some dependencies based on its name.
But, suppose I have this kind of rule, which I specify one by one the targets I want:
OBJECTS=abc.o bca.o cba.o
$(OBJECTS): $(SRC)/%.c
$(CC) -c -o $# $< $(CFLAGS)
How do I make the % stem actually work for the current target name make is executing? Just using % doesn't work, neither $#.
Note that I'm trying to write the actual target name to its own dependency. For example, when make is executing the rule for abc.o, it would include $(SRC)/abc.c and just it (something like $(patsubst %.o, $(SRC)/%.c, MAGIC_TARGET_NAME_VARIABLE)).
You can just replace this rule:
$(OBJECTS): $(SRC)/%.c
with:
$(OBJECTS) : %.o : $(SRC)/%.c
You will need to add the $(OBJ) to the -o part of the recipe if you still want them built there:
$(OBJECTS) : %.o : $(SRC)/%.c
$(CC) -c -o $(OBJ)/$# $< $(CFLAGS)
I’m not completely clear on what you’re asking, but I think this accomplishes what you’re trying to do:
OBJECTS=abc.o bca.o cba.o
.PHONY: all
all: $(OBJECTS:%=obj/%)
$(OBJ)/%.o: $(SRC)/%.c
echo $(CC) -c -o $# $< $(CFLAGS)
All .o files are built; each .o file is built using only the .c file corresponding to it; and if you want to refer to the list of all object files or source files in the command for compiling a .o file, then you can reference ${OBJECTS} directly.
If this isn’t what you’re trying to do, you’ll be able to get a better answer by listing the input files you have, the output files you want to make, the input dependencies of each output file, and what compilation command you want to execute for each output file.