Make Tips and Tricks

Special Macros

1. $@

$@ is the name of the target. This allows you to easily write a generic action that can be used for multiple different targets that produce different output files. For example, the following two targets produce output files named client and server respectively.

client: client.c
    $(CC) client.c -o $@
 
server: server.c
    $(CC) server.c -o $@

1. $?

The $? macro stores the list of dependents more recent than the target (i.e., those that have changed since the last time make was invoked for the given target). We can use this to make the build commands from the above example even more general:

client: client.c
    $(CC) $? -o $@
 
server: server.c
    $(CC) $? -o $@

3. $^

$^ gives you all dependencies, regardless of whether they are more recent than the target. Duplicate names, however, will be removed. This might be useful if you produce transient output (such as displaying a result to the screen rather than saving it to a file).

# print the source to the screen
viewsource: client.c server.c
    less $^

4. $+

$+ is like $^, but it keeps duplicates and gives you the entire list of dependencies in the order they appear.

# print the source to the screen
viewsource: client.c server.c
    less $+

5. $<

If you only need the first dependency, then $< is for you. Using $< can be safer than relying on $^ when you have only a single dependency that needs to appear in the commands executed by the target. If you start by using $^ when you have a single dependency, if you then add a second, it may be problematic, whereas if you had used $< from the beginning, it will continue to work. (Of course, you may want to have all dependencies show up. Consider your needs carefully.)

Wildcards

The percent sign % can be used to perform wildcard matching to write more general targets. When a % appears in the dependencies list, it replaces the same string of text throughout the command in makefile target. If you wish to use the matched text in the target itself, use the special variable $*. For instance, the following example will let you type make name of .c file to build an executable file with the given name:

%:
    gcc -o $* $*.c

E.g. make test would run `gcc -o test test.c

Replacing text

It is possible to create a new macro based on replacing part of an old macro. For instance, given a list of source files, called SRC, you might wish to generate the corresponding object files, stored in a macro called OBJ. To do so, you can specify that OBJ is equivalent to SRC, except with the .c extension replaced with a .o extension:

BIN = $(SRC:.c=.o)

$(BIN): $(SRC)
    gcc -o $@ $^

Note that this is effectively saying that in the macro SRC, .c should be replaced with .o.

Reference

Silent execution

Makefile commands can be suppressed with @.

greet:
    @echo hello
    echo bye

When running:

$ make greet
hello
echo bye
bye

The other way is to silence all of the makefile commands. For the above example,

$ make -s greet
hello
bye

Creating folders

BIN_DIR=./bin

run: | $(BIN_DIR)

$(BIN_DIR):
  @echo "Folder $(BIN_DIR) does not exist!"
  mkdir $@

Makefile cookbook

Taken from makefiletutorial:

# Thanks to Job Vranish (https://spin.atomicobject.com/2016/08/26/makefile-c-projects/)
TARGET_EXEC := final_program

BUILD_DIR := ./build
SRC_DIRS := ./src

# Find all the C and C++ files we want to compile
# Note the single quotes around the * expressions. Make will incorrectly expand these otherwise.
SRCS := $(shell find $(SRC_DIRS) -name '*.cpp' -or -name '*.c' -or -name '*.s')

# String substitution for every C/C++ file.
# As an example, hello.cpp turns into ./build/hello.cpp.o
OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)

# String substitution (suffix version without %).
# As an example, ./build/hello.cpp.o turns into ./build/hello.cpp.d
DEPS := $(OBJS:.o=.d)

# Every folder in ./src will need to be passed to GCC so that it can find header files
INC_DIRS := $(shell find $(SRC_DIRS) -type d)
# Add a prefix to INC_DIRS. So moduleA would become -ImoduleA. GCC understands this -I flag
INC_FLAGS := $(addprefix -I,$(INC_DIRS))

# The -MMD and -MP flags together generate Makefiles for us!
# These files will have .d instead of .o as the output.
CPPFLAGS := $(INC_FLAGS) -MMD -MP

# The final build step.
$(BUILD_DIR)/$(TARGET_EXEC): $(OBJS)
    $(CXX) $(OBJS) -o $@ $(LDFLAGS)

# Build step for C source
$(BUILD_DIR)/%.c.o: %.c
    mkdir -p $(dir $@)
    $(CC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@

# Build step for C++ source
$(BUILD_DIR)/%.cpp.o: %.cpp
    mkdir -p $(dir $@)
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@


.PHONY: clean
clean:
    rm -r $(BUILD_DIR)

# Include the .d makefiles. The - at the front suppresses the errors of missing
# Makefiles. Initially, all the .d files will be missing, and we don't want those
# errors to show up.
-include $(DEPS)