Examining the rule we find the target is defined as 'main' with a prerequisite of 'main.o'. This simply means that in order to create 'main' the 'main.o' file must exist. The absence of a recipe relies on the implicit rules. This can be observed by looking at the output when running
make as below;
$ make
cc -c -o main.o main.c
cc main.o -o main
The existence of implicit rules comes with some disadvantages, namely it's easy to not understand what is being done for you. Conceptually, the implicit rule that generates the object files takes the form of the prefix rule below;
$ cat Makefile
main: main.o
.c.o:
${CC} ${CPPFLAGS} ${CFLAGS} -c $^
Understanding what is going on allows tailoring the behavior without explicitly defining a rule. Note the usage of the CPPFLAGS and CFLAGS variables. Tweaking the original makefile will allow us to add debugging info and specifying an optimization level 3 as below;
$ cat Makefile
CFLAGS += -g -o3
main: main.o
This results in a slight difference when we run make;
$ make
cc -g -o3 -c -o main.o main.c
cc main.o -o main
The foundation of make is detecting changes to the prerequisites and determining when the targets need to be remade. This can be observed by re-running make immediately after running make, the result is a notification that "'main' is up to date". Affecting the main.c file timestamp by modifying the file or simply touching it will result in the need for the rule to be applied once again.
$ make
cc -g -o3 -c -o main.o main.c
cc main.o -o main
user@kaylee:~/make.blog/C$ make
make: `main' is up to date.
user@kaylee:~/make.blog/C$ touch main.c
user@kaylee:~/make.blog/C$ make
cc -g -o3 -c -o main.o main.c
cc main.o -o main
Likely, you've seen this all before, but stay with me I assure you there's more interesting things to come.
Often, it's preferred to have a target that cleans up the directory and allows building from scratch. The convention is to name such a target clean. Below is a modified makefile that defines a clean target that simply deletes the executable and the object files.
CFLAGS += -g -o3
main: main.o
clean:
${RM} main main.o
Executing 'make clean' will result in deleting main and main.o files. Adding a file to your project can be accomplished by adding the object file to the prerequisites for main and recipe for the clean target or we can make use of pattern. We'll do this by explicitly defining each of the C source files in a variable, then perform a list replacement substituting the *.c with *.o extensions to get our object file list. The object file list can then be used in the target prerequisites and in the clean target recipe. Adding a file to the SRCS variable rather than duplication in multiple locations.
$ cat Makefile
CFLAGS += -g -o3
SRCS=main.c
OBJS=$(subst .c,.o,${SRCS})
main: ${OBJS}
clean:
${RM} main ${OBJS}
Still however there is duplication, namely the multiple references of main, that can be addressed by a new variable definition.
$ cat Makefile
CFLAGS += -g -o3
PROGS=main
SRCS=main.c
OBJS=$(subst .c,.o,${SRCS})
${PROGS}: ${OBJS}
clean:
${RM} ${PROGS} ${OBJS}
Definitely on the right path, but the addition of a file requires modification to the makefile. The wildcard expansion demonstrated in the following makefile. The addition or removal of a file with the .c extension in the current directory will take effect in the wildcard expansion.
$ cat Makefile
CFLAGS += -g -o3
PROGS=main
SRCS=${wildcard *.c}
OBJS=$(subst .c,.o,${SRCS})
${PROGS}: ${OBJS}
clean:
${RM} ${PROGS} ${OBJS}
Let's look at some less typical usages of make which gives us a bit more insight into the creation of targets, prerequisites, and recipes. Imagemagick is a common utility that we'll be making use in the following examples.
We'll build up the makefile as we go, incorporating what we've learned above. We'll be satisfying the same objectives using two forms of makefiles; one that makes use of suffix rules, one that makes use of pattern rules.
Let's begin by defining our objectives. Suppose our project requires taking in a list of JPG files and converting each into a series of other image file formats, namely PNG, GIF, JP2 and XWD files.
Using the suffix rule syntax, the makefile can begin taking the following form;
$ cat Makefile.suffix
.SUFFIXES:
.SUFFIXES: .jpg .png .gif .jp2 .xwd
all: image.xwd
.jpg.png:
${SH} convert $< $@
.png.gif:
${SH} convert $< $@
.gif.jp2:
${SH} convert $< $@
.jp2.xwd:
${SH} convert $< $@
clean:
${RM} *.gif *.jp2 *.xwd
The all target consists of the default target, the prerequisite of image.xwd. In other words, make is complete when an up-to-date image.xwd file exists. How it arrives at it is make magic, more precisely a series of suffix rules. A series of prefix rules chaining is required to get to the final XWD file, each target we can kick off by explicitly specifying on the command line. Specifying 'make -f Makefile.suffix image.png' results in firing of the .jpg.png suffix rule. The suffix rules are chained as each must fire to arrive at the final XWD file. Running 'make' performs this by stepping through a series of recipes; JPG => PNG => GIF => JP2 => XWD.
$ make -f Makefile.suffix
convert image.jpg image.png
convert image.png image.gif
convert image.gif image.jp2
convert image.jp2 image.xwd
rm image.jp2 image.gif image.png
Notice the final step removes intermediate files which can be preserved which can be prevented by adding ".PRECIOUS: %.jpg %.png %.gif %.jp2 %.xwd" line which tells make not to remove the intermediate files with the specified extensions. The example is a bit fictional but done to demonstrate suffix rules and chaining. Modifying the source file image.jpg followed by rerunning make will result in converting the new file to each of the alternative file formats.
As is, each prerequisite is generated via suffix rule chaining to completion before moving on to the next prerequisite. In other words, if you specified image.xwd and image01.xwd the image.xwd would be generated to completion (ie. JPG => PNG => GIF => JP2 => XWD) before moving on to image01.xwd.
Meeting the same goals, let's utilize pattern rules rather than suffix rules which are somewhat dated in use.
$ cat Makefile.pattern
.PRECIOUS: %.jpg %.png %.gif %.jp2 %.xwd
all: image.xwd
%.png:%.jpg
${SH} convert $< $@
%.gif:%.png
${SH} convert $< $@
%.jp2:%.gif
${SH} convert $< $@
%.xwd:%.jp2
${SH} convert $< $@
clean:
${RM} *.gif *.jp2 *.xwd
The most noteworthy difference between the target/prerequisites. The prefix rules define a target, the pattern rule defines a target and prerequisite making the illusion of the rules being reversed.
What if you want to convert each input images to Jpgs before moving on to Gifs before moving on to Jp2s before the Xwds. This can be done by specifying a wildcard expansion for the source files and using the substitution expression for each of the formats then specifying each of the formats in as prerequisites for the all target, as follows;
$ cat Makefile.pattern
.PRECIOUS: %.jpg %.png %.gif %.jp2 %.xwd
SRCS=${wildcard *.jpg}
PNGS=$(subst .jpg,.png,${SRCS})
GIFS=$(subst .jpg,.gif,${SRCS})
JP2S=$(subst .jpg,.jp2,${SRCS})
XWDS=$(subst .jpg,.xwd,${SRCS})
all: ${PNGS} ${GIFS} ${JP2S} ${XWDS}
%.png:%.jpg
${SH} convert $< $@
%.gif:%.png
${SH} convert $< $@
%.jp2:%.gif
${SH} convert $< $@
%.xwd:%.jp2
${SH} convert $< $@
clean:
${RM} ${PNGS} ${GIFS} ${JP2S} ${XWDS}
This way, each format is fully satisfied before moving on to the next. Perhaps less necessary for image files, more applicable for generating source files. For example, the Protobuf message compiler allows generation of header/c++ files which also allows interdependencies between message files. This requires all the message files to be converted to C/H files before firing the compilation, otherwise a source file may reference a header file that hasn't been created yet.
Hope you find this useful. Good luck with your make projects!
