1. gnu make in detail for deginners
<以下为gnu make in detail for beginners的删减版,完整的在这里>
Have you ever peeked into the source code of any of the applications you run every day? Ever used make install to install some application? You will see make in most projects. It enables developers to easily compile large and complex programs with many components. It’s also used for writing maintenance scripts based on timestamps. This article shows you how to have fun with make.
Large projects can contain thousands of lines of code, distributed in multiple source files, written by many developers and arranged in several subdirectories. A project may contain several component divisions. These components may have complex inter-dependencies — for example, in order to compile component X, you have to first compile Y; in order to compile Y, you have to first compile Z; and so on. For a large project, when a few changes are made to the source, manually recompiling the entire project each time is tedious, error-prone and time-consuming.
Make is a solution to these problems. It can be used to specify dependencies between components, so that it will compile components in the order required to satisfy dependencies. An important feature is that when a project is recompiled after a few changes, it will recompile only the files which are changed, and any components that are dependent on it. This saves a lot of time. Make is, therefore, an essential tool for a large software project.
Each project needs a Makefile — a script that describes the project structure, namely, the source code files, the dependencies between them, compiler arguments, and how to produce the target output (normally, one or more executables). Whenever the make command is executed, the Makefile in the current working directory is interpreted, and the instructions executed to produce the target outputs. The Makefile contains a collection of rules, macros, variable assignments, etc. (‘Makefile’ or ‘makefile’ are both acceptable.)
1.1. Installing GNU Make
Most distributions don’t ship make as part of the default installation. You have to install it, either using the package-management system, or by manually compiling from source. To compile and build from source, download the tarball, extract it, and go through the README file. (If you’re running Ubuntu, you can install make as well as some other common packages required for building from source, by running: sudo apt-get install build-essential.)
1.2. A sample project
module.h:1
2#include <stdio.h>
void sample_func();
module.c:1
2
3
4
5#include "module.h"
void sample_func()
{
printf("Hello world!");
}
main.c:1
2
3
4
5
6
7#include "module.h"
void sample_func();
int main()
{
sample_func();
return 0;
}
The following are the manual steps to compile the project and produce the target binary:1
2
3slynux@freedom:~$ gcc -I . -c main.c # Obtain main.o
slynux@freedom:~$ gcc -I . -c module.c # Obtain module.o
slynux@freedom:~$ gcc main.o module.o -o target_bin # Obtain target binary
(-I is used to include the current directory (.) as a header file location.)
1.3. Writing a Makefile from scratch
By convention, all variable names used in a Makefile are in upper-case. A common variable assignment in a Makefile is CC = gcc, which can then be used later on as ${CC} or $(CC). Makefiles use # as the comment-start marker, just like in shell scripts.
The general syntax of a Makefile rule is as follows:1
2
3
4target: dependency1 dependency2 ...
[TAB] action1
[TAB] action2
...
a simple sample project:1
2
3
4
5
6
7
8
9all: main.o module.o
gcc main.o module.o -o target_bin
main.o: main.c module.h
gcc -I . -c main.c
module.o: module.c module.h
gcc -I . -c module.c
clean:
rm -rf *.o
rm target_bin
- When we ran make without specifying a target on the command line, it defaulted to the first target in our Makefile
There are different ways of assigning variables in a Makefile. They are (type of assignment, followed by the operator in parentheses):
Simple assignment (:=)For example, when a CC := ${GCC} ${FLAGS} is first encountered, CC is set to gcc -W and wherever ${CC} occurs in actions, it is replaced with gcc -W.Recursive assignment (=)
Recursive assignment (the operator used is =) involves variables and values that are not evaluated immediately on encountering their definition, but are re-evaluated every time they are encountered in an action that is being executed. As an example, say we have:1
2GCC = gcc
FLAGS = -WWith the above lines, CC = ${GCC} {FLAGS} will be converted to gcc -W only when an action like ${CC} file.c is executed somewhere in the Makefile. With recursive assignation, if the GCC variable is changed later (for example, GCC = c++), then when it is next encountered in an action line that is being updated, it will be re-evaluated, and the new value will be used; ${CC} will now expand to c++ -W.
We will also have an interesting and useful application further in the article, where this feature is used to deal with varying cases of filename extensions of image files.Conditional assignment (?=)Conditional assignment statements assign the given value to the variable only if the variable does not yet have a value.Appending (+=)The appending operation appends texts to an existing variable. For example:1
2CC = gcc
CC += -WCC now holds the value gcc -W. Though variable assignments can occur in any part of the Makefile, on a new line, most variable declarations are found at the beginning of the Makefile.
- Using patterns and special variables
The % character can be used for wildcard pattern-matching, to provide generic targets. For example:1
2%.o: %.c
[TAB] actions
When % appears in the dependency list, it is replaced with the same string that was used to perform substitution in the target.
Inside actions, we can use special variables for matching filenames. Some of them are:
1 | $@ (full target name of the current target) |
Action modifiers
We can change the behaviour of the actions we use by prefixing certain action modifiers to the actions. Two important action modifiers are:- (minus)— Prefixing this to any action causes any error that occurs while executing the action to be ignored. By default, execution of a Makefile stops when any command returns a non-zero (error) value. If an error occurs, a message is printed, with the status code of the command, and noting that the error has been ignored. Looking at the Makefile from our sample project: in the clean target, the rm target_bin command will produce an error if that file does not exist (this could happen if the project had never been compiled, or if make clean is run twice consecutively). To handle this, we can prefix the rm command with a minus, to ignore errors: -rm target_bin.@ (at)suppresses the standard print-action-to-standard-output behaviour of make, for the action/command that is prefixed with @. For example, to echo a custom message to standard output, we want only the output of the echo command, and don’t want to print the echo command line itself. @echo Message will print “Message” without the echo command line being printed.
Use PHONY to avoid file-target name conflicts
Remember the all and clean special targets in our Makefile? What happens when the project directory has files with the names all or clean? The conflicts will cause errors. Use the .PHONY directive to specify which targets are not to be treated as files — for example: .PHONY: all clean.- Simulating make without actual execution
At times, maybe when developing the Makefile, we may want to trace the make execution (and view the logged messages) without actually running the actions, which is time consuming. Simply use make -n to do a “dry run”. - Using the shell command output in a variable
Sometimes we need to use the output from one command/action in other places in the Makefile — for example, checking versions/locations of installed libraries, or other files required for compilation. We can obtain the shell output using the shell command. For example, to return a list of files in the current directory into a variable, we would run: LS_OUT = $(shell ls). - Nested Makefiles
Nested Makefiles (which are Makefiles in one or more subdirectories that are also executed by running the make command in the parent directory) can be useful for building smaller projects as part of a larger project. To do this, we set up a target whose action changes directory to the subdirectory, and invokes make again:
1 | subtargets: |
Instead of running the make command, we used $(MAKE), an environment variable, to provide flexibility to include arguments. For example, if you were doing a “dry run” invocation: if we used the make command directly for the subdirectory, the simulation option (-n) would not be passed, and the commands in the subdirectory’s Makefile would actually be executed. To enable use of the -n argument, use the $(MAKE) variable.
Now let’s improve our original Makefile using these advanced features:
1 | CC = gcc # Compiler to use |
Run make on the modified Makefile and test it; also run make with the new list target. Observe the output.
- Make in non-compilation contexts
I hope you’re now well informed about using make in a programming context. However, it’s also useful in non-programming contexts, due to the basic behaviour of checking the modification timestamps of target files and dependencies, and running the specified actions when required. For example, let’s write a Makefile that will manage an image store for us, doing thumbnailing when required. Our scenario is as follows:
We have a directory with two subdirectories, images and thumb.
The images subdirectory contains many large image files; thumb contains thumbnails of the images, as .jpg files, 100x100px in image size.
When a new image is added to the images directory, creation of its thumbnail in the thumb directory should be automated. If an image is modified, its thumbnail should be updated.
The thumbnailing process should only be done for new or updated images, and not images that have up-to-date thumbnails.
This problem can be solved easily by creating a Makefile in the top-level directory, as follows:
1 | FILES = $(shell find images -type f -iname "*.jpg" | sed 's/images/thumb/g') |
In the above Makefile, FILES = $(shell find images -type f -iname “.jpg” | sed ‘s/images/thumb/g’) is used to generate a list of dependency filenames. JPEG files could have the extension .jpg or .JPG (that is, differing in case). The -iname parameter to find (find images -type f -iname “.jpg”) will do a case-insensitive search on the names of files, and will return files with both lower-case and upper-case extensions — for example, images/1.jpg, images/2.jpg, images/3.JPG and so on. The sed command replaces the text “images” with “thumb”, to get the dependency file path.
When make is invoked, the all target is executed first. Since FILES contains a list of thumbnail files for which to check the timestamp (or if they exist), make jumps down to the thumb/%.jpg wildcard target for each thumbnail image file name. (If the extension is upper-case, that is, thumb/3.JPG, then make will look for, and find, the second wildcard target, thumb/%.JPG.)
For each thumbnail file in the thumb directory, its dependency is the image file in the images directory. Hence, if any file (that’s expected to be) in the thumb directory does not exist, or its timestamp is older than the dependency file in the images directory, the action (calling $(CONVERT_CMD) to create a thumbnail) is run.
Using the features we described earlier, CONVERT_CMD is defined before targets are specified, but it uses recursive assignment. Hence, the input and target filenames passed to the convert command are substituted from the first dependency ($<) and the target ($@) every time the action is invoked, and thus will work no matter from which action target (thumb/%.JPG or thumb/%.jpg) the action is invoked.
Naturally, the “Updating thumbnail” message is also defined using recursive assignment for the same reasons, ensuring that $(MSG) is re-evaluated every time the actions are executed, and thereby able to cope with variations in the case of the filename extension.
slynux@freedom:~$ make
Updating thumbnail 1.jpg
convert -resize “100x100” images/1.jpg thumb/1.jpg
Updating thumbnail 4.jpg
convert -resize “100x100” images/4.jpg thumb/4.jpg
If I edit 4.jpg in images and rerun make, since only 4.jpg‘s timestamp has changed, a thumbnail is generated for that image:
slynux@freedom:~$ make
Updating thumbnail 4.jpg
convert -resize “100x100” images/4.jpg thumb/4.jpg
Writing a script (shell script or Python, etc) to maintain image thumbnails by monitoring timestamps would have taken many lines of code. With make, we can do this in just 8 lines of Makefile. Isn’t make awesome?
That’s all about the basics of using the make utility. Happy hacking till we meet again!
This article was originally published in September 2010 issue of the print magazine.