man make: a Primer on the Make Utility
In a compiled language, the makefile is arguably the most important part of any programming project. To compile your project, you first have to compile each source file into an object file, which in turn needs to be linked with system libraries into the final executable file. Each command can have a considerable number of arguments added in. That's a lot of typing and a lot of potential for mistakes. The more source files you have, the more complex the compilation process becomes, unless you use makefiles. Most Linux users have at least a cursory knowledge of make and makefiles (because that's how we build software packages for our systems), but not much more than that. Most developers probably don't have too much in-depth experience with makefiles, because most Integrated Development Environments (IDEs) have the capability of managing makefiles for them. Although this is convenient most of the time, knowing more about how make works and what goes into makefiles can help you troubleshoot compilation errors down the road.
According to make's man page, "The purpose of the make utility is to determine automatically which pieces of a large program need to be recompiled, and issue the commands to recompile them." Essentially, make is used to determine efficiently (and without user error) which portions of the source code have been updated since the last compilation and recompile them. It can be used for more than just compiling programs. Because it isn't limited to any particular language, you can use it for anything you can come up with that relates to the modified date of a group of files.
Running make is a straightforward process. The more convoluted portion of using make is constructing the makefile. The makefile is a file that consists of a series of rules that define the dependencies of your project. These rules govern the behavior of make during execution.
Command lines must be indented with tab characters; spaces cause funky errors. This has been a design flaw in make for decades. Empty lines must still have a tab character or else make will throw a fit.
Comments start with a pound sign (#).
Continuation of a line is denoted by a back slash (\).
Lines containing equal signs (=) are variable definitions.
Each command line typically is executed in a separate Bourne shell—that is, sh1.
To execute more than one command line in the same shell, type them on the same line, separated by semicolons. Use a \ to continue the line if necessary.
Listing 1. Example Makefile
SOURCES=main.cpp hello.cpp factorial.cpp
all: $(SOURCES) $(EXECUTABLE)
$(CC) $(LDFLAGS) $(OBJECTS) -o $@
$(CC) $(CFLAGS) $< -o $@
Rules and Targets
<![CDATA[ CC=g++ CFLAGS=-c -Wall LDFLAGS= SOURCES=main.cpp hello.cpp factorial.cpp OBJECTS=$(SOURCES:.cpp=.o) EXECUTABLE=hello all: $(SOURCES) $(EXECUTABLE) $(EXECUTABLE): $(OBJECTS) $(CC) $(LDFLAGS) $(OBJECTS) -o $@ .cpp.o: $(CC) $(CFLAGS) $< -o $@ ]]>
Rules and Targets
Each rule in the makefile is an independent series of commands that are executed in order to build a target. Make does not necessarily run each rule in order. Make will run through the rules recursively, building each target in turn, based on modification. Rules are formatted like this:
target: dependency list ... commands ...
The target is typically the name of a file, but it can be a phony target (discussed later in this article). The dependency list is a space-separated list of files that designate whether the target needs to be rebuilt. The commands can be any shell command, so long as the target is up to date at the end of them. It is imperative that you indent the commands with a tab character and not spaces. This is a design flaw in make that has yet to be fixed, and it will cause some strange and obscure errors should you use spaces instead of tabs in your makefile.
When make encounters a rule, it first checks the files listed in the dependency list to ensure that they haven't changed. If one of them has, make looks through the makefile for the rule containing that file as the target. This recursion continues until a rule is found where all the dependencies are unchanged or rebuilt (or have no further dependencies), and then make executes the listed commands for that rule before returning to the previous rule, and so on, until the root rule has been satisfied and its commands run.
You may use pattern-matching characters to describe dependencies in the dependency list or in commands, but they may not be used in the target.
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