Chapter 1. How to Write a Simple Makefile

     

The mechanics of programming usually follow a fairly simple routine of editing source files, compiling the source into an executable form, and debugging the result. Although transforming the source into an executable is considered routine, if done incorrectly a programmer can waste immense amounts of time tracking down the problem. Most developers have experienced the frustration of modifying a function and running the new code only to find that their change did not fix the bug. Later they discover that they were never executing their modified function because of some procedural error such as failing to recompile the source, relink the executable, or rebuild a jar. Moreover, as the program's complexity grows these mundane tasks can become increasingly error-prone as different versions of the program are developed, perhaps for other platforms or other versions of support libraries, etc.

The make program is intended to automate the mundane aspects of transforming source code into an executable. The advantages of make over scripts is that you can specify the relationships between the elements of your program to make , and it knows through these relationships and timestamps exactly what steps need to be redone to produce the desired program each time. Using this information, make can also optimize the build process avoiding unnecessary steps.

GNU make (and other variants of make ) do precisely this. make defines a language for describing the relationships between source code, intermediate files, and executables. It also provides features to manage alternate configurations, implement reusable libraries of specifications, and parameterize processes with user -defined macros. In short, make can be considered the center of the development process by providing a roadmap of an application's components and how they fit together.

The specification that make uses is generally saved in a file named makefile . Here is a makefile to build the traditional "Hello, World" program:

 hello: hello.c         gcc hello.c -o hello 

To build the program execute make by typing:

 $  make  

at the command prompt of your favorite shell. This will cause the make program to read the makefile and build the first target it finds there:

 $  make  gcc hello.c -o hello 

If a target is included as a command-line argument, that target is updated. If no command-line targets are given, then the first target in the file is used, called the default goal .

Typically the default goal in most makefile s is to build a program. This usually involves many steps. Often the source code for the program is incomplete and the source must be generated using utilities such as flex or bison . Next the source is compiled into binary object files ( .o files for C/C++, .class files for Java, etc.). Then, for C/C++, the object files are bound together by a linker (usually invoked through the compiler, gcc ) to form an executable program.

Modifying any of the source files and reinvoking make will cause some, but usually not all, of these commands to be repeated so the source code changes are properly incorporated into the executable. The specification file, or makefile , describes the relationship between the source, intermediate, and executable program files so that make can perform the minimum amount of work necessary to update the executable.

So the principle value of make comes from its ability to perform the complex series of commands necessary to build an application and to optimize these operations when possible to reduce the time taken by the edit-compile-debug cycle. Furthermore, make is flexible enough to be used anywhere one kind of file depends on another from traditional programming in C/C++ to Java, TEX, database management, and more.



Managing Projects with GNU make
Managing Projects with GNU Make (Nutshell Handbooks)
ISBN: 0596006101
EAN: 2147483647
Year: 2003
Pages: 131

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