How to write MakeFiles?

Compilation and Linking:

There are generally three distinct steps in compiling some code to an executable. They are:

  1. Compiling the code to Assembly code
    • This step is usually done transparently as most compilers perform it and then invoke the assembler themselves, so you don’t really have to worry about it. It can be useful later on if you’re trying to perform optimizations and you want to see how your compiler actually implements your code, if you can get your compiler to dump the assembly code that it generates. Other than that, just be aware that it happens and don’t bother about it for a while.
  2. Assembling the Assembly code to object code
    • As you should know by now, a computer executes binary codes in its CPU. In order to run your program, your program will need to be translated into these binary codes. This is what this stage is about. The generated assembly (normally never actually present on your system but just passed from the compiler to the assembler) is translated into binary code.
    • Some compilers will translate the assembly code directly into an executable file, and others will leave it as what is known as object code. We’re only concerned with those that create object code.
    • Object code is executable code which isn’t properly organized to run. It may be missing functions that it calls, it may be missing global variables, things like that. Generally, in C and C++, object code is stored in files with the extension .o. Thus if you compiled your code into object code but didn’t link it, you would go from a program called hello_world.c to hello_world.o.
  3. Linking the object code together into an executable
    • If you have several object files (though you technically only need one), you can perform what is known as linking. This is the process of resolving dependencies between the object files, resolving dependencies of the object files on external libraries, and resolving addresses of global variables and such. In simple terms, linking is the process of taking code which is in executable form and turn it into code which can actually be run by your computer on your operating system.

Example: Suppose the source code for an executable binary file named project.exe is stored in a file named project.cpp. The program #includes a library’s header file named lib.h and the library’s implementation file is named lib.cpp.

Make a folder named project that contains project.cpp, lib.h and lib.cpp files.

Here is project.cpp file:

#include <stdio.h>
#include "lib.h"
int main() {
printf("Doing something to 3 gives: %d\n", doSomething(3));


 #ifndef __LIB_H
 #define __LIB_H
 int doSomething(int x);


 #include "lib.h"
 int doSomething(int x) {
 return x*3;

The directive #include "lib.h" causes the compiler (actually, the preprocessor) to insert the declaration of the prototype for function doSomething(int x) into project.cpp. However, the definition of doSomething(int x) is in lib.cpp, not in project.cpp.

Translating a program like project.cpp to produce a binary executable program involves three phases:

  1. Compiling, in which C++ code is translated into assembly;
  2. Assembling, in which assembly is translated into machine language; and
  3. Linking, in which calls to functions outside the main file are linked to their definitions.

All implementation files being used must be compiled as well as the main program. The resulting object files must then be linked together to produce a binary executable.

Step 1:
Separately compile the program file name.cpp to produce an object file named name.o in some systems. This may require using a special compiler switch, as in the GNU C++ command:
      g++ -c project.cpp
project.cpp project.o
Step 2:
Separately compile each library implementation file in the same manner; for example:
      g++ -c lib.cpp
lib.cpp lib.o
Step 3:
Link the object files together into a binary executable program — for example, in GNU C++ with the command:
      g++ project.o lib.o -o project.exe
project.o project.exe

Why MAKEFILE is to be used?

If a program involves several different libraries, it can be very difficult to keep track of which files are out of date, which need to be recompiled, and so on. Single commands are very usefull in this regard. With GNU C++, UNIX’s make utility can be used to execute a file named Makefile that contains the commands for the compilations and linking.

What is make?

The make utility eliminates the complexity of separate compilation by determining what files are out of date and re-making them. Learning to use it effectively can save a great deal of time, especially on projects that have several files.

Using the make utility requires a programmer to create a special file named Makefile, from which the make program reads information on how to build your program and when.

There are three important parts to a Makefile: the target, the dependencies, and the instructions. Just so that you know what this will look like, it’s of the form:

TargetFile: DependencyFile1 DependencyFile2 ... DependencyFilen

where TargetFile is the file that needs to be updated, and each Dependencyi is a file upon which TargetFile depends.

Instruction is a UNIX command to make TargetFile.

The command/instruction must be preceded by a TAB and end with a Return.

Example: The first line-pair in our Makefile appears as follows:

project.exe: project.o lib.o
      g++ project.o lib.o -o project.exe

Note that the first line specifies the dependencies of project.exe, and the second line is the UNIX command to make project.exe.

Of course, project.o won’t exist the first time we compile, so we should specify a line-pair for it, too:

project.o: project.cpp lib.h
         g++ -c project.cpp

We should then do the same thing for lib.o:

lib.o: lib.cpp lib.h
         g++ -c lib.cpp

The Makefile thus appears as follows:

    project.exe: project.o lib.o
          g++ project.o lib.o -o project.exe

    project.o: project.cpp lib.h
             g++ -c project.cpp

    lib.o: lib.cpp lib.h
             g++ -c lib.cpp

Now user go to project folder and give command make.

priyanka@priyanka:~$cd project


The program reads the Makefile, and:

  1. Sees that project.exe depends upon project.o, and
    1. Checks project.o, which depends on project.cpp and lib.h;
    2. Determines whether project.o is out of date;
    3. If so, it executes the command to make project.o:
      g++ -c project.cpp;
  2. Sees that project.exe also depends upon lib.o, and
    1. Checks lib.o, which depends on lib.cpp and lib.h;
    2. Determines whether lib.o is out of date;
    3. If so, it executes the command to make lib.o:
      g++ -c lib.cpp;
  3. Sees that everything on which project.exe depends is now up to date, and so executes the command to make project.exe:
    g++ project.o lib.o -o project.exe

Details About make.

  1. While a Makefile usually consists of pairs of lines, there can in fact be any number of commands after the line specifying the dependencies.

    Example: We could write,

    project.exe: project.o lib.o
          g++ project.o lib.o -o project.exe
          rm project.o
          rm lib.o

    This would automatically remove the object files after project.exe is made.

  2. The make utility also allows a user to specify what is to be made:
        uname% make lib.o

    will operate using lib.o as its primary TargetFile instead of project.exe.

  3. A TargetFile need not have any dependencies. This, combined with (1) and (2) allows make to be used for all kinds of non-compilation activities.

    Example: Suppose our Makefile contains the following lines:

              rm -f project.exe *.o *~ *#

    and the user types

        uname% make clean

    What happens? (This is a fool-proof way to clean up a messy directory.)

  4. make is coordinated with emacs. When an emacs user types the command:
        M-x compile

    emacs responds with

        Compile command: make -k

    If a Makefile is in the directory containing the file on which you are working, then pressing the Return key will execute make using that Makefile.

Tip to remember:
There can be only one file named Makefile in a directory.
Advantage of using Makefile:
1. Since a Makefile coordinates the translation of one project, each project should be stored in its own dedicated directory, with a separate Makefile to coordinate its translation. Doing so allows you to remove the object files, binary executables, etc., because to remake the project, you need only cd to the directory and type make.
2. An added benefit is that all the files for one project are confined within one directory, making it easier to port the project to a different machine. (Just copy the directory to the new machine, cd to the directory, and type make).

About Priyanka Kapoor

Simple, Hardworking & friendly.....
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