CSCI 1300 - Exercise 6
Using Make for G++ Compilations

What You'll Get from This Exercise

This week's lab introduces another important tool: The make facility. This facility allows you to easily maintain and keep your files up to date when you are working on a project that is split accross several cxx files (and their associated header files). For each of your future assignments, you should keep all of the files for that assignment in one directory, and create a make file to manage the files in that directory.

PC Labs in the
Engineering Center
CR 235    CR 239
CR 244    CR 252 (24 hours)
CH 107    ME 107
Other campus sites are
listed at www.colorado.edu/its/labs.

Installing the
CS1300 Software
Open a DOS window. If the CS1300 software is not permanently
installed on your machine, then use one of these methods:
If you get the message "Out of environment space", then:
  • Click on the MS-DOS icon in the top left corner of the window.
  • Select Properties from the pop-up menu.
  • Select the Memory tab from the command box.
  • Click the arrow on the Initial Environment box. Move down in the box as far as possible (by clicking the downward arrow). Click on the biggest number that you see in this box.
  • Click OK in the command box.
  • Click OK in the MS-DOS Prompt information box.
  • Stop the DOS Session by clicking the X in the top-right or by typing the command "exit".
  • Restart a new DOS session and try running your commands again.
Copy an Example Makefile
This week's lab exercise starts in your working directory. You'll need to copy four files into that directory from the installed cs1300\lab\ directory: 
    makefile
    sinewave
    intarray.cxx
    intarray.h
What's in a Makefile?
The file makefile is the usual input file for a tool called make. The purpose of make is to help you maintain and update a collection of related program files. The collection usually has header files, cxx files, and compiled files--all of which depend on each other. For example, consider last week's sinewave program. The final product is an executable file named sinewave.exe, which is created by linking together two other compiled files, sinewave.o and intarray.o. The two compiled files were created by compiling sinewave.cxx and intarray.cxx (both of which used the header file intarray.h). The complete dependencies among the files can be drawn like this:

The upward arrows in the file express how each file is created. For example, the file sinewave.o is created by compiling sinewave.cxx and also includes intarray.h. To state the matter simply:

If sinewave.cxx or intarray.h changes, then sinewave.o must be regenerated by giving the compiler command:
g++ -Wall -c -gstabs sinewave.cxx

This requirement to sometimes regenerate sinewave.o is one of the dependencies that the example makefile expresses. To see this dependency, use emacs to open up the file named makefile.

Near the bottom of the file you'll find these two lines: 

sinewave.o: sinewave.cxx intarray.h
        g++ -Wall -c -gstabs sinewave.cxx

The first line is called a target line, which begins with a file name and a colon. After the colon is a list of more file names. Here's how to interpret the line: The file before the colon (called the target file) depends on the other files (after the colon). Whenever one of the files after the colon changes, the make tool knows that the target file needs to be regenerated. After the target line, there is a series of commands that tell exactly how to regenerate the target file. For the case of sinewave.o, we only need the one g++ command to regenerate the file. (Notice that we included the -c flag to indicate that we should only compile and not create an executable file yet. We also included the -gstabs flag in case we want to use the debugger.)

There is one other peculiar requirement: The command lines (such as the g++ command) must each begin with a tab (not with 8 spaces!).

As a second example of a dependency, the executable file sinewave.exe is created by compiling together the object files sinewave.o and intarray.o. If either of these two object files should change, then sinewave.exe also needs to be recreated. Here is the appropriate target line and command from our makefile: 

sinewave.exe: sinewave.o intarray.o
        g++ -Wall -gstabs sinewave.o intarray.o -o sinewave

This target line says that if sinewave.o or intarray.o should happen to change, then the sinewave.exe must be regenerated with the g++ command that is shown.

Using Make to Regenerate a Specified Target File
In order to illustrate how the make facility works, start by getting rid of all the object files and executable files. You can do this with the delete command: 

    del  intarray.o 
    del  sinewave.o 
    del  sinewave.exe
There are two simple ways to use the make facility to automatically regenerate your files. The first approach regenerates a specific file. For example, suppose you want to regenerate intarray.o. Then you can use the make command, as shown here:

make -k intarray.o

The make command will find the dependency information in the makefile file. It sees that intarray.o depends on other files, so it will first ensure that those files are present (and regenerate them if necessary). In this example, the two files intarray.h and intarray.cxx are necessary for generating intarray.o. These two files are present, so the make command proceeds to generate intarray.o, using the g++ command that is specified in the makefile. When the g++ command is executed, it is displayed on the screen, so you will see this appear on the screen:

g++ -Wall -c -gstabs intarray.cxx

After this command finishes, you should list the files in your directory, where you will find the object file intarray.o is once again present.

Using Make without Specifying a Target File
You may also use the make command without specifying a file, like this:

make -k

Without a specified file, the make command will regenerate the first target that it finds in makefile. Try this now, and you will see that the executable file sinewave.exe is regenerated, since sinewave.exe is the first target file in makefile. During the process of regenerating the sinewave.exe file, the make command had to carry out several steps. In the first step, the make command realizes that sinewave.exe depends on intarray.o and also on sinewave.o. But the file sinewave.o is not present. So, the make command first regenerates sinewave.o, and then it can proceed to regenerate the executable file sinewave. On the screen you'll see the two steps displayed:

g++ -Wall -c -gstabs sinewave.cxx
g++ -Wall -gstabs sinewave.o intarray.o -lm -o sinewave

Using Make from within Emacs
The best feature of the make command is how it automatically keeps track of exactly which object files and executable files need to be recompiled. As an example, you should now change one of your source files. I suggest that you use emacs to make a small change to the sinewave.cxx program, perhaps adding another small output statement. Then save the new sinewave.cxx (CTRL-x s) and from within emacs give the compile command (ESC x compile RETURN).

What happens? The emacs compile command automatically issues the "make -k" command. The make facility realizes that sinewave.cxx has changed, and therefore the object file sinewave.o is regenerated with the command:

g++ -Wall -c -gstabs sinewave.cxx

Next, the make facility realizes that sinewave.o has just changed, and therefore the executable file sinewave.exe is regenerated with:

g++ -Wall -gstabs sinewave.o intarray.o -lm -o sinewave

But, notice that the object file intarray.o was not recompiled. The dependencies in the makefile were sufficient to show that intarray.o did not need recreation.

Using Make with Special Targets
A makefile can also have special target lines that carry out special actions rather than regenerate files. There are two such target lines at the bottom of the example makefile: 

clean:
        rm $(EXPENDABLES)

all:
        @make $(EXPENDABLES)

The special target clean simply removes all of the files that can be generated. (These are called the "expendable" files because they are easy to replace by regenerating them.) It does the removal with the rm command (which is part of the compiler files that you downloaded). The rm command removes one or more files. The term $(EXPENDABLES) is a macro expansion--meaning that the make command will look elsewhere in the file for the meaning of the word EXPENDABLES. It will find this at the top of the file, where EXPENDABLES is defined to be the sequence of all files that we can regenerate.

The special target "all" generates all expendable files. The @ symbol in front of make command suppresses printing of the command, so that "make. . ." will not be printed on the screen. The two special targets can be activated just like any other target. For example:

make -k all

The make all command is useful when you want to make certain that all files are up to date. It will only regenerate files that need to be regenerated because the corresponding cxx or h files have changed.

Michael Main (main@colorado.edu)