makefiles_test, a C++ code which illustrates how a "makefile" can be used to manage the compilation and loading of a C++ code.
A makefile is usually stored in a file named makefile.
The purpose of a makefile is to record or infer the commands necessary to compile and load a program (or, more generally, to "build" a program or object that depends on objects), to intelligently automate this task, and to efficiently update all objects that depend on an object that has itself just been updated.
The most natural example for which a makefile is useful would involve the relationship between several text files containing C++ routines (with extension ".C"), the object files created by compiling separately each C++ file (with extension ".o"), and the executable program that can be created by loading them all together (which, by default, is called "a.out", but which we will rename to "cpp_simple".
We suppose we start with the C++ source code files cpp_simple.C, midpoint.C, and f.C, as well as an include file, cpp_simple.H. If we wished to build the executable cpp_simple, we need to create cpp_simple.o, midpoint.o, f.o, and then load them together, and rename the result to cpp_simple.
The "dependencies" or relationships between these files can be thought of as follows:
cpp_simple needs cpp_simple.o, midpoint.o and f.o.
The commands to create cpp_simple are
g++ cpp_simple.o midpoint.o sub2.o
mv a.out cpp_simple
cpp_simple.o needs cpp_simple.C and cpp_simple.H.
The command to create cpp_simple.o is
g++ -c cpp_simple.C
midpoint.o needs midpoint.C and cpp_simple.H..
The command to create midpoint.o is
g++ -c midpoint.C
f.o needs f.C and cpp_simple.H..
The command to create f.o is
g++ -c f.C
The corresponding makefile records these relationships. Each dependency line lists a "target" (something you want to make), followed by a colon, and then a list of the components on which that target depends. There follow one or more command lines that tell how to put the components together to make the target. Note that each command line must begin with a TAB character. We will use the symbol --TAB--> to suggest this. Here is what the makefile would look like
cpp_simple : cpp_simple.o midpoint.o f.o
--TAB--> g++ cpp_simple.o midpoint.o f.o
--TAB--> mv a.out cpp_simple
cpp_simple.o : cpp_simple.C cpp_simple.H
--TAB--> g++ -c cpp_simple.C
midpoint.o : midpoint.C cpp_simple.H
--TAB--> g++ -c midpoint.C
f.o : f.C cpp_simple.H
--TAB--> g++ -c f.C
To create the program, type make cpp_simple. If you just edited midpoint.C and want only to recompile it, type make midpoint.o. But if you just edited midpoint.C, and you want to recompile it, and then also recreate the program, then type make cpp_simple. The make program will notice that midpoint.C has been updated, and automatically recompile it, and then rebuild cpp_simple.
The computer code and data files described and made available on this web page are distributed under the MIT license
makefiles_test is available in a C version, a C++ version and a FORTRAN90 version.