CS 215 - Fundamentals of Programming IISpring 2010 - Very Basic make"make is a command generator."1 This document explains a small portion of the functionality of make sufficient for most courses. If you want to know more, see the books cited in the footnote. make generally is used for managing software projects (usually in UNIX environments). It allows a programmer to specify the dependencies between various program files and attach commands that generate these files. This can save time and effort since only the files that are affected by a change need to be regenerated, rather than the entire project. To use make you create a makefile that describes the dependency relationships among the files in your project and states the commands for generating each file that are not source files. The name of this file often is Makefile, but you can name it anything you want. If there are to be multiple makefiles in one directory, generally they are named Makefile.ProjectName where ProjectName is different for each makefile. A makefile is a plain text file with the following syntax. # Comments start with hash character to the end of line target1 : dependencies1 <tab> command1 target2 : dependencies2 <tab> command2 ...A target represents something that is to be generated. The dependencies are those files which are used to create the target. The command is used to create the target when needed. The target must start in the first column of a line. There can be more than one target listed in a line separated by spaces. This would mean that all of the targets depend on the same files. The dependencies are a list of files separated by spaces upon which the target depends. There may be more than one command to perform for each target, each on a separate line. Each command line must start with the TAB character (not spaces). For example, in file named Makefile, we might have: vidstore : vidstore.o store.o tape.o <tab> g++ -Wall -o vidstore vidstore.o store.o tape.oThis says that the target vidstore depends on the files vidstore.o, store.o, and tape.o. The command g++ -Wall -o vidstore vidstore.o store.o tape.o is performed to create 1 Andrew Oram and Steve Talbott, Managing Projects with make, O'Reilly Media, Inc. 1991 (out of print). The current reference book for make is Robert Mecklenburg, Managing Projects with GNU Make, O'Reilly Media, Inc. 2004.01/12/10 1 of 4vidstore. This command links together the .o object files into an executable file named vidstore. A .o object file is the result of compiling a single source file, so it usually depends on a .cpp source file and some .h header files. Thus it should also be listed as a target with dependencies. For example, vidstore.o : vidstore.cpp store.h tape.h <tab> g++ -c -Wall vidstore.cppcould be the next target and says that vidstore.o depends on the files vidstore.cpp, store.h, and tape.h. To complete the makefile for this project, there would also be targets for store.o and tape.o. To run make you just type makeat the shell prompt. By default, it looks for a file in the current directory named makefile or Makefile in that order, though most people recommend Makefile so that it will be distinctive among lowercase file names. By default, make will try to create the first target. You also can ask it to create a particular target explicitly by giving it as a command-line argument. Thus, for our example makefile, typing just make will try to create vidstore, while typing make vidstore.owill try to create just vidstore.o. Making intermediate targets is useful if you are trying to get a file like vidstore.cpp to compile when the rest of your project is not done yet. When you run make the following happens: If the target has dependencies, make first checks if the dependencies are up-to-date, before checking the original target. If they are, then it checks if the target is older than the dependencies. If so, it runs the command to create a new version of the target. If not, everything is up-to-date and nothing happens. If the dependencies are not up-to-date, then they are recreated first. Thus a target that depends on one file that depends on another file that has been modified eventually will cause the first target to be updated. Suppose for the above example, you modify store.h. Then when make is invoked, it assumes you want to make vidstore. Since dependency vidstore.o is also a target, make checks if the files vidstore.cpp, store.h, or tape.h are newer. Since we've modified store.h, make will compile a new vidstore.o. (This probably also will happen with store.o.) Since vidstore.o (and store.o) is now newer than any existing vidstore file, make will then also link a new executable. Choosing targets and dependencies is something of an art. When you write a target, it clearly depends on anything that is listed in the command to bring it up-to-date, but you only need to list the ones that can be changed by you. In addition, for code files, any user-defined include files should also be listed as a dependency. For C++ programs, this generally means you'll have a makefile of the form: 01/12/10 2 of 4# Makefile for program program : program.o class1.o class2.o ... <tab> g++ -Wall -o program program.o class1.o class2.o ... program.o : program.cpp class1.h class2.h ... <tab> g++ -Wall -c program.cpp class1.o : class1.cpp class1.h <tab> g++ -Wall -c class1.cpp ...Since typing make by itself assumes that you want to check the first target, generally the first target in a makefile should generate the entire program. Anything that is listed as a dependency that is generated by a program, rather than something you wrote explicitly, should also be a target with its own dependencies and a command line that will generate it, and so forth. (For most programs, there's usually only two steps, compiling and linking, but an example of where a C++ source file could be generated is if you use a special purpose code generator like lex or yacc.) If you have multiple makefiles or would rather give one a more descriptive name (e.g., Makefile.vidstore), you can have make read a specified file using the -f option. For example, if the makefile for the video store application is named Makefile.vidstore, then typing make -f Makefile.vidstorewill create vidstore. This can be combined with the explicitly named target as follows: make -f Makefile.vidstore vidstore.oAnother
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