The Makefile utility

1. The Makefile utility

2. Motivation • Small programs single file • “Not so small” programs : • Many lines of code • Multiple components • More than one programmer

3. Motivation – continued • Problems: • Long files are harder to manage (for both programmers and machines)‏ • Every change requires long compilation • Many programmers cannot modify the same file simultaneously • Division into components is desired

4. Motivation – continued • Solution: divide project into multiple files • Goals: • Good division into components • Minimum compilation when something is changed • Easy maintenance of project structure, dependencies and creation

5. Project maintenance • Done in Unix by the Makefile mechanism • A makefile is a file (script) containing : • Project structure (files, dependencies)‏ • Instructions for file creation • The make command reads a makefile, understands the project structure and generates the executable • Note that the Makefile mechanism is not limited to C programs

6. Project structure • Project structure and dependencies can be represented as a DAG (Directed Acyclic Graph) • Example : • Program contains 3 files • main.c., sum.c, sum.h • sum.h included in both .c files • Executable should be the file sum

7. sum (exe)‏ sum.o main.o main.c sum.h sum.c sum.h

8. makefile sum: main.o sum.o gcc –o sum main.o sum.o main.o: main.c sum.h gcc –c main.c sum.o: sum.c sum.h gcc –c sum.c

9. Rule syntax target main.o: main.c sum.h gcc –c main.c tab dependency action Rule

10. Equivalent makefiles • .o depends (by default) on corresponding .c file. Therefore, equivalent makefile is: sum: main.o sum.o gcc –o sum main.o sum.o main.o: sum.h gcc –c main.c sum.o: sum.h gcc –c sum.c

11. make operation • Project dependencies tree is constructed • Target of first rule should be created • We go down the tree to see if there is a target that should be recreated. This is the case when the target file is older than one of its dependencies • In this case we recreate the target file according to the action specified, on our way up the tree. Consequently, more files may need to be recreated • If something is changed, linking is usually necessary

12. make operation - continued • make operation ensures minimum compilation, when the project structure is written properly • Do not writesomething like: prog: main.c sum1.c sum2.c gcc –o prog main.c sum1.c sum2.c which requires compilation of the whole project when something is changed

13. Make operation - example FileLast Modified sum 10:03 main.o 09:56 sum.o 09:35 main.c 10:45 sum.c 09:14 sum.h 08:39

14. Make operation - example • Operations performed: gcc –c main.c gcc –o sum main.o sum.o • main.o should be recompiled (main.c is newer). • Consequently, main.o is newer than sum and therefore sum should be recreated (by re-linking).

15. Another makefile example • Example : • Program contains 4 files • vm.c, lex.c, parser.c, compiler.c • Executable should be the file compiler • Each project file can have an additional .h file.

16. Compiler Assignment compiler (exe) compiler.o vm.o lex.o parser.o vm.c lex.c compiler.c parser.c

17. makefile CC=gcc compiler: compiler.o vm.o lex.o parser.o $(CC) –o compiler compiler.o vm.o lex.o parser.o compiler.o: compiler.c vm.o lex.o parser.o $(CC) –c compiler.c vm.o lex.o parser.o

18. Makefile cont. parser.o: parser.c $(CC) –c parser.c lex.o: lex.c $(CC) –c lex.c vm.o: vm.c $(CC) –c vm.c

19. Example - continued • We can define multiple targets in a makefile • We can use variables such as CC to quickly update the makefile. • In our example this is good if we are using new c++11 features and want to compile using gcc-4.6 or gcc-4.7 • make • Will create the compiler executable

20. Reference • Good tutorial for makefiles http://www.gnu.org/software/make/manual/make.html