Makefiles

What is make?

• Make is a utility to track dependencies in a system of files, and to automate compilations
• Examples of dependencies - C program
  • Objs depend on source for that object
  • Objs depend on headers
  • Binary depends on objs
  • Libs depend on objs
• Uses files called Makefiles as configuration
• Hierarchical dependencies determined by mod time - if any dependency is newer than target, remake target with command
• Many things described here are specific to GNU make (gmake in the gnu locker on Athena).

Makefiles - basic

• naming - GNUmakefile, makefile, Makefile in that order
  • "Makefile" is recommended - fairly standard, appears at top of dir
  • use GNUmakefile only for gmake-specific makefiles.
• explicit rules (basic)

  	TARGET... : DEPENDENCIES... [; COMMAND]
  		COMMAND
  		...
  		...
  	

• Command lines MUST start with tabs
• \newline syntax for breaking up lines
• Phony targets i.e. clean, all
  • No dependencies
  • Executed every time target is goal
  • (gmake specific) declare as phony to avoid name conflicts with special target keyword .PHONY:

    	.PHONY : clean
    	clean :
    			rm $(OBJS)
    	

• Default goal is first rule that does not start with a period
• Comments - `#' starts

Commands

• Each line is run in a different shell, if you want to cause a command to affect the next, use ; in the command so that make interprets it as a single command:

  	foo:
  		cd foodir ; doit
  	

  or

  	foo:
  		cd foodir ; \
  		doit
  	

• To ignore errors from a command, prefix command with "-"

  	foo:
  		-rm -f *~ *.o
  	

Variables

• Naming
  • Case sensitive
  • Any characters except trailing whitespace, `:', `#', or `='
  • Should avoid all but alphanumerics and underscore
• Setting
  • VARNAME = VARVALUE (note: trailing spaces not truncated)
  • make VARNAME=value
  • inherits most environment variables except $SHELL and a few others
• Referencing: $(VARNAME)
• Standard variables for program Makefiles
  • $(OBJS)
  • $(SHELL)
  • $(MAKE)
  • $(CC)
  • $(CFLAGS)
  • $(INCLUDE)
  • $(LDFLAGS)
• Automatic variables - make sets these in special cases for substitutions and other purposes. More later

Multiple targets and rules

• Multiple targets equivalent to list of rules for each target

  	foo bar baz: blah
  		doit
  
  	and
  
  	foo: blah
  		doit
  	bar: blah
  		doit
  	baz: blah
  		doit
  	

  are equivalent
• Multiple rules for a single target are acceptable - make will check all files that depend on a given updated file. Useful for adding dependencies.

  	FILES = foo bar baz
  	foo: bar
  		doit
  	bar baz: blah
  		dothat
  	$(FILES): gag
  	# no commands allowed here
  	

  will add dependency on gag to $(FILES) (ie, if gag changes, files will get rebuild)
• Multiple commands are NOT acceptable - there can be only one set of commands for a given target; all other rules using the target must not have commands.
  • exception - double-colon rules' commands are independent.

Implicit rules

• Default rules, either internal to make or user-defined, which set dependencies based on classes of filename.
• Pattern rules

  	TARGET-PATTERN: DEP-PATTERN
  		COMMANDS
  
  	eg
  
  	%.o: %.c
  		$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@
  	

• Can be chained, i.e. .y -> .c -> .o -> the .c file does not exist, but make considers it for dependency purposes. Make deletes the temp files when it's done.
• A number of useful implicit rules are predefined e.g. .c -> .o as above.

Static pattern rules

• Used to take analagous dependencies and execute commands constructed from names

  	TARGETS ...: TARGET-PATTERN: DEP-PATTERNS ...
  		COMMANDS
  	...
  	

• Example:

  	OBJS = foo.o bar.o
  	$(OBJS): %.o: %.c
  		$(CC) -c $(CFLAGS) $< -o $@
  	

• TARGET-PATTERN is used to construct names to place into DEP-PATTERNS
• Special Variables
  • `%' - is the variable which matches the stem of the targets, which is then substituted into the `%' in the dependency pattern to make the actual command.
  • `$<' is the automatic variable which holds the name of the dependency.
  • `$@' is the automatic variable which holds the name of the target.
  • `$*' is an automatic variable which will expand to the stem (what matched the %)
• Different from implicit rules in that it only applies to the list of targets you specify.

Including other Makefiles

• directive: include FILENAMES
• FILENAMES can include shell globs and make vars
• useful for including global rules and vars

makedepend

• Parses .c files for other files they include, then writes them in the form of a dependency rule in a makefile.
• For example, if foo.c #includes foo.h, bar.h, and quux.c, it will add a rule of the form:

  foo.c: foo.h bar.h quux.c

  to the makefile
• old way to use:

  	
  	dep:
  		makedepend $(CPPFLAGS) *.c
  	

  will remake all dependencies when make dep is called, and write them to the end of the current makefile
• gmake way to use (as suggested by Mike Whitson):

  	include *.d
  	%.d: %.c
  		$(SHELL) -ec '$(CC) -M \
  			$(CPPFLAGS) $< | \
  			sed '\''s/$*.o/& $@/g'\'' \
  			> $@'
  	

  (gmake considers all included makefiles as targets, and, if any needed to be recreated, reruns itself with the updated makefiles.)

Recursion

• Useful if you want to have subsections of code in subdirs with their own makefiles
• Basic syntax:

  	subsystem:
  		cd subdir; $(MAKE)
  	

• Always use $(MAKE) rather than passing `make' - inherits path to make and command line arguments
• Exporting variables to sub-makes normally only happens for variables defined in the environment or the command line.
  • export VARIABLE
  • unexport VARIABLE

Archives (libraries)

• A specific member of an archive can be target or dependency:

  ARCHIVE(MEMBERS)

  eg

  	libfoo.a(foo.o bar.o): foo.o bar.o
  		ar -r libfoo.a foo.o bar.o
  	

Other functionality

• GNU gmake supports conditionals, loops, functions, and more
• Look in info pages (M-x info in emacs) for more information

Other options

• imake and xmkmf
• autoconf and configure