Letting make Deduce the Commands

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Letting make deduce the commands

It is not necessary to spell out the commands for compiling the individual C source files, because make can figure them out: it has an implicit rule for updating a ‘.o’ file from a correspondingly named ‘.c’ file using a ‘cc -c’ command.

For example, it will use the command ‘cc -c main.c -o main.o’ to compile ‘main.c’ into ‘main.o’. We can therefore omit the commands from the rules for the object files. See Implicit rules.

When a ‘.c’ file is used automatically in this way, it is also automatically added to the list of dependencies. We can therefore omit the ‘.c’ files from the dependencies, provided we omit the commands. The following is the entire example, with both of these changes, and a variable, objects (as previously suggested in Variables make makefiles simpler).

objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o

edit : $(objects)
cc -o edit $(objects)
main.o : defs.h
kbd.o : defs.h command.h
command.o : defs.h command.h
display.o : defs.h buffer.h
insert.o : defs.h buffer.h
search.o : defs.h buffer.h
files.o : defs.h buffer.h command.h
utils.o : defs.h

.PHONY : clean
clean :
-rm edit $(objects)

The example in Another style of makefile shows how to write the makefile in actual practice. (The complications associated with ‘clean’ are described in Phony targets and in Errors in commands.)

Because implicit rules are so convenient, they are used frequently.

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