GNU `make'

Node:Variables Simplify, Next:make Deduces, Previous:How Make Works, Up:Introduction

### Variables Make Makefiles Simpler

In our example, we had to list all the object files twice in the rule for `edit` (repeated here):

         edit : main.o kbd.o command.o display.o \
                       insert.o search.o files.o utils.o
                 cc -o edit main.o kbd.o command.o display.o \
                            insert.o search.o files.o utils.o

Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. Variables allow a text string to be defined once and substituted in multiple places later (see How to Use Variables).

It is standard practice for every makefile to have a variable named `objects`, `OBJECTS`, `objs`, `OBJS`, `obj`, or `OBJ` which is a list of all object file names. We would define such a variable `objects` with a line like this in the makefile:

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

Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)` (see How to Use Variables).

Here is how the complete simple makefile looks when you use a variable for the object files:

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