make Processes a Makefile
By default, make starts with the first target (not targets whose
names start with ‘.’). This is called the default goal.
(Goals are the targets that make strives ultimately to
update. You can override this behavior using the command line
(see Arguments to Specify the Goals) or with the
.DEFAULT_GOAL special variable (see Other Special Variables).
In the simple example of the previous section, the default goal is to
update the executable program edit; therefore, we put that rule
first.
Thus, when you give the command:
make
make reads the makefile in the current directory and begins by
processing the first rule. In the example, this rule is for relinking
edit; but before make can fully process this rule, it
must process the rules for the files that edit depends on,
which in this case are the object files. Each of these files is
processed according to its own rule. These rules say to update each
‘.o’ file by compiling its source file. The recompilation must
be done if the source file, or any of the header files named as
prerequisites, is more recent than the object file, or if the object
file does not exist.
The other rules are processed because their targets appear as
prerequisites of the goal. If some other rule is not depended on by the
goal (or anything it depends on, etc.), that rule is not processed,
unless you tell make to do so (with a command such as
make clean).
Before recompiling an object file, make considers updating its
prerequisites, the source file and header files. This makefile does not
specify anything to be done for them—the ‘.c’ and ‘.h’ files
are not the targets of any rules—so make does nothing for these
files. But make would update automatically generated C programs,
such as those made by Bison or Yacc, by their own rules at this time.
After recompiling whichever object files need it, make decides
whether to relink edit. This must be done if the file
edit does not exist, or if any of the object files are newer than
it. If an object file was just recompiled, it is now newer than
edit, so edit is relinked.
Thus, if we change the file insert.c and run make,
make will compile that file to update insert.o, and then
link edit. If we change the file command.h and run
make, make will recompile the object files kbd.o,
command.o and files.o and then link the file edit.