make: How Make Works
2.3 How 'make' Processes a Makefile
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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
DONTPRINTYET behavior using the command line (
Arguments to Specify the Goals
Goals.) or with the '.DEFAULT_GOAL' special variable (*noteOther
DONTPRINTYET behavior using the command line (Arguments to Specify the Goals
Goals.) or with the '.DEFAULT_GOAL' special variable (Other
Special Variables 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'.
Info Catalog
make: Simple Makefile
make: Introduction
make: Variables Simplify