= GNU Make

[[http://www.gnu.org/software/make/manual/html_node/|Manual]]

== Anatomy of a Make Rule 

{{{
target: prerequisite prerequisite prerequisite ...
    recipe
}}}

[[https://www.gnu.org/software/make/manual/html_node/Rule-Introduction.html|Source]]

== Misc

* A line beginning with ''#'' is a comment
* '@' at the beginning of a command runs the command but does not copy the line to stdout
* '-' at the beginning of a command runs the command but does not abort make if command fails
* ''%%make -n%%'' or ''%%make --dry-run%%'' - Don't actually run any commands; just print them.
* If a target begins with ''%'', the whole rule is a definition of an implicit rule, called //pattern rule//

== Automatic variables

* ''$@'' contains the name of the target. It does oviously not work in prerequsites, only in recipie!
* ''$<'' The name of the first prerequisite. Useful in the recipe
* ''$^'' The names of all prerequisites, with spaces between them
* ''$*'' is useful for constructing names of related files. FIXME

See [[https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html|Automatic-Variables]]

== Rules for overwriting variables

# Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value.
# An explicit assignment in the makefile overrides the environment
# An assignment in the makefile can be overridden by the command line argument, eg  {{{make CFLAGS='-g -O' [target] }}}

Sources: [[https://www.gnu.org/software/make/manual/html_node/Overriding.html|1]] [[https://www.gnu.org/software/make/manual/html_node/DESTDIR.html|2]]

== Variable assignment

<code bash>
a = Peter
c =  $(a)
d := $(a)
a = Franz

test:
        @echo c expands to $(c) # Franz
        @echo d expands to $(d) # Peter
</code>

== CFLAGS usage

> If there are C compiler options that must be used for proper compilation of certain files, do not include them in CFLAGS. **Users expect to be able to specify CFLAGS freely themselves**. Instead, arrange to pass the necessary options to the C compiler independently of CFLAGS, by writing them explicitly in the compilation commands or by defining an implicit rule. 

See also: [[https://www.gnu.org/software/make/manual/html_node/Command-Variables.html|See Command-Variables]], [[gnu_gcc]]

== Implicit rules

Example: ''file.o'' is made automatically from ''file.c'' with a command of the form `$(CC) -c $(CPPFLAGS) $(CFLAGS)'