File: make.info,  Node: Overriding,  Next: Testing,  Prev: Avoiding Compilation,  Up: Running

9.5 Overriding Variables
========================

An argument that contains '=' specifies the value of a variable: 'V=X'
sets the value of the variable V to X.  If you specify a value in this
way, all ordinary assignments of the same variable in the makefile are
ignored; we say they have been "overridden" by the command line
argument.

   The most common way to use this facility is to pass extra flags to
compilers.  For example, in a properly written makefile, the variable
'CFLAGS' is included in each recipe that runs the C compiler, so a file
'foo.c' would be compiled something like this:

     cc -c $(CFLAGS) foo.c

   Thus, whatever value you set for 'CFLAGS' affects each compilation
that occurs.  The makefile probably specifies the usual value for
'CFLAGS', like this:

     CFLAGS=-g

   Each time you run 'make', you can override this value if you wish.
For example, if you say 'make CFLAGS='-g -O'', each C compilation will
be done with 'cc -c -g -O'.  (This also illustrates how you can use
quoting in the shell to enclose spaces and other special characters in
the value of a variable when you override it.)

   The variable 'CFLAGS' is only one of many standard variables that
exist just so that you can change them this way.  *Note Variables Used
by Implicit Rules: Implicit Variables, for a complete list.

   You can also program the makefile to look at additional variables of
your own, giving the user the ability to control other aspects of how
the makefile works by changing the variables.

   When you override a variable with a command line argument, you can
define either a recursively-expanded variable or a simply-expanded
variable.  The examples shown above make a recursively-expanded
variable; to make a simply-expanded variable, write ':=' or '::='
instead of '='.  But, unless you want to include a variable reference or
function call in the _value_ that you specify, it makes no difference
which kind of variable you create.

   There is one way that the makefile can change a variable that you
have overridden.  This is to use the 'override' directive, which is a
line that looks like this: 'override VARIABLE = VALUE' (*note The
'override' Directive: Override Directive.).