|GNU Make Manual||www.imodulo.com · 2003-04-05|
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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 command 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:
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.)
CFLAGS is only one of many standard variables that exist just so that you can change them this way. Variables Used by Implicit Rules, 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 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
:= 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 (The
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