Variable values in
make are usually global; that is, they are the
same regardless of where they are evaluated (unless they’re reset, of
course). Exceptions to that are variables defined with the
function (see The
let Function) or the
foreach Function, and automatic variables
(see Automatic Variables).
Another exception are target-specific variable values. This
feature allows you to define different values for the same variable,
based on the target that
make is currently building. As with
automatic variables, these values are only available within the context
of a target’s recipe (and in other target-specific assignments).
Set a target-specific variable value like this:
target … : variable-assignment
Target-specific variable assignments can be prefixed with any or all of the
private; these apply their normal behavior to this instance of the
Multiple target values create a target-specific variable value for each member of the target list individually.
The variable-assignment can be any valid form of assignment; recursive (‘=’), simple (‘:=’ or ‘::=’), immediate (‘::=’), appending (‘+=’), or conditional (‘?=’). All variables that appear within the variable-assignment are evaluated within the context of the target: thus, any previously-defined target-specific variable values will be in effect. Note that this variable is actually distinct from any “global” value: the two variables do not have to have the same flavor (recursive vs. simple).
Target-specific variables have the same priority as any other makefile
variable. Variables provided on the command line (and in the
environment if the ‘-e’ option is in force) will take precedence.
override directive will allow the target-specific
variable value to be preferred.
There is one more special feature of target-specific variables: when you define a target-specific variable that variable value is also in effect for all prerequisites of this target, and all their prerequisites, etc. (unless those prerequisites override that variable with their own target-specific variable value). So, for example, a statement like this:
prog : CFLAGS = -g prog : prog.o foo.o bar.o
CFLAGS to ‘-g’ in the recipe for prog, but
it will also set
CFLAGS to ‘-g’ in the recipes that create
prog.o, foo.o, and bar.o, and any recipes which
create their prerequisites.
Be aware that a given prerequisite will only be built once per invocation of make, at most. If the same file is a prerequisite of multiple targets, and each of those targets has a different value for the same target-specific variable, then the first target to be built will cause that prerequisite to be built and the prerequisite will inherit the target-specific value from the first target. It will ignore the target-specific values from any other targets.