6.11 Target-specific Variable Values

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 let function (see The let Function) or the foreach function (see 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 special keywords export, unexport, override, or private; these apply their normal behavior to this instance of the variable only.

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. Specifying the 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

will set 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.