File: make.info,  Node: Target-specific,  Next: Pattern-specific,  Prev: Environment,  Up: Using Variables

6.11 Target-specific Variable Values
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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 (*note Let Function::) or the 'foreach' function (*note Foreach
Function::, and automatic variables (*note 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.