make: Secondary Expansion
3.8 Secondary Expansion
=======================
In the previous section we learned that GNU 'make' works in two distinct
phases: a read-in phase and a target-update phase (
How 'make'
Reads a Makefile Reading Makefiles.). GNU make also has the ability to
enable a _second expansion_ of the prerequisites (only) for some or all
targets defined in the makefile. In order for this second expansion to
occur, the special target '.SECONDEXPANSION' must be defined before the
first prerequisite list that makes use of this feature.
If that special target is defined then in between the two phases
mentioned above, right at the end of the read-in phase, all the
prerequisites of the targets defined after the special target are
expanded a _second time_. In most circumstances this secondary
expansion will have no effect, since all variable and function
references will have been expanded during the initial parsing of the
makefiles. In order to take advantage of the secondary expansion phase
of the parser, then, it's necessary to _escape_ the variable or function
reference in the makefile. In this case the first expansion merely
un-escapes the reference but doesn't expand it, and expansion is left to
the secondary expansion phase. For example, consider this makefile:
.SECONDEXPANSION:
ONEVAR = onefile
TWOVAR = twofile
myfile: $(ONEVAR) $$(TWOVAR)
After the first expansion phase the prerequisites list of the
'myfile' target will be 'onefile' and '$(TWOVAR)'; the first (unescaped)
variable reference to ONEVAR is expanded, while the second (escaped)
variable reference is simply unescaped, without being recognized as a
variable reference. Now during the secondary expansion the first word
is expanded again but since it contains no variable or function
references it remains the value 'onefile', while the second word is now
a normal reference to the variable TWOVAR, which is expanded to the
value 'twofile'. The final result is that there are two prerequisites,
'onefile' and 'twofile'.
Obviously, this is not a very interesting case since the same result
could more easily have been achieved simply by having both variables
appear, unescaped, in the prerequisites list. One difference becomes
apparent if the variables are reset; consider this example:
.SECONDEXPANSION:
AVAR = top
onefile: $(AVAR)
twofile: $$(AVAR)
AVAR = bottom
Here the prerequisite of 'onefile' will be expanded immediately, and
resolve to the value 'top', while the prerequisite of 'twofile' will not
be full expanded until the secondary expansion and yield a value of
'bottom'.
This is marginally more exciting, but the true power of this feature
only becomes apparent when you discover that secondary expansions always
take place within the scope of the automatic variables for that target.
This means that you can use variables such as '$@', '$*', etc. during
the second expansion and they will have their expected values, just as
in the recipe. All you have to do is defer the expansion by escaping
the '$'. Also, secondary expansion occurs for both explicit and
implicit (pattern) rules. Knowing this, the possible uses for this
feature increase dramatically. For example:
.SECONDEXPANSION:
main_OBJS := main.o try.o test.o
lib_OBJS := lib.o api.o
main lib: $$($$@_OBJS)
Here, after the initial expansion the prerequisites of both the
'main' and 'lib' targets will be '$($@_OBJS)'. During the secondary
expansion, the '$@' variable is set to the name of the target and so the
expansion for the 'main' target will yield '$(main_OBJS)', or 'main.o
try.o test.o', while the secondary expansion for the 'lib' target will
yield '$(lib_OBJS)', or 'lib.o api.o'.
You can also mix in functions here, as long as they are properly
escaped:
main_SRCS := main.c try.c test.c
lib_SRCS := lib.c api.c
.SECONDEXPANSION:
main lib: $$(patsubst %.c,%.o,$$($$@_SRCS))
This version allows users to specify source files rather than object
files, but gives the same resulting prerequisites list as the previous
example.
Evaluation of automatic variables during the secondary expansion
phase, especially of the target name variable '$$@', behaves similarly
to evaluation within recipes. However, there are some subtle
differences and "corner cases" which come into play for the different
types of rule definitions that 'make' understands. The subtleties of
using the different automatic variables are described below.
Secondary Expansion of Explicit Rules
-------------------------------------
During the secondary expansion of explicit rules, '$$@' and '$$%'
evaluate, respectively, to the file name of the target and, when the
target is an archive member, the target member name. The '$$<' variable
evaluates to the first prerequisite in the first rule for this target.
'$$^' and '$$+' evaluate to the list of all prerequisites of rules _that
have already appeared_ for the same target ('$$+' with repetitions and
'$$^' without). The following example will help illustrate these
behaviors:
.SECONDEXPANSION:
foo: foo.1 bar.1 $$< $$^ $$+ # line #1
foo: foo.2 bar.2 $$< $$^ $$+ # line #2
foo: foo.3 bar.3 $$< $$^ $$+ # line #3
In the first prerequisite list, all three variables ('$$<', '$$^',
and '$$+') expand to the empty string. In the second, they will have
values 'foo.1', 'foo.1 bar.1', and 'foo.1 bar.1' respectively. In the
third they will have values 'foo.1', 'foo.1 bar.1 foo.2 bar.2', and
'foo.1 bar.1 foo.2 bar.2 foo.1 foo.1 bar.1 foo.1 bar.1' respectively.
Rules undergo secondary expansion in makefile order, except that the
rule with the recipe is always evaluated last.
The variables '$$?' and '$$*' are not available and expand to the
empty string.
Secondary Expansion of Static Pattern Rules
-------------------------------------------
Rules for secondary expansion of static pattern rules are identical to
those for explicit rules, above, with one exception: for static pattern
rules the '$$*' variable is set to the pattern stem. As with explicit
rules, '$$?' is not available and expands to the empty string.
Secondary Expansion of Implicit Rules
-------------------------------------
As 'make' searches for an implicit rule, it substitutes the stem and
then performs secondary expansion for every rule with a matching target
pattern. The value of the automatic variables is derived in the same
fashion as for static pattern rules. As an example:
.SECONDEXPANSION:
foo: bar
foo foz: fo%: bo%
%oo: $$< $$^ $$+ $$*
When the implicit rule is tried for target 'foo', '$$<' expands to
'bar', '$$^' expands to 'bar boo', '$$+' also expands to 'bar boo', and
'$$*' expands to 'f'.
Note that the directory prefix (D), as described in Implicit
Rule Search Algorithm Implicit Rule Search, is appended (after
expansion) to all the patterns in the prerequisites list. As an
example:
.SECONDEXPANSION:
/tmp/foo.o:
%.o: $$(addsuffix /%.c,foo bar) foo.h
@echo $^
The prerequisite list printed, after the secondary expansion and
directory prefix reconstruction, will be '/tmp/foo/foo.c /tmp/bar/foo.c
foo.h'. If you are not interested in this reconstruction, you can use
'$$*' instead of '%' in the prerequisites list.
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