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eval
Function
The eval
function is very special: it allows you to define new
makefile constructs that are not constant; which are the result of
evaluating other variables and functions. The argument to the
eval
function is expanded, then the results of that expansion
are parsed as makefile syntax. The expanded results can define new
make
variables, targets, implicit or explicit rules, etc.
The result of the eval
function is always the empty string;
thus, it can be placed virtually anywhere in a makefile without
causing syntax errors.
It's important to realize that the eval
argument is expanded
twice; first by the eval
function, then the results of
that expansion are expanded again when they are parsed as makefile
syntax. This means you may need to provide extra levels of escaping
for "$" characters when using eval
. The value
function (see section 8.7 The value
Function) can sometimes be useful in these
situations, to circumvent unwanted expansions.
Here is an example of how eval
can be used; this example
combines a number of concepts and other functions. Although it might
seem overly complex to use eval
in this example, rather than
just writing out the rules, consider two things: first, the template
definition (in PROGRAM_template
) could need to be much more
complex than it is here; and second, you might put the complex,
"generic" part of this example into another makefile, then include
it in all the individual makefiles. Now your individual makefiles are
quite straightforward.
PROGRAMS = server client server_OBJS = server.o server_priv.o server_access.o server_LIBS = priv protocol client_OBJS = client.o client_api.o client_mem.o client_LIBS = protocol # Everything after this is generic .PHONY: all all: $(PROGRAMS) define PROGRAM_template $(1): $$($(1)_OBJ) $$($(1)_LIBS:%=-l%) ALL_OBJS += $$($(1)_OBJS) endef $(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog)))) $(PROGRAMS): $(LINK.o) $^ $(LDLIBS) -o $@ clean: rm -f $(ALL_OBJS) $(PROGRAMS) |
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