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### The `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 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)