File: make.info,  Node: Eval Function,  Next: Origin Function,  Prev: Value Function,  Up: Functions

8.10 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 (*note 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): $$($(1)_OBJS) $$($(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)