A description for Makefile syntax used in OpenWRT

1. Makefile
Antony
1

2. call

The call can be used to create new
parameterized functions.

$(call variable,param,param,…)
reverse = $(2) $(1)
foo = $(call reverse,a,b)

This would be equal to
foo = b a

A function to manipulate, then return a
string.
2

3. define

define’ in makefile is like ‘=‘ operator.

It allows multiple lines and it is ended with
endef.

Eg.
define PROGRAM_template =
$(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
ALL_OBJS += $$($(1)_OBJS)
endef
$(call PROGRAM_template,$(prog))
3

4. eval

dynamic makefile construction 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.

A function, to use the input string as a part
of Makfile.
4

5. Example for call, define, and eval
PROGRAMS = server client
server_OBJS = server.o
client_OBJS = client.o
# Everything after this is generic
.PHONY: all
all: $(PROGRAMS)
define PROGRAM =
$(1): $$($(1)_OBJS)
ALL_OBJS += $$($(1)_OBJS)
endef
$(foreach prog,$(PROGRAMS),
$(eval $(call PROGRAM,$(prog))))
--expand 1.
prog = server client
server: $server_OBJS
ALL_OBJS += $(server_OBJS)
client: $client_OBJS
ALL_OBJS += $(client_OBJS)
--expand2.
server : server.o
ALL_OBJS += server.o
client: client.o
ALL_OBJS += client.o
5

6. Appendix: Variables

$? ： updated dependencies, which is newer
than target itself.

$@ ： the value of targets.

$< ： the first value of dependencies.

$* : all files (w/o extension) in targets.
6

7. Reference

https://www.gnu.org/software/make/manual/html
_node/Call-Function.html

https://www.gnu.org/software/make/manual/html
_node/Eval-Function.html

http://kevincrazy.pixnet.net/blog/post/29780477-
makefile
%E7%B0%A1%E6%98%93%E6%95%99%E5%AD
%B8...

http://wiki.ubuntu.org.cn/index.php?title=
%E8%B7%9F
%E6%88%91%E4%B8%80%E8%B5%B7%E5%86
%99Makefile:%E4%BD%BF
%E7%94%A8%E5%87%BD 7