Simplest way to reverse the order of strings in a make variable

Question

Let's say you have a variable in a makefile fragment like the following:

MY_LIST=a b c d

How do I then reverse the order of that list? I need:

$(warning MY_LIST=${MY_LIST}) 

to show

MY_LIST=d c b a

Edit: the real problem is that

ld -r some_object.o ${MY_LIST}

produces an a.out with undefined symbols because the items in MY_LIST are actually archives, but in the wrong order. If the order of MY_LIST is reversed, it will link correctly (I think). If you know a smarter way to get the link order right, clue me in.

Answer 1

A solution in pure GNU make:

default: all

foo = please reverse me

reverse = $(if $(1),$(call reverse,$(wordlist 2,$(words $(1)),$(1)))) $(firstword $(1))

all : @echo $(call reverse,$(foo))

Gives:

$ make

me reverse please

Answer 2

Doh! I could have just used a shell script-let:

(for d in ${MY_LIST}; do echo $$d; done) | tac

Answer 3

You can also define search groups with ld:

ld -r foo.o -( a.a b.a c.a -)

Will iterate through a.a, b.a, and c.a until no new unresolved symbols can be satisfied by any object in the group.

If you're using gnu ld, you can also do:

ld -r -o foo.o --whole-archive bar.a

Which is slightly stronger, in that it will include every object from bar.a regardless of whether it satisfies an unresolved symbol from foo.o.

Answer 4

An improvement to the GNU make solution:

reverse = $(if $(wordlist 2,2,$(1)),$(call reverse,$(wordlist 2,$(words $(1)),$(1))) $(firstword $(1)),$(1))

• better stopping condition, original uses the empty string wasting a function call
• doesn't add a leading space to the reversed list, unlike the original

Answer 5

Playing off of both Ben Collins' and elmarco's answers, here's a punt to bash which handles whitespace "properly"¹

reverse = $(shell printf "%s\n" $(strip $1) | tac)

which does the right thing, thanks to $(shell) automatically cleaning whitespace and printf automatically formatting each word in its arg list:

$(info [ $(call reverse,  one two   three four  ) ] )

yields:

[ four three two one ]

¹...according to my limited test case (i.e., the $(info ...) line, above).