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I'm trying to organize a Makefile with complex commands into something more readable.

I'm trying to break down a long list of parameters, but even single parameters are too bad, hence I wanted to break them too. But I cannot find a way to avoid unwanted spaces for these cases

# network and forward ports
QEMU_NET_FLAGS=-nic user
QEMU_NET_FLAGS+=,id=n1,restrict=on,ipv6=off,hostname=$(VMNAME)
# port forwards:
# hostfwd=[tcp|udp]:[hostaddr]:hostport-[guestaddr]:guestport
QEMU_NET_FLAGS+=,hostfwd=tcp::$(VMSSH)-:22
QEMU_NET_FLAGS+=,hostfwd=tcp::8000-:8000
QEMU_NET_FLAGS+=,hostfwd=tcp::8080-:8080
QEMU_NET_FLAGS+=,hostfwd=tcp::8081-:8081

Will cause -nic user ,id=1... instead of -nic user,id=1...

Tried ::=

First suggestion by @steeldriver:

QEMU_NET_FLAGS::=-nic user
QEMU_NET_FLAGS::=$(QEMU_NET_FLAGS),id=n1,restrict=on,ipv6=off,hostname=$(VMNAME)
QEMU_NET_FLAGS::=$(QEMU_NET_FLAGS),hostfwd=tcp::$(VMSSH)-:22

causes another problem since these are "templates", and variables like VMNAME and VMSSH should resolve late (recursively) when the resulting command line is used. And if I use :::= to then += values recursively, we are back to the unwanted space problem.

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    The KISS solution would probably be a simple (non-recursive i.e. :=) assignment of the form QEMU_NET_FLAGS := $(QEMU_NET_FLAGS),id=n1,... Commented May 14, 2024 at 21:31
  • @steeldriver that's actually a good overal solution. I'd rather repeat the assignment part like so instead of mashing everything in a single line. Commented May 15, 2024 at 7:36
  • @steeldriver almost, this only creates a problem since those are "template" commands that require late substitution for things like $(VMNAME)... trying to fiddle a solution to answer here Commented May 15, 2024 at 9:02

1 Answer 1

Reset to default
1 
# helper for subst
null :=
space := ${null} ${null}

QEMU_NET_FLAGS = user
QEMU_NET_FLAGS += ,id=n1,restrict=on,ipv6=off,hostname=$(VMNAME)
QEMU_NET_FLAGS += ,hostfwd=tcp::$(VMSSH)-:22

QEMU_RUN_FLAGS+=-nic $(subst ${space},${null},$(QEMU_NET_FLAGS))

now when later I use this by setting the VMNAME and VMSSH variables and use $(QMEU_RUN_FLAGS) i will get the correct value, late/recursively substituted, and without spaces.

But I will leave this question open as it is not the best syntax experience. Notably, i had to change QEMU_NET_FLAGS = -nic user (with an intentional space) to add the parts with intentional space on the final concatenation after the subst in QEMU_RUN_FLAGS+=-nic $(subst ...

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