[libvirt PATCH] build: workaround behaviour regression in gnu make 4.3
Eric Blake
eblake at redhat.com
Fri Mar 13 19:02:35 UTC 2020
On 3/13/20 1:42 PM, Daniel P. Berrangé wrote:
> We need the "$(space)" variable to contain a single whitespace
> character. We do this by assigning and then appending an empty
> string to the variable. Variable appends get separated by a
> single whitespace historically, but GNU make 4.3 introduced a
> behaviour regression.
>
> https://lists.gnu.org/archive/html/bug-make/2020-01/msg00057.html
>
> [quote]
> * WARNING: Backward-incompatibility!
> Previously appending using '+=' to an empty variable would
> result in a value starting with a space. Now the initial
> space is only added if the variable already contains some
> value. Similarly, appending an empty string does not
> add a trailing space.
> [/quote]
>
> This patch tries a new trick to get a single whitespace by
> getting make to expand two non-existant variables separated
> by a space.
Reviewed-by: Eric Blake <eblake at redhat.com>
https://blog.jgc.org/2007/06/escaping-comma-and-space-in-gnu-make.html
needs to be updated to cover this change in GNU make, but it also points
out that GNU make permits:
$(space) = $(null) $(null)
as a way to then write [$( )] or [$ ] instead of [$(space)] (all three
producing "[ ]"), if you really want to go crazy with odd macro names.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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