Dear make maintainer,
I am a happy user of make (various implementations) for 25 years. Today I
learned that "multiple target rules" don't do what I thought they do.
While dealing with this, I think I've found a bug, either in code or in
documentation. Plus I have a related suggestion for the doc.
1. multi target with "&:" ran twice
Attached is a minimal test case. When run with -j2, it executes the "foo1
foo2" rule twice, in parallel. The related documentation is:
https://www.gnu.org/software/make/manual/make.html#Multiple-Targets
Section "Rules with Grouped Targets" says: "When make builds any one of
the grouped targets, it understands that all the other targets in the
group are also updated as a result of the invocation of the recipe.
Furthermore, if only some of the grouped targets are out of date or
missing make will realize that running the recipe will update all of the
targets."
This suggests GNU make would understand that both foo1 and foo2 will be
produced by the rule, which implies it wouldn't run it twice, in parallel.
Since the attached test case does run it twice in parallel, I think it's
either a bug in the implementation or in the documentation
2. Doc clarification in the normal ":" syntax
It was entirely my fault that I misunderstood what a multiple target rule
with the plain ":" separator would do. While initially searching the web
for why my ":" multi target rule ran parallel, I found some other users
had similar problems.
I think it's generally surprising that with the plain ":" syntax multi
target rules won't get Make to understand that all targets are created.
The documentation is clear about this in an implicit way, but I had to
read it multiple times to make sure.
Would it be possible to mention this explicitly in the doc?
Thank you in advance,
Tibor 'Igor2' Palinkas
all: bar1 bar2
foo1 foo2 &:
date > foo1
sleep 1
date > foo2
bar1: foo1 foo2
cat foo1 foo2 > bar1
bar2: foo1 foo2
cat foo1 foo2 > bar2
clean:
rm foo1 foo2 bar1 bar2
