What are double-colon rules in a Makefile for?
Section 4.13 of the GNU Make manual describes the so-called double-colon rules:
Double-colon rules are rules written with ‘::’ instead of ‘:’ after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule.
When a target appears in multiple rules, all the rules must be the same type: all ordinary, or all double-colon. If they are double-colon, each of them is independent of the others. Each double-colon rule's commands are executed if the target is older than any prerequisites of that rule. If there are no prerequisites for that rule, its commands are always executed (even if the target already exists). This can result in executing none, any, or all of the double-colon rules.
Double-colon rules with the same target are in fact completely separate from one another. Each double-colon rule is processed individually, just as rules with different targets are processed.
The double-colon rules for a target are executed in the order they appear in the makefile. However, the cases where double-colon rules really make sense are those where the order of executing the commands would not matter.
Double-colon rules are somewhat obscure and not often very useful; they provide a mechanism for cases in which the method used to update a target differs depending on which prerequisite files caused the update, and such cases are rare.
Each double-colon rule should specify commands; if it does not, an implicit rule will be used if one applies. See section Using Implicit Rules.
I kinda grok the meaning of each sentence of this section individually, but it's still not clear to me what double-colon rules are for. As for being rare, I have not yet seen any open-source project whose Makefile did not begin with
all::
Therefore: What's the intended purpose of double-colon rules in Makefiles?
Answer
Each :: rule is processed independently, so it can be simpler. For example, the single rule:
libxxx.a : sub1.o sub2.o
ar rv libxxx.a sub1.o
ar rv libxxx.a sub2.o
can be replaced with two simpler rules:
libxxx.a :: sub1.o
ar rv libxxx.a sub1.o
libxxx.a :: sub2.o
ar rv libxxx.a sub2.o
Utilities like AutoMake have an easier time spitting out many simple rules than a few complex ones.
A great answer with more examples was posted, then taken down, then found here:
Thanks to R.K. Owen for writing it, and Edward Minnix for finding it again!