File: make.info,  Node: Double-Colon,  Next: Automatic Prerequisites,  Prev: Static Pattern,  Up: Rules

4.13 Double-Colon Rules
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"Double-colon" rules are explicit 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.  Pattern rules
with double-colons have an entirely different meaning (*note
Match-Anything Rules::).

   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
recipe is executed if the target is older than any prerequisites of that
rule.  If there are no prerequisites for that rule, its recipe is 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 recipes
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 a recipe; if it does not, an
implicit rule will be used if one applies.  *Note Using Implicit Rules:
Implicit Rules.