Tuesday, June 15, 2010

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

4.13 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. *Note Using Implicit Rules:
Implicit Rules.

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