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* doc/make.texi (Prerequisite Types): [SV 58056] Clarify ordering.

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Paul Smith 2022-10-02 09:30:20 -04:00
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doc/make.texi
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@ -2075,26 +2075,25 @@ extra features (@pxref{Recipes, ,Writing Recipes in Rules}).
@cindex normal prerequisites
@cindex prerequisites, order-only
@cindex order-only prerequisites
There are actually two different types of prerequisites understood by
GNU @code{make}: normal prerequisites such as described in the
previous section, and @dfn{order-only} prerequisites. A normal
prerequisite makes two statements: first, it imposes an order in which
recipes will be invoked: the recipes for all prerequisites of a target
will be completed before the recipe for the target is run. Second, it
imposes a dependency relationship: if any prerequisite is newer than
the target, then the target is considered out-of-date and must be
There are two different types of prerequisites understood by GNU @code{make}:
normal prerequisites, described in the previous section, and @dfn{order-only}
prerequisites. A normal prerequisite makes two statements: first, it imposes
an order in which recipes will be invoked: the recipes for all prerequisites
of a target will be completed before the recipe for the target is started.
Second, it imposes a dependency relationship: if any prerequisite is newer
than the target, then the target is considered out-of-date and must be
rebuilt.
Normally, this is exactly what you want: if a target's prerequisite is
updated, then the target should also be updated.
Occasionally, however, you have a situation where you want to impose a
specific ordering on the rules to be invoked @emph{without} forcing
the target to be updated if one of those rules is executed. In that
case, you want to define @dfn{order-only} prerequisites. Order-only
prerequisites can be specified by placing a pipe symbol (@code{|})
in the prerequisites list: any prerequisites to the left of the pipe
symbol are normal; any prerequisites to the right are order-only:
Occasionally you may want to ensure that a prerequisite is built before a
target, but @emph{without} forcing the target to be updated if the
prerequisite is updated. @dfn{Order-only} prerequisites are used to create
this type of relationship. Order-only prerequisites can be specified by
placing a pipe symbol (@code{|}) in the prerequisites list: any prerequisites
to the left of the pipe symbol are normal; any prerequisites to the right are
order-only:
@example
@var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites}