diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/make.texi | 48 |
1 files changed, 46 insertions, 2 deletions
diff --git a/doc/make.texi b/doc/make.texi index 20fc1c8..6fac114 100644 --- a/doc/make.texi +++ b/doc/make.texi @@ -158,6 +158,7 @@ Writing Rules * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. +* Prerequisite Types:: There are two types of prerequisites. * Wildcards:: Using wildcard characters such as `*'. * Directory Search:: Searching other directories for source files. * Phony Targets:: Using a target that is not a real file's name. @@ -1473,6 +1474,7 @@ the makefile (often with a target called @samp{all}). @menu * Rule Example:: An example explained. * Rule Syntax:: General syntax explained. +* Prerequisite Types:: There are two types of prerequisites. * Wildcards:: Using wildcard characters such as `*'. * Directory Search:: Searching other directories for source files. * Phony Targets:: Using a target that is not a real file's name. @@ -1524,7 +1526,7 @@ added to the prerequisites. @end itemize @end ifinfo -@node Rule Syntax, Wildcards, Rule Example, Rules +@node Rule Syntax, Prerequisite Types, Rule Example, Rules @section Rule Syntax @cindex rule syntax @@ -1593,7 +1595,49 @@ How to update is specified by @var{commands}. These are lines to be executed by the shell (normally @samp{sh}), but with some extra features (@pxref{Commands, ,Writing the Commands in Rules}). -@node Wildcards, Directory Search, Rule Syntax, Rules +@node Prerequisite Types, Wildcards, Rule Syntax, Rules +@comment node-name, next, previous, up +@section Types of Prerequisites +@cindex prerequisite types +@cindex types of prerequisites + +@cindex prerequisites, normal +@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 actually makes two statements: first, it imposes an order +of execution of build commands: any commands necessary to build any of +a target's prerequisites will be fully executed before any commands +necessary to build the target. 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it imposes a dependency +relationship: if any prerequisite is newer than the target, then the +target is considered out-of-date and must be rebuilt.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: + +@example +@var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites} +@end example + +The normal prerequisites section may of course be empty. Also, you +may still declare multiple lines of prerequisites for the same target: +they are appended appropriately. Note that if you declare the same +file to be both a normal and an order-only prerequisite, the normal +prerequisite takes precedence (since they are a strict superset of the +behavior of an order-only prerequisite). + +@node Wildcards, Directory Search, Prerequisite Types, Rules @section Using Wildcard Characters in File Names @cindex wildcard @cindex file name with wildcards |