diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/make.texi | 83 |
1 files changed, 53 insertions, 30 deletions
diff --git a/doc/make.texi b/doc/make.texi index 82df90a..8f86c0c 100644 --- a/doc/make.texi +++ b/doc/make.texi @@ -3190,9 +3190,11 @@ to precisely the targets specified. @cindex multiple rules for one target (@code{::}) @cindex @code{::} rules (double-colon) -@dfn{Double-colon} rules are rules written with @samp{::} instead of -@samp{:} after the target names. They are handled differently from -ordinary rules when the same target appears in more than one rule. +@dfn{Double-colon} rules are explicit rules written with @samp{::} +instead of @samp{:} 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 (@pxref{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 @@ -9148,6 +9150,10 @@ in fact any prerequisites at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. @xref{Last Resort}. +More than one pattern rule may match a target. In this case +@code{make} will choose the ``best fit'' rule. @xref{Pattern Match, +,How Patterns Match}. + @c !!! The end of of this paragraph should be rewritten. --bob Pattern rules may have more than one target. Unlike normal rules, this does not act as many different rules with the same prerequisites @@ -9163,33 +9169,6 @@ updated themselves. @cindex multiple targets, in pattern rule @cindex target, multiple in pattern rule -It is possible that several pattern rules can be used to update a -target. In this case @code{make} considers rules which produce -shorter stems first. This results in more specific rules being -preferred to the more generic ones, for example: - -@example -%.o: %.c - $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@ - -lib/%.o: lib/%.c - $(CC) -fPIC -c $(CFLAGS) $(CPPFLAGS) $< -o $@@ - -all: foo.o lib/bar.o -@end example - -In this example the second rule will be used to update @file{lib/bar.o} -even though the first rule can also be used. - -Pattern rules which result in the same stem length are considered in -the order in which they appear in the makefile. Of equally applicable -rules, only the first one found is used. The rules you write take -precedence over those that are built in. Note however, that a rule whose -prerequisites actually exist or are mentioned always takes priority over a -rule with prerequisites that must be made by chaining other implicit rules. -@cindex pattern rules, order of -@cindex order of pattern rules - @node Pattern Examples, Automatic Variables, Pattern Intro, Pattern Rules @subsection Pattern Rule Examples @@ -9502,6 +9481,50 @@ rest of the stem is substituted for the @samp{%}. The stem @samp{src/a} with a prerequisite pattern @samp{c%r} gives the file name @file{src/car}.@refill +@cindex pattern rules, order of +@cindex order of pattern rules +A pattern rule can be used to build a given file only if there is a +target pattern that matches the file name, @emph{and} all +prerequisites in that rule either exist or can be built. The rules +you write take precedence over those that are built in. Note however, +that a rule whose prerequisites actually exist or are mentioned always +takes priority over a rule with prerequisites that must be made by +chaining other implicit rules. + +@cindex stem, shortest +It is possible that more than one pattern rule will meet these +criteria. In that case, @code{make} will choose the rule with the +shortest stem (that is, the pattern that matches most specifically). +If more than one pattern rule has the shortest stem, @code{make} will +choose the first one found in the makefile. + +This algorithm results in more specific rules being preferred over +more generic ones; for example: + +@example +%.o: %.c + $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@ + +%.o : %.f + $(COMPILE.F) $(OUTPUT_OPTION) $< + +lib/%.o: lib/%.c + $(CC) -fPIC -c $(CFLAGS) $(CPPFLAGS) $< -o $@@ +@end example + +Given these rules and asked to build @file{bar.o} where both +@file{bar.c} and @file{bar.f} exist, @code{make} will choose the first +rule and compile @file{bar.c} into @file{bar.o}. In the same +situation where @file{bar.c} does not exist, then @code{make} will +choose the second rule and compile @file{bar.f} into @file{bar.o}. + +If @code{make} is asked to build @file{lib/bar.o} and both +@file{lib/bar.c} and @file{lib/bar.f} exist, then the third rule will +be chosen since the stem for this rule (@samp{bar}) is shorter than +the stem for the first rule (@samp{lib/bar}). If @file{lib/bar.c} +does not exist then the third rule is not eligible and the second rule +will be used, even though the stem is longer. + @node Match-Anything Rules, Canceling Rules, Pattern Match, Pattern Rules @subsection Match-Anything Pattern Rules |