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| -rw-r--r-- | make.texinfo | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/make.texinfo b/make.texinfo index 2583b05..8c91c5e 100644 --- a/make.texinfo +++ b/make.texinfo @@ -2090,6 +2090,7 @@ sets @samp{bar} to @samp{a.c b.c c.c}. @xref{Setting}. @subsection Recursive References @cindex recursive variable reference +@cindex variable reference, recursive Variables may be referenced inside a variable reference. This is called a @dfn{recursive variable reference}. For example, @@ -2140,6 +2141,74 @@ but it works: @samp{$($($(z)))} expands to @samp{$($(y))} which becomes @samp{variable2} in @samp{$(x)} and finally expands to @samp{$(variable2)}, a simple variable reference that becomes @samp{Hello}.@refill +Recursive variable references need not be as simple as @samp{$($(a))}. +They can contain several variable references: + +@example +a_dirs := dira dirb +1_dirs := dir1 dir2 + +a_files := filea fileb +1_files := file1 file2 + +ifeq "$(use_a)" "yes" +a1 := a +else +a1 := 1 +endif + +ifeq "$(use_dirs)" "yes" +df := dirs +else +df := files +endif + +dirs := $($(a1)_$(df)) +@end example + +@noindent +will give @samp{dirs} the value of @samp{a_dirs}, @samp{1_dirs}, +@samp{a_files} or @samp{1_files} depending on the settings of @samp{use_a} +and @samp{use_dirs}.@refill + +Recursive variable references can also be part of a modified reference: + +@example +a_objects := a.o b.o c.o +1_objects := 1.o 2.o 3.o + +sources := $($(a1)_object:.o=.c) +@end example + +@noindent +defines @samp{sources} as either @samp{a.c b.c c.c} or @samp{1.c 2.c 3.c}, +depending on the value of @samp{a1}. + +The only restriction on this sort of use of recursive variable references +is that they cannot specify part of a function invokation. This is because +the search for an initial word matching defined function is done before the +check for a variable name containing a dollar sign. Thus, + +@example +ifdef do_sort +func := sort +else +func := strip +endif + +bar := a d b g q c + +foo := $($(func) $(bar)) +@end example + +@noindent +attempts to give @samp{foo} the value of the variable @samp{sort a d b g +q c} or @samp{strip a d b g q c}, rather than giving @samp{a d b g +q c} as the argument to either the @code{sort} or the @code{strip} +function. This is an implementation restriction which may be removed in +the future if the need arises, but this is not likely since there are not +really any two functions you would want to give the same arguments.@refill + Recursive variable references are a complicated concept needed only for very complex makefile programming. You need not worry about them in general, except to know that making a variable with a dollar sign in its @@ -4659,6 +4728,9 @@ Intermediate implicit files. @xref{Chained Rules}. Special search method for library dependencies written in the form @samp{-l@var{name}}. @xref{Libraries/Search}. +@item +Allowing suffixes for suffix rules (@pxref{Suffix Rules}) to contain +any characters. In other version of @code{make}, they must begin with @samp{.} and not contain any @samp{/} characters. @item |