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Diffstat (limited to 'doc/make.texi')
| -rw-r--r-- | doc/make.texi | 62 |
1 files changed, 46 insertions, 16 deletions
diff --git a/doc/make.texi b/doc/make.texi index 526aec6..cc5f061 100644 --- a/doc/make.texi +++ b/doc/make.texi @@ -1612,22 +1612,31 @@ static pattern rules, and simple prerequisite definitions. @cindex secondary expansion @cindex expansion, secondary +@findex .SECONDEXPANSION In the previous section we learned that GNU @code{make} works in two distinct phases: a read-in phase and a target-update phase -(@pxref{Reading Makefiles, , How @code{make} Reads a Makefile}). -There is an extra wrinkle that comes in between those two phases, -right at the end of the read-in phase: at that time, all the -prerequisites of all of the targets are expanded a @emph{second time}. -In most circumstances this secondary expansion will have no effect, -since all variable and function references will have been expanded -during the initial parsing of the makefiles. In order to take -advantage of the secondary expansion phase of the parser, then, it's -necessary to @emph{escape} the variable or function reference in the -makefile. In this case the first expansion merely un-escapes the -reference but doesn't expand it, and expansion is left to the -secondary expansion phase. For example, consider this makefile: - -@example +(@pxref{Reading Makefiles, , How @code{make} Reads a Makefile}). GNU +make also has the ability to enable a @emph{second expansion} of the +prerequisites (only) for some or all targets defined in the makefile. +In order for this second expansion to occur, the special target +@code{.SECONDEXPANSION} must be defined before the first prerequisite +list that makes use of this feature. + +If that special target is defined then in between the two phases +mentioned above, right at the end of the read-in phase, all the +prerequisites of the targets defined after the special target are +expanded a @emph{second time}. In most circumstances this secondary +expansion will have no effect, since all variable and function +references will have been expanded during the initial parsing of the +makefiles. In order to take advantage of the secondary expansion +phase of the parser, then, it's necessary to @emph{escape} the +variable or function reference in the makefile. In this case the +first expansion merely un-escapes the reference but doesn't expand it, +and expansion is left to the secondary expansion phase. For example, +consider this makefile: + +@example +.SECONDEXPANSION: ONEVAR = onefile TWOVAR = twofile myfile: $(ONEVAR) $$(TWOVAR) @@ -1651,6 +1660,7 @@ appear, unescaped, in the prerequisites list. One difference becomes apparent if the variables are reset; consider this example: @example +.SECONDEXPANSION: AVAR = top onefile: $(AVAR) twofile: $$(AVAR) @@ -1670,10 +1680,11 @@ target. This means that you can use variables such as @code{$@@}, expected values, just as in the command script. All you have to do is defer the expansion by escaping the @code{$}. Also, secondary expansion occurs for both explicit and implicit (pattern) rules. -Knowing this, the possible uses for this feature are almost endless. -For example: +Knowing this, the possible uses for this feature increase +dramatically. For example: @example +.SECONDEXPANSION: main_OBJS := main.o try.o test.o lib_OBJS := lib.o api.o @@ -1694,6 +1705,7 @@ You can also mix functions here, as long as they are properly escaped: main_SRCS := main.c try.c test.c lib_SRCS := lib.c api.c +.SECONDEXPANSION: main lib: $$(patsubst %.c,%.o,$$($$@@_SRCS)) @end example @@ -1723,6 +1735,8 @@ the same target (@code{$$+} with repetitions and @code{$$^} without). The following example will help illustrate these behaviors: @example +.SECONDEXPANSION: + foo: foo.1 bar.1 $$< $$^ $$+ # line #1 foo: foo.2 bar.2 $$< $$^ $$+ # line #2 @@ -1763,6 +1777,8 @@ target pattern. The value of the automatic variables is derived in the same fashion as for static pattern rules. As an example: @example +.SECONDEXPANSION: + foo: bar foo foz: fo%: bo% @@ -1781,6 +1797,8 @@ expansion) to all the patterns in the prerequisites list. As an example: @example +.SECONDEXPANSION: + /tmp/foo.o: %.o: $$(addsuffix /%.c,foo bar) foo.h @@ -2880,6 +2898,18 @@ intermediate files, except that they are never automatically deleted. as secondary (i.e., no target is removed because it is considered intermediate). +@findex .SECONDEXPANSION +@item .SECONDEXPANSION + +If @code{.SECONDEXPANSION} is mentioned as a target anwyeren in the +makefile, then all prerequisite lists defined @emph{after} it appears +will be expanded a second time after all makefiles have been read in. +@xref{Secondary Expansion, ,Secondary Expansion}. + +The prerequisites of the special target @code{.SUFFIXES} are the list +of suffixes to be used in checking for suffix rules. +@xref{Suffix Rules, , Old-Fashioned Suffix Rules}. + @findex .DELETE_ON_ERROR @item .DELETE_ON_ERROR @cindex removing targets on failure |