GNU make manual 翻译(九十四)
继续翻译
4.5.4 Writing Recipes with Directory Search ------------------------------------------- When a prerequisite is found in another directory through directory search, this cannot change the recipe of the rule; they will execute as written. Therefore, you must write the recipe with care so that it will look for the prerequisite in the directory where `make' finds it. This is done with the "automatic variables" such as `$^' (*note Automatic Variables::). For instance, the value of `$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus: foo.o : foo.c cc -c $(CFLAGS) $^ -o $@ (The variable `CFLAGS' exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; *note Variables Used by Implicit Rules: Implicit Variables.) Often the prerequisites include header files as well, which you do not want to mention in the recipe. The automatic variable `$<' is just the first prerequisite: VPATH = src:../headers foo.o : foo.c defs.h hack.h cc -c $(CFLAGS) $< -o $@
4.5.4 写目录搜索相关的片段
-------------------------------------------
当一个前提条件通过目录搜索在另一个目录中被发现,这并不会改变规则的片段;片段将按照所写的被执行。因此,你必须仔细地书写片段这样它可以在make找到的目录中搜索前提条件。
这是用 类似$^ 之类的自动变量来实现的(*note Automatic Variables::)。例如,$^的值是规则中所有前提条件的列表,其中也包括它们所在目录的名字,而$@是目的的名字。因此:
foo.o : foo.c
cc -c $(CFLAGS) $^ -o $@
(变量 CFLAGS 已存在,所以你可以用隐式规则来为C编译指定此标志;我们为一贯性考虑而是用它,所以它将影响到所有的C 编译; *note Variables Used by Implicit Rules: Implicit Variables)
通常,前提条件中也包括头文件,你可能不想在片段中提及此事。自动变量 $< 指代第一个前提条件:
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
cc -c $(CFLAGS) $< -o $@
后文待续