GNU Make 规则 hello.c 和 %.c 做了一些奇怪的事情

Question

我有一个程序 hello.c 和一个使用隐式规则构建它的 makefile：

hello:

构建按预期工作：

cc -c -o hello.o hello.c
cc hello.o -o hello

然后我在 hello.c 上放了一个食谱，事情就变得疯狂了。新的makefile：

hello:
hello.c:
    : secret recipe

构建输出：

: secret recipe
cc hello.c -o hello

这让我想到问题1：为什么隐式规则不再创建目标文件？

然后当我将模式 %.c 用于 hello.c 时，它变得更奇怪了。生成文件：

hello:
%.c:
    : secret recipe

输出：

cc hello.c -o hello

没有目标文件，而且 hello.c 的配方也没有运行。如何？

现在是真正超级奇怪的东西。使用 %.c 规则在 makefile 上运行 make -B 会给出以下输出：

: secret recipe
cc -c -o makefile.o makefile.c
cc: error: makefile.c: No such file or directory

makefile.c 来自哪里？这与手动触摸移除强制配方有何不同？

（整理这些问题，因为它们都与 *.c 文件上的规则有关，并且可能以某种方式相关）

Answer 1

makefile.c 来自哪里？

结合 a) 默认后缀列表，b) %.c 模式规则，以及 c) GNU make 还考虑更新其 makefile 的事实（这是重新执行 makefile 读取所需的深奥功能，例如生成包含的目标。）

您可以启用调试选项 (make -B -d) 来查看：

GNU Make 3.82
Built for x86_64-redhat-linux-gnu
Copyright (C) 2010  Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Reading makefiles...
Reading makefile `makefile'...
Updating makefiles....
 Considering target file `makefile'.
  Looking for an implicit rule for `makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile.o'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile.c'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile.cc'.
  Trying pattern rule with stem `makefile'.
  [...]
  Trying implicit prerequisite `makefile.sh'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile,v'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `RCS/makefile,v'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `RCS/makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `s.makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `SCCS/s.makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile.o'.
  Looking for a rule with intermediate file `makefile.o'.
   Avoiding implicit rule recursion.
   Trying pattern rule with stem `makefile'.
   Trying implicit prerequisite `makefile.c'.
   Trying pattern rule with stem `makefile'.
   Trying implicit prerequisite `makefile.cc'.
   Trying pattern rule with stem `makefile'.
   Trying implicit prerequisite `makefile.C'.
   Trying pattern rule with stem `makefile'.
   Trying implicit prerequisite `makefile.cpp'.
   Trying pattern rule with stem `makefile'.
   [...]
   Trying implicit prerequisite `RCS/makefile.o'.
   Trying pattern rule with stem `makefile.o'.
   Trying implicit prerequisite `s.makefile.o'.
   Trying pattern rule with stem `makefile.o'.
   Trying implicit prerequisite `SCCS/s.makefile.o'.
   Trying pattern rule with stem `makefile'.
   Trying implicit prerequisite `makefile.c'.
   Looking for a rule with intermediate file `makefile.c'.
    Avoiding implicit rule recursion.
    Avoiding implicit rule recursion.
    Trying pattern rule with stem `makefile'.
  Found an implicit rule for `makefile'.
  Considering target file `makefile.o'.
   File `makefile.o' does not exist.
   Considering target file `makefile.c'.
    File `makefile.c' does not exist.
    Finished prerequisites of target file `makefile.c'.
   Must remake target `makefile.c'.
Invoking recipe from makefile:3 to update target `makefile.c'.
: secret recipe
Putting child 0xbe74b0 (makefile.c) PID 13836 on the chain.
Live child 0xbe74b0 (makefile.c) PID 13836
Reaping winning child 0xbe74b0 PID 13836
Removing child 0xbe74b0 PID 13836 from chain.
   Successfully remade target file `makefile.c'.
   Finished prerequisites of target file `makefile.o'.
  Must remake target `makefile.o'.
Invoking builtin recipe to update target `makefile.o'.
cc    -c -o makefile.o makefile.c
Putting child 0xbdb910 (makefile.o) PID 13837 on the chain.
Live child 0xbdb910 (makefile.o) PID 13837
cc: error: makefile.c: No such file or directory
cc: fatal error: no input files
compilation terminated.
Reaping losing child 0xbdb910 PID 13837
gmake: *** [makefile.o] Error 4
Removing child 0xbdb910 PID 13837 from chain.

你可以通过将.SUFFIXES:作为你的makefile的第一行来删除后缀列表，然后你会得到：

GNU Make 3.82
Built for x86_64-redhat-linux-gnu
Copyright (C) 2010  Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Reading makefiles...
Reading makefile `makefile'...
Updating makefiles....
 Considering target file `makefile'.
  Looking for an implicit rule for `makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `makefile,v'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `RCS/makefile,v'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `RCS/makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `s.makefile'.
  Trying pattern rule with stem `makefile'.
  Trying implicit prerequisite `SCCS/s.makefile'.
  No implicit rule found for `makefile'.
  Finished prerequisites of target file `makefile'.
 No need to remake target `makefile'.
Updating goal targets....
Considering target file `hello'.
 Looking for an implicit rule for `hello'.
 Trying pattern rule with stem `hello'.
 Trying implicit prerequisite `hello,v'.
 Trying pattern rule with stem `hello'.
 Trying implicit prerequisite `RCS/hello,v'.
 Trying pattern rule with stem `hello'.
 Trying implicit prerequisite `RCS/hello'.
 Trying pattern rule with stem `hello'.
 Trying implicit prerequisite `s.hello'.
 Trying pattern rule with stem `hello'.
 Trying implicit prerequisite `SCCS/s.hello'.
 No implicit rule found for `hello'.
 Finished prerequisites of target file `hello'.
No need to remake target `hello'.
gmake: Nothing to be done for `hello'.

您甚至可以使用make -B -d --no-builtin-rules 避免这些“匹配任何规则”：

Reading makefiles...
Reading makefile `makefile'...
Updating makefiles....
 Considering target file `makefile'.
  Looking for an implicit rule for `makefile'.
  No implicit rule found for `makefile'.
  Finished prerequisites of target file `makefile'.
 No need to remake target `makefile'.
Updating goal targets....
Considering target file `hello'.
 Looking for an implicit rule for `hello'.
 No implicit rule found for `hello'.
 Finished prerequisites of target file `hello'.
No need to remake target `hello'.
gmake: Nothing to be done for `hello'.