c - GNU Make 规则 hello.c 和 %.c 做奇怪的事情

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，然后一切变得疯狂起来。新的 makefile:

hello:
hello.c:
    : secret recipe

构建输出:

: secret recipe
cc hello.c -o hello

这让我想到了问题 1:为什么隐式规则不再创建目标文件？

当我对 hello.c 使用模式 %.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 super 奇怪的东西。使用 %.c 规则在 makefile 上运行 make -B 会得到以下输出:

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

makefile.c 是从哪里来的？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 文件的规则相关，并且可能以某种方式相关)

Answer 1

Where does makefile.c come from?

来自 a) 默认后缀列表、b) %.c 模式规则和 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.

您可以通过将 .SUFFIXES: 作为 makefile 的第一行来删除后缀列表，然后您将得到:

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'.