GNU Make rule hello.c и% .c делают странные вещи

Question

У меня есть программа hello.c и make-файл для ее сборки с использованием неявных правил:

hello:

Сборка работает как положено:

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

Тогда я положил рецепт на hello.c, и все стало дико. Новый make-файл:

hello:
hello.c:
    : secret recipe

Результат сборки:

: secret recipe
cc hello.c -o hello

Что подводит меня к вопросу 1: почему неявные правила больше не создают объектный файл?

А потом становится страннее, когда я использую шаблон% .c для hello.c. Makefile:

hello:
%.c:
    : secret recipe

Выход:

cc hello.c -o hello

Нет объектного файла, но и рецепт для hello.c не запускается. Как?

А теперь действительно супер странные вещи. Запуск make -B в make-файле с правилом %.c дает следующий вывод:

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

Откуда берется makefile.c? И чем это может отличаться от ручного снятия прикосновения к форсированным рецептам?

(Собраны эти вопросы, так как они все связаны с правилами в * .c-файлах и предположительно как-то связаны)

Answer 1

Откуда берется makefile.c?

Из комбинации a) списка суффиксов по умолчанию, b) правила шаблона% .c и c) тот факт, что GNU make также рассматривает свой make-файл для обновления (это эзотерическая функция, необходимая для повторного выполнения чтения make-файла, например сгенерированные включенные цели.)

Вы можете включить опцию отладки (make -B -d), чтобы увидеть это:

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.

Вы можете удалить список суффиксов, поместив .SUFFIXES: в качестве первой строки в вашем make-файле, затем вы получите:

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'.

Вы даже можете избежать этих "правил совпадения с чем-либо" с помощью make -B -d --no-builtin-rules:

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'.