Re: [PATCH v2 2/4] compat/regex: include alloca.h before undef it

From: Ramsay Jones <ramsay@ramsayjones.plus.com>
Date: Apr 27, 2020, 16:28 UTC
Message-ID: <f3af49c7-6c35-d2f6-c87c-e7342b1c2b6f@ramsayjones.plus.com>
In-Reply-To: <20200427010821.GD14800@danh.dev>

On 27/04/2020 02:08, Danh Doan wrote: [snip]

Show 8 quoted lines

>>> musl's alloca.h is available here:
>>>
>>> https://git.musl-libc.org/cgit/musl/tree/include/alloca.h
>>
>> Hmm, OK, so that partly explains the problem. I wonder if the
>> musl guys would accept a bug report?
> 
> I don't think they have a policy of no `#undef` whatsoever.

That's fair enough.

> But, I think they're picky when come to C-correctly and
> POSIX-correctly.
> Does C or POSIX define alloca(3) at all?

No alloca() is not in either the POSIX or C standard(s). This was an extension from the early days of BSD Unix.

For some reason, I thought you had to explicitly '#include <alloca.h>' to use it, but it appears that (by default) you get a bonus include from the <stdlib.h> header, unless you restrict the headers using the various macros and/or compiler command-line options.

As it happens, even on glibc systems, the <alloca.h> header is included by the <stdlib.h> header, unless you take steps to suppress it. So, we would have had the same issue, if it wasn't for the aforementioned '#undef alloca' the the glibc header file.

When I need to look at pp output, while debugging things like this, I cherry-pick a patch to the Makefile:

  $ git diff
  diff --git a/Makefile b/Makefile
  index 6d5cee270c..cd8753bf54 100644
  --- a/Makefile
  +++ b/Makefile
  @@ -2423,6 +2423,9 @@ $(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs)
   %.s: %.c GIT-CFLAGS FORCE
          $(QUIET_CC)$(CC) -o $@ -S $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
   
  +%.i: %.c GIT-CFLAGS FORCE
  +       $(QUIET_CC)$(CC) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) -E $< >$*.i
  +
   ifdef USE_COMPUTED_HEADER_DEPENDENCIES
   # Take advantage of gcc's on-the-fly dependency generation
   # See <http://gcc.gnu.org/gcc-3.0/features.html>.
  @@ -2474,7 +2477,7 @@ http-walker.sp http-walker.s http-walker.o: EXTRA_CPPFLAGS = -DNO_EXPAT
   endif
   
   ifdef NO_REGEX
  -compat/regex/regex.sp compat/regex/regex.o: EXTRA_CPPFLAGS = \
  +compat/regex/regex.i compat/regex/regex.sp compat/regex/regex.o: EXTRA_CPPFLAGS = \
          -DGAWK -DNO_MBSUPPORT
   endif
   
  $

[The second hunk above is not actually part of the cherry-picked patch, but I needed it in this instance to get GAWK and NO_MBSUPPORT passed to the compiler!]

  $ make NO_REGEX=1 compat/regex/regex.i
      CC compat/regex/regex.i
  $ vim compat/regex/regex.i
  $

... which shows <alloca.h> is indeed being #included from <stdlib.h>. [it is protected by a __USE_MISC pp variable, but I didn't bother to track it down! ;-)]

ATB, Ramsay Jones