Re: [uml-devel] [PATCH 2/3] um: Do not use stdin and stdout names for variables

[Hans-Werner Hilse <hwhilse@gmail.com>]

Am 2015-06-11 10:09, schrieb Richard Weinberger:
> On Thu, Jun 11, 2015 at 9:54 AM, Hans-Werner Hilse <hwhilse@gmail.com> 
> wrote:
>> About forbidden use of names/identifiers from the stdlib, the C 
>> standard
>> is a bit hard for me to grasp fully. However, I think C99 7.1.3
>> paragraph 1, 3rd point is relevant here: "Each macro name in any of 
>> the
>> following subclauses (including the future library directions) is
>> reserved for use as specified if any of its associated headers is
>> included; unless explicitly stated otherwise (see 7.1.4)."
> 
> Yeah, but we're talking about structure members.

Indeed, and I can't find more detail about the "reserved for use as 
specified" part.
I think it is saying, though, that those identifiers are macros (and as 
such will get expanded). The preprocessor will not care if that's in a 
struct member name.

>> So aside from renaming struct members, another option would be to lift
>> the macro definition by an "#undef" in relevant places. I have a
>> feeling, though, that this faces new problems with the C99 standard,
>> 7.1.3 paragraph 3, which says that "If the program removes (with 
>> #undef)
>> any macro definition of an identifier in the first group listed above,
>> the behavior is undefined."
> 
> So, what exactly is the build error you're facing and how can I 
> reproduce?

Of course I should have told you in the first place:

[hw@hilses linux-uml]$ make ARCH=um CC="musl-gcc"
...
   CC      arch/um/drivers/slip_user.o
In file included from arch/um/drivers/slip_user.c:12:0:
/usr/lib/musl/include/sys/termios.h:1:2: warning: #warning redirecting 
incorrect #include <sys/termios.h> to <termios.h> [-Wcpp]
  #warning redirecting incorrect #include <sys/termios.h> to <termios.h>
   ^
In file included from arch/um/drivers/slip_user.c:6:0:
arch/um/drivers/slip_user.c: In function ‘slip_pre_exec’:
arch/um/drivers/slip_user.c:67:12: error: expected identifier before ‘(’ 
token
   if (data->stdin >= 0)
             ^
arch/um/drivers/slip_user.c:68:14: error: expected identifier before ‘(’ 
token
    dup2(data->stdin, 0);
               ^
arch/um/drivers/slip_user.c:68:3: error: too few arguments to function 
‘dup2’
    dup2(data->stdin, 0);
    ^

(We can ignore the warning for now/this discussion, I think.)

You can reproduce without actually installing a musl toolchain when 
changing glibc's macro definition to musl's:

make ARCH=um CFLAGS="-Dstdout=\\(stdout\\)"

Will trigger the same error in the same place. Of course, this is 
artificially producing it. Using musl, however, leads to the same 
behaviour.

I used the defconfig, that's why it triggers in slip_user.c first - 
other places do use stdin/stdout as struct member name, too.

-hwh

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