Re: [PATCH 2/2] mingw: avoid const warning

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 6, 2014, 19:13 UTC
Message-ID: <xmqq1tv1rguh.fsf@gitster.dls.corp.google.com>
In-Reply-To: <5391D956.7050903@gmail.com>

Karsten Blees <karsten.blees@gmail.com> writes:

Show 41 quoted lines

> Am 29.05.2014 12:47, schrieb Stepan Kasal:
>> Fix const warnings in http-fetch.c and remote-curl.c main() where is
>> argv declared as const.
>> 
>> The fix should work for all future declarations of main, no matter
>> whether the second parameter's type is "char**", "const char**", or
>> "char *[]".
>
> I'm 100% in favor of a solution that doesn't restrict main to non-const char**! Thanks.
>
>> Signed-off-by: Stepan Kasal <kasal@ucw.cz>
>> ---
>>  compat/mingw.h | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/compat/mingw.h b/compat/mingw.h
>> index 15f0c9d..8745d19 100644
>> --- a/compat/mingw.h
>> +++ b/compat/mingw.h
>> @@ -369,10 +369,11 @@ extern CRITICAL_SECTION pinfo_cs;
>>  void mingw_startup();
>>  #define main(c,v) dummy_decl_mingw_main(); \
>>  static int mingw_main(c,v); \
>> -int main(int argc, char **argv) \
>> +int main(c, char **main_argv_not_used) \
>>  { \
>> +	typedef v, **argv_type; \
>>  	mingw_startup(); \
>> -	return mingw_main(__argc, __argv); \
>> +	return mingw_main(__argc, (argv_type)__argv); \
>>  } \
>>  static int mingw_main(c,v)
>>  
>
> I have to admit I had trouble understanding what 'typedef v,
> **arv_type;' does (looks invalid at first glance), and why you
> would need main_argv_not_used instead of just main(c,v).
>
> So, I'd like to award +10 points for cleverness, but -10 for
> obscurity ;-) Probably deserves a comment or an explanation in the
> commit message.

Agreed.  The "typedef" one is a cute hack.

I am wondering why the solution is not a more obvious "drop const that is not ANSI C", though. I only have a ready-access to N1570 draft but in it I find:

    5.1.2.2.1 Program startup

    The function called at program startup is named main. The
    implementation declares no prototype for this function. It shall
    be deﬁned with a return type of int and with no parameters:

    int main(void) { /* ... */ }

    or with two parameters (referred to here as argc and argv,
    though any names may be used, as they are local to the function
    in which they are declared):

    int main(int argc, char *argv[]) { /* ... */ }

    or equivalent;10) or in some other implementation-deﬁned manner.

    ---
    10) Thus, int can be replaced by a typedef name deﬁned as int,
    or the type of argv can be written as char ** argv, and so on.

> A simpler solution that works with all definitions of main() is to
> cast to void* (tell the compiler all responsibility is on
> us).

Can you cast away the constness that way, though?

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