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

Am 06.06.2014 21:13, schrieb Junio C Hamano:

Karsten Blees [off-list ref] writes:

quoted

Am 29.05.2014 12:47, schrieb Stepan Kasal:

quoted

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.

quoted

Signed-off-by: Stepan Kasal <redacted>
---
 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:

Actually, that was the original solution ($gmane/247535). I just complained because it was slightly different from what we had in msysgit for quite some time [1], causing merge conflicts.

I guess compilers probably won't complain if you declare argv const, even if the standard is more strict. After all, you aren't supposed to modify argv.

quoted

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?

Not 'away'. This passes a non-const value to a const parameter, which is typically not a problem. Its just 'char**' to 'const char**' that produces the warning, because 'const char' hides behind a non-const pointer, see [2]. 'void*' to 'const char**' works, though.

[1] https://github.com/msysgit/git/commit/6949537a
[2] http://c-faq.com/ansi/constmismatch.html