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Date:	Wed, 04 Jan 2012 19:31:02 +0100
From:	Christian Franke <Christian DOT Franke AT t-online DOT de>
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To:	cygwin AT cygwin DOT com
Subject:	mingw64-i686-gcc-4.5.3-4: -Wformat warnings broken in C++
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When printf/scanf functions from MinGW runtime are selected via 
__USE_MINGW_ANSI_STDIO, then format string checking is broken. This only 
affects the C++ compiler:


$ cygcheck -f /usr/bin/i686-w64-mingw32-g++
mingw64-i686-gcc-g++-4.5.3-4

$ cat testfmt.c
#define __USE_MINGW_ANSI_STDIO 1
#include <stdio.h>

void myprintf(const char *, ...)
   __attribute__((format(gnu_printf,1,2)));

int main()
{
   long long x = 42;
   printf("%lld\n", x); // C++: Bogus warning
   myprintf("%lld\n", x); // No warning (OK)
   printf("%I64d\n", x); // Warning (OK)
   return 0;
}

$ i686-w64-mingw32-gcc -Wformat -c testfmt.c
testfmt.c: In function 'main':
testfmt.c:12:3: warning: format '%I64d' expects type 'int', but argument 
2 has type 'long long int'

$ i686-w64-mingw32-g++ -Wformat -c testfmt.c
testfmt.c: In function 'int main()':
testfmt.c:10:21: warning: unknown conversion type character 'l' in format
testfmt.c:10:21: warning: too many arguments for format
testfmt.c:12:22: warning: format '%I64d' expects type 'int', but 
argument 2 has type 'long long int'


Interestingly the bogus warning only occurs for standard functions like 
printf(). These are replaced by inline functions in MinGW stdio.h if 
__USE_MINGW_ANSI_STDIO is set. Probably a subtle bug in the handling of 
functions known by the compiler.


Another observation: Include files from C++ standard library (e.g. 
<string>) now silently set __USE_MINGW_ANSI_STDIO:

/usr/lib/gcc/i686-w64-mingw32/4.5.3/include/c++/i686-w64-mingw32/bits/os_defines.h:
...
#undef __USE_MINGW_ANSI_STDIO
#define __USE_MINGW_ANSI_STDIO 1


Both are new issues not seen in previous releases. If desired, I could 
resend this report to mingw-w64 list.

Christian


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