Re: [Mingw-users] snprintf and __USE_MINGW_ANSI_STDIO
- Date: Wed, 16 Nov 2016 18:28:00 +0000
- From: "Burkhardt, Glenn B UTAS" <Glenn.Burkhardt@xxxxxxxxxxxx>
- Subject: Re: [Mingw-users] snprintf and __USE_MINGW_ANSI_STDIO
On 16/11/16 13:50, Burkhardt, Glenn B UTAS wrote:
> It looks like "snprintf" and friends don't have the same definitions
> to use the ANSI stdio functions like printf, fprintf, and sprintf do.
> The result is that if __USE_MINGW_ANSI_STDIO is defined, and -Wall is
> used, there are compile time warnings if specifications like "%lld"
> are used.
On Wed 11/16/2016 11:28, Keith Marshall wrote:
> This is nothing whatsoever to do with __USE_MINGW_ANSI_STDIO, (which you really SHOULDN'T define yourself, BTW; use appropriate
> _POSIX_C_SOURCE, or _XOPEN_SOURCE definitions instead). This is just evidence of the inherent dumbness of GCC's printf format warnings.
I have to disagree with you. The problem is the omission of the correct definition for "snprintf" with __USE_MINGW_ANSI_STDIO defined. It works for "printf", and "sprintf". Look at stdio.h - the static definition for "sprintf" is there to use __mingw_vsprintf(), but is missing for "snprintf".
This small program:
#define __USE_MINGW_ANSI_STDIO 1
long long x = 0x100000000L;
sprintf(buff, "%lld\n", x);
snprintf(buff, sizeof(buff), "%lld\n", x);
gives these warnings:
$ gcc -g -Wall tt.c -o tt
tt.c: In function 'main':
tt.c:12:5: warning: unknown conversion type character 'l' in format [-Wformat]
tt.c:12:5: warning: too many arguments for format [-Wformat-extra-args]
But the "snprintf" call works properly. The output is:
> > The function actually works correctly with "%lld". Is this a bug, or
> > am I missing something?
> If the function works correctly, it's hardly a bug, is it? GCC's printf format warning feature is unlikely to deliver sane diagnostics, while it assumes that
> only MSVC semantics are applicable. (FWIW, the second most prevalent diagnostic emitted during a build of GCC itself, is a swathe of bogus
> warnings about MSVC format specifiers).
Call me fastidious, but I find that compiler warnings often alert me to real errors in the code. False positives waste my time.
> If they bother you, disable the -Wformat (-Wno-format) messages, (or, maybe better, use the format specifier macros from <inttypes.h>).
BTW, the correct format specifiers for int64_t, viz, PRId64, are wrong when using __USE_MINGW_ANSI_STDIO. That's another bug.
Also, for what it's worth, this all works properly with the headers from the MinGW-w64 fork of the MinGW code, and on every Linux system I use. :-)
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