Date: Mon, 21 May 2018 16:43:06 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Dimitry Andric <dim@FreeBSD.org>
Cc: Thomas Munro <munro@ip9.org>, freebsd-hackers@freebsd.org
Subject: Re: printf("%m") doesn't generate a warning -- shouldn't it?
Message-ID: <20180521134306.GT6887@kib.kiev.ua>
In-Reply-To: <BAD690AA-C51A-4125-8F23-B637758C8EB6@FreeBSD.org>
References: <CADLWmXU__ZRaS=gg_Y18WYbJJgk2Rs065RhkYmAZCFY8mp1Wjg@mail.gmail.com> <BAD690AA-C51A-4125-8F23-B637758C8EB6@FreeBSD.org>
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On Mon, May 21, 2018 at 01:19:01PM +0200, Dimitry Andric wrote:
> On 21 May 2018, at 04:38, Thomas Munro <munro@ip9.org> wrote:
> >
> > As discussed on the PostgreSQL[1] and NetBSD mailing lists[2][3],
> > syslog-like printf("%m") is a GNU extension that doesn't generate a
> > warning from Clang or GCC on other operating systems even though when
> > it doesn't actually work. That's because the __printf__ attribute
> > that our __printflike macro in /usr/include/stdio.h expands to
> > effectively means "like printf in glibc, allowing %m", not like POSIX
> > or our actual libc which just prints out "m" when it sees it.
> >
> > It'd sure be nice to get a compiler warning on FreeBSD when porting
> > software that uses that if it doesn't actually work (or ... to support
> > it).
>
> Please submit upstream bug(s) for clang and gcc. It turns out clang has
> a -Wformat-non-iso warning flag which should warn about this, but I
> can't get it to emit it:
>
> $ cat printf-m.c
> #include <stdio.h>
>
> int main(void)
> {
> printf("error: %m\n");
> return 0;
> }
>
> $ clang -std=c99 -Wformat-non-iso -c printf-m.c
> <nothing>
Why not add %m instead ? It is very easy and several people did it in
round-about ways.
diff --git a/lib/libc/stdio/vfprintf.c b/lib/libc/stdio/vfprintf.c
index 29fbd95b399..70f5074e2f7 100644
--- a/lib/libc/stdio/vfprintf.c
+++ b/lib/libc/stdio/vfprintf.c
@@ -317,6 +317,7 @@ __vfprintf(FILE *fp, locale_t locale, const char *fmt0, va_list ap)
int ret; /* return value accumulator */
int width; /* width from format (%8d), or 0 */
int prec; /* precision from format; <0 for N/A */
+ int saved_errno;
char sign; /* sign prefix (' ', '+', '-', or \0) */
struct grouping_state gs; /* thousands' grouping info */
@@ -466,6 +467,7 @@ __vfprintf(FILE *fp, locale_t locale, const char *fmt0, va_list ap)
savserr = fp->_flags & __SERR;
fp->_flags &= ~__SERR;
+ saved_errno = errno;
convbuf = NULL;
fmt = (char *)fmt0;
argtable = NULL;
@@ -776,6 +778,11 @@ reswitch: switch (ch) {
}
break;
#endif /* !NO_FLOATING_POINT */
+ case 'm':
+ cp = strerror(saved_errno);
+ size = (prec >= 0) ? strnlen(cp, prec) : strlen(cp);
+ sign = '\0';
+ break;
case 'n':
/*
* Assignment-like behavior is specified if the
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