Date: Mon, 23 Aug 2004 02:34:14 +0300 From: Giorgos Keramidas <keramida@linux.gr> To: Sean McNeil <sean@mcneil.com> Cc: freebsd-current@freebsd.org Subject: Re: bsdtar core dumps Message-ID: <20040822233414.GA79931@gothmog.gr> In-Reply-To: <1093213755.72863.0.camel@server.mcneil.com> References: <1092777586.92327.9.camel@server.mcneil.com> <20040817213813.GE3827@gothmog.gr> <1092951447.1167.12.camel@server.mcneil.com> <4127841D.6050104@freebsd.org> <1093213755.72863.0.camel@server.mcneil.com>
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On 2004-08-22 15:29, Sean McNeil <sean@mcneil.com> wrote:
> On Sat, 2004-08-21 at 10:19, Tim Kientzle wrote:
> > The code you've pointed to above concerns me because of the part about:
> > if (n == 0) {
> > ...
> > n = 1;
> > }
> >
> > That ain't right: If I told vsnprintf the buffer size was zero, it
> > should treat it as such. If I meant "one", I would have said "one."
> >
> > On the other hand, the vsnprintf.3 man page does explicitly state
> > that "the output is always null-terminated," which would preclude
> > passing a zero-length buffer, which is exactly what libarchive is
> > doing in this situation. It is bogus, but at least it's documented
> > bogosity. ;-)
The vsnprintf() function cannot pass a zero-length buffer to __vfprintf()
because the __vfprintf() function is expected return the number of bytes it
would need to do the real printing. It's not illegal to pass a zero-length
bugger to vsnprintf(); at least it's not specifically prohibited by the
manpage. The following program *DOES* pass zero as the length of the
buffer to vsnprintf() and a NULL pointer as the buffer address but doesn't
fault on an i386 machine:
1 #include <stdarg.h>
2 #include <stdio.h>
3
4 size_t koko(const char *_fmt, ...);
5
6 int
7 main(void)
8 {
9 size_t foo;
10
11 foo = koko("%ld", 5);
12 printf("rc = %lu\n", (unsigned long)foo);
13 return 0;
14 }
15
16 size_t
17 koko(const char *fmt, ...)
18 {
19 size_t rc;
20 va_list ap;
21
22 va_start(ap, fmt);
23 rc = vsnprintf(NULL, 0, fmt, ap);
24 va_end(ap);
25 return rc;
26 }
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