Re: alternative form flag with zero octal value

[Rich Felker <dalias@libc.org>]

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On Wed, Jan 10, 2018 at 09:02:46PM -0500, Rich Felker wrote:
> On Thu, Jan 11, 2018 at 11:43:05AM +1100, Peter Wang wrote:
> > Hi,
> > 
> > I'm not certain it is a bug, but this program produces "00" instead of "0":
> > 
> >     #include <stdio.h>
> >     int main(void)
> >     {
> >         printf("%#o\n", 0);
> >         return 0;
> >     }
> > 
> > Please Cc: any replies as I am not subscribed to this list.
> 
> Indeed, this is a bug. I'm not sure whether it was introduced in
> commit 78897b0dc00b7cd5c29af5e0b7eebf2396d8dce0 or already present,
> but it was not present in Dmitry Levin's original more complex version
> of the patch. I'm going to essentially revert this commit and replace
> it with an alternate one-line fix using infrastructure that wasn't
> available at the time but added later in commit
> 167dfe9672c116b315e72e57a55c7769f180dffa.
> 
> Thanks for the report.

Here's the patch I'm going to push if I don't find any problems with
it.

Rich

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From b64539ae06aa91a407359238f4e909adb9bfab3d Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Wed, 10 Jan 2018 20:45:02 -0500
Subject: [PATCH] fix printf alt-form octal with value 0 and no explicit
 precision

commit 78897b0dc00b7cd5c29af5e0b7eebf2396d8dce0 wrongly simplified
Dmitry Levin's original submitted patch fixing alt-form octal with the
zero flag and field width present, omitting the special case where the
value is zero. as a result, printf("%#o",0) wrongly prints "00" rather
than "0".

the logic prior to this commit was actually better, in that it was
aligned with how the alt-form flag (#) for printf is specified ("it
shall increase the precision"). at the time there was no good way to
avoid the zero flag issue with the old logic, but commit
167dfe9672c116b315e72e57a55c7769f180dffa added tracking of whether an
explicit precision was provided.

revert commit 78897b0dc00b7cd5c29af5e0b7eebf2396d8dce0 and switch to
using the explicit precision indicator for suppressing the zero flag.
---
 src/stdio/vfprintf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
index 15356f5..50fb55c 100644
--- a/src/stdio/vfprintf.c
+++ b/src/stdio/vfprintf.c
@@ -559,7 +559,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			if (0) {
 		case 'o':
 			a = fmt_o(arg.i, z);
-			if ((fl&ALT_FORM) && p<z-a+1) prefix+=5, pl=1;
+			if ((fl&ALT_FORM) && p<z-a+1) p=z-a+1;
 			} if (0) {
 		case 'd': case 'i':
 			pl=1;
@@ -574,7 +574,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			a = fmt_u(arg.i, z);
 			}
 			if (xp && p<0) goto overflow;
-			if (p>=0) fl &= ~ZERO_PAD;
+			if (xp) fl &= ~ZERO_PAD;
 			if (!arg.i && !p) {
 				a=z;
 				break;
-- 
2.10.0