Re: Bugs in strftime

[Rich Felker <dalias@libc.org>]

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On Mon, Feb 05, 2018 at 12:51:24PM -0500, Rich Felker wrote:
> > This bug is causing these failures:
> > > "%+3C": expected "+20", got "020"
> > > "%+11F": expected "+2016-01-03", got "02016-01-03"
> > > "%+5G": expected "+2015", got "02015"
> > > "%+5Y": expected "+2016", got "02016"
> > > "%+5Y": expected "+0000", got "00000"
> 
> I'll need to look over how to change the logic to match the desired
> behavior but it shouldn't be hard.

Attaching a patch I intend to push if I don't find any problems, and
the one I've already got queued up for the %F issue. Let me know if
you see any issues with them.

Rich

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From 596207aa38b0db33f222c9924a1310fee3de88b5 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Mon, 5 Feb 2018 13:36:04 -0500
Subject: [PATCH 1/2] fix strftime field widths with %F format and zero year
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the code to strip initial sign and leading zeros inadvertently
stripped all the zeros and the subsequent '-' separating the month.
instead, only strip sign characters from the very first position, and
only strip zeros when they are followed by another digit.

based on testing by Dennis Wölfing.
---
 src/time/strftime.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/time/strftime.c b/src/time/strftime.c
index d1ca7ca..16b3bb2 100644
--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -251,7 +251,8 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 		t = __strftime_fmt_1(&buf, &k, *f, tm, loc, pad);
 		if (!t) break;
 		if (width) {
-			for (; *t=='+' || *t=='-' || (*t=='0'&&t[1]); t++, k--);
+			if (*t=='+' || *t=='-') t++, k--;
+			for (; *t=='0' && t[1]-'0'<10U; t++, k--);
 			width--;
 			if (plus && tm->tm_year >= 10000-1900)
 				s[l++] = '+';
-- 
2.10.0


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From c7f0da4134d4e7f2efd295e7fb738c65c469fbd1 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Tue, 6 Feb 2018 12:31:06 -0500
Subject: [PATCH 2/2] adjust strftime + modifier to match apparent intent of
 POSIX
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it's unclear from the specification whether the word "consumes" in
"consumes more than four bytes to represent a year" refers just to
significant places or includes leading zeros due to field width
padding. however the examples in the rationale indicate that the
latter was the intent. in particular, the year 270 is shown being
formatted by %+5Y as +0270 rather than 00270.

previously '+' prefixing was implemented just by comparing the year
against 10000. instead, count the number of significant digits and
padding bytes to be added, and use the total to determine whether to
apply the '+' prefix.

based on testing by Dennis Wölfing.
---
 src/time/strftime.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/src/time/strftime.c b/src/time/strftime.c
index 16b3bb2..708875e 100644
--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -251,15 +251,21 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 		t = __strftime_fmt_1(&buf, &k, *f, tm, loc, pad);
 		if (!t) break;
 		if (width) {
+			/* Trim off any sign and leading zeros, then
+			 * count remaining digits to determine behavior
+			 * for the + flag. */
 			if (*t=='+' || *t=='-') t++, k--;
 			for (; *t=='0' && t[1]-'0'<10U; t++, k--);
-			width--;
-			if (plus && tm->tm_year >= 10000-1900)
-				s[l++] = '+';
-			else if (tm->tm_year < -1900)
+			if (width < k) width = k;
+			size_t d;
+			for (d=0; t[d]-'0'<10U; d++);
+			if (tm->tm_year < -1900) {
 				s[l++] = '-';
-			else
-				width++;
+				width--;
+			} else if (plus && d+(width-k) >= (*p=='C'?3:5)) {
+				s[l++] = '+';
+				width--;
+			}
 			for (; width > k && l < n; width--)
 				s[l++] = '0';
 		}
-- 
2.10.0