[PATCH 1/1] tzset.3: Add description for posixrules file

[J William Piggott <elseifthen-KK0ffGbhmjU@public.gmane.org>]

Based upon reviewing the glibc source, the posixrules file is being used
for very specific TZ strings that can be represented as:

[:]stdoffsetdst[offset][,]

If anything follows the above string, even invalid data, posixrules will
not be used. Below is some shell output demonstrating this.

$ TZ="NZST-12:00:00NZDT-13:00:00,ANYTHING" \
> strace -eopen date 2>&1 | grep -Ei 'posixrules|jan'
Thu Jan 29 06:53:35 NZDT 2015

$ TZ="NZST-12:00:00NZDT-13:00:00," \
> strace -eopen date 2>&1 | grep -Ei 'posixrules|jan'
open("/usr/share/zoneinfo/posixrules", O_RDONLY|O_CLOEXEC) = 3
Thu Jan 29 05:54:58 NZST 2015

Signed-off-by: J William Piggott <elseifthen-KK0ffGbhmjU@public.gmane.org>
---
 man3/tzset.3 | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/man3/tzset.3 b/man3/tzset.3
index 9011e5f..2e91fd8 100644
--- a/man3/tzset.3
+++ b/man3/tzset.3
@@ -201,6 +201,11 @@ The system timezone file.
 The system timezone database directory.
 .TP
 .B /usr/share/zoneinfo/posixrules
+When a TZ string includes a dst timezone without anything following it,
+then this file is used for the start/end rules. It is in the
+.B tzfile(5)
+format. By default, the zoneinfo Makefile hard links it to the
+.IR America/New_York " tzfile."
 .PP
 Above are the current standard file locations, but they are
 configurable when glibc is compiled.
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