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2017-09-28 15:54:58 -05:00

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Y2K compliance in libpng:
September 28, 2017
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.
This is your unofficial assurance that libpng from version 0.71 and
upward through 1.2.59 are Y2K compliant. It is my belief that earlier
versions were also Y2K compliant.
Libpng only has three year fields. One is a 2-byte unsigned integer
that will hold years up to 65535. The other two hold the date in text
format, and will hold years up to 9999.
The integer is
"png_uint_16 year" in png_time_struct.
The strings are
"png_charp time_buffer" in png_struct and
"near_time_buffer", which is a local character string in png.c.
There are seven time-related functions:
png_convert_to_rfc_1123() in png.c
(formerly png_convert_to_rfc_1152() in error)
png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c
png_convert_from_time_t() in pngwrite.c
png_get_tIME() in pngget.c
png_handle_tIME() in pngrutil.c, called in pngread.c
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
All appear to handle dates properly in a Y2K environment. The
png_convert_from_time_t() function calls gmtime() to convert from system
clock time, which returns (year - 1900), which we properly convert to
the full 4-digit year. There is a possibility that applications using
libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
function, or that they are incorrectly passing only a 2-digit year
instead of "year - 1900" into the png_convert_from_struct_tm() function,
but this is not under our control. The libpng documentation has always
stated that it works with 4-digit years, and the APIs have been
documented as such.
The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
integer to hold the year, and can hold years as large as 65535.
zlib, upon which libpng depends, is also Y2K compliant. It contains
no date-related code.
Glenn Randers-Pehrson
libpng maintainer
PNG Development Group