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