| 1 | Y2K compliance in libpng:
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| 2 | =========================
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| 3 |
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| 4 | February 26, 2015
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| 5 |
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| 6 | Since the PNG Development group is an ad-hoc body, we can't make
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| 7 | an official declaration.
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| 8 |
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| 9 | This is your unofficial assurance that libpng from version 0.71 and
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| 10 | upward through 1.2.53 are Y2K compliant. It is my belief that earlier
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| 11 | versions were also Y2K compliant.
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| 12 |
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| 13 | Libpng only has three year fields. One is a 2-byte unsigned integer
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| 14 | that will hold years up to 65535. The other two hold the date in text
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| 15 | format, and will hold years up to 9999.
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| 16 |
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| 17 | The integer is
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| 18 | "png_uint_16 year" in png_time_struct.
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| 19 |
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| 20 | The strings are
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| 21 | "png_charp time_buffer" in png_struct and
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| 22 | "near_time_buffer", which is a local character string in png.c.
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| 23 |
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| 24 | There are seven time-related functions:
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| 25 |
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| 26 | png_convert_to_rfc_1123() in png.c
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| 27 | (formerly png_convert_to_rfc_1152() in error)
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| 28 | png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c
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| 29 | png_convert_from_time_t() in pngwrite.c
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| 30 | png_get_tIME() in pngget.c
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| 31 | png_handle_tIME() in pngrutil.c, called in pngread.c
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| 32 | png_set_tIME() in pngset.c
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| 33 | png_write_tIME() in pngwutil.c, called in pngwrite.c
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| 34 |
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| 35 | All appear to handle dates properly in a Y2K environment. The
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| 36 | png_convert_from_time_t() function calls gmtime() to convert from system
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| 37 | clock time, which returns (year - 1900), which we properly convert to
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| 38 | the full 4-digit year. There is a possibility that applications using
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| 39 | libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
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| 40 | function, or that they are incorrectly passing only a 2-digit year
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| 41 | instead of "year - 1900" into the png_convert_from_struct_tm() function,
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| 42 | but this is not under our control. The libpng documentation has always
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| 43 | stated that it works with 4-digit years, and the APIs have been
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| 44 | documented as such.
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| 45 |
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| 46 | The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
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| 47 | integer to hold the year, and can hold years as large as 65535.
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| 48 |
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| 49 | zlib, upon which libpng depends, is also Y2K compliant. It contains
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| 50 | no date-related code.
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| 51 |
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| 52 |
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| 53 | Glenn Randers-Pehrson
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| 54 | libpng maintainer
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| 55 | PNG Development Group
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