1*593dc095SDavid du Colombier Y2K compliance in libpng: 2*593dc095SDavid du Colombier ========================= 3*593dc095SDavid du Colombier 4*593dc095SDavid du Colombier December 3, 2004 5*593dc095SDavid du Colombier 6*593dc095SDavid du Colombier Since the PNG Development group is an ad-hoc body, we can't make 7*593dc095SDavid du Colombier an official declaration. 8*593dc095SDavid du Colombier 9*593dc095SDavid du Colombier This is your unofficial assurance that libpng from version 0.71 and 10*593dc095SDavid du Colombier upward through 1.2.8 are Y2K compliant. It is my belief that earlier 11*593dc095SDavid du Colombier versions were also Y2K compliant. 12*593dc095SDavid du Colombier 13*593dc095SDavid du Colombier Libpng only has three year fields. One is a 2-byte unsigned integer 14*593dc095SDavid du Colombier that will hold years up to 65535. The other two hold the date in text 15*593dc095SDavid du Colombier format, and will hold years up to 9999. 16*593dc095SDavid du Colombier 17*593dc095SDavid du Colombier The integer is 18*593dc095SDavid du Colombier "png_uint_16 year" in png_time_struct. 19*593dc095SDavid du Colombier 20*593dc095SDavid du Colombier The strings are 21*593dc095SDavid du Colombier "png_charp time_buffer" in png_struct and 22*593dc095SDavid du Colombier "near_time_buffer", which is a local character string in png.c. 23*593dc095SDavid du Colombier 24*593dc095SDavid du Colombier There are seven time-related functions: 25*593dc095SDavid du Colombier 26*593dc095SDavid du Colombier png_convert_to_rfc_1123() in png.c 27*593dc095SDavid du Colombier (formerly png_convert_to_rfc_1152() in error) 28*593dc095SDavid du Colombier png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c 29*593dc095SDavid du Colombier png_convert_from_time_t() in pngwrite.c 30*593dc095SDavid du Colombier png_get_tIME() in pngget.c 31*593dc095SDavid du Colombier png_handle_tIME() in pngrutil.c, called in pngread.c 32*593dc095SDavid du Colombier png_set_tIME() in pngset.c 33*593dc095SDavid du Colombier png_write_tIME() in pngwutil.c, called in pngwrite.c 34*593dc095SDavid du Colombier 35*593dc095SDavid du Colombier All appear to handle dates properly in a Y2K environment. The 36*593dc095SDavid du Colombier png_convert_from_time_t() function calls gmtime() to convert from system 37*593dc095SDavid du Colombier clock time, which returns (year - 1900), which we properly convert to 38*593dc095SDavid du Colombier the full 4-digit year. There is a possibility that applications using 39*593dc095SDavid du Colombier libpng are not passing 4-digit years into the png_convert_to_rfc_1123() 40*593dc095SDavid du Colombier function, or that they are incorrectly passing only a 2-digit year 41*593dc095SDavid du Colombier instead of "year - 1900" into the png_convert_from_struct_tm() function, 42*593dc095SDavid du Colombier but this is not under our control. The libpng documentation has always 43*593dc095SDavid du Colombier stated that it works with 4-digit years, and the APIs have been 44*593dc095SDavid du Colombier documented as such. 45*593dc095SDavid du Colombier 46*593dc095SDavid du Colombier The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned 47*593dc095SDavid du Colombier integer to hold the year, and can hold years as large as 65535. 48*593dc095SDavid du Colombier 49*593dc095SDavid du Colombier zlib, upon which libpng depends, is also Y2K compliant. It contains 50*593dc095SDavid du Colombier no date-related code. 51*593dc095SDavid du Colombier 52*593dc095SDavid du Colombier 53*593dc095SDavid du Colombier Glenn Randers-Pehrson 54*593dc095SDavid du Colombier libpng maintainer 55*593dc095SDavid du Colombier PNG Development Group 56