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November 2010
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2010.11.30 19:26 "Re: Win32 ReadFile() / WriteFile() heads-up", by Toby Thain
On 30/11/10 2:15 PM, John Emmas wrote: > You guys are probably already aware of this but I thought I'd flag it up > anyway.... > > As you know, libTIFF is widely used by 3rd party products and libraries. > Among them is the cross-platform library libgtk. On the Windows platform, > this is called libgtk-win32. LibTIFF is actually used by libgdk_pixbuf > which is a graphical image library, supporting many different image types > (BMP. PNG, JPEG etc). > > Because it interfaces with so many different types it generically uses the > CRT function _wfopen() to open the image files. The FILE* returned by > _wfopen() is then passed to _fileno() to obtain a file handle. That file > handle then gets used to manipulate the file (reading / writing etc) using > the interface provided by each specific library. This all works very well > as long as you don't use the win32 versions of libgdk_pixbuf and libTIFF. > > The problem is that in libTIFF (_tiffReadProc() for example, in > tif_win32.c) reading gets handled by the Windows function, ReadFile() and > similarly for WriteFile(). Unfortunately, those functions seem to only > work with handles returned by CreateFile(). They don't work for handles > obtained from fopen() or _wfopen(). Of course, if one sticks entirely > with libTIFF and uses the recommended open function TIFFOpen(), all will > be fine (because that does call CreateFile() and thereby obtains a > suitable handle). However, any library that's cross-platform probably > wouldn't even consider CreateFile() / ReadFile() / WriteFile() etc. It > would tend to prefer generic CRT functions, such as read() and write(). > > I'm not saying there's anything wrong with libTIFF. LibTIFF works fine as > long as you use its own interface function to open TIFF files - but > libgdk_pixbuf is an important library and it'd be nice if libTIFF could > work seamlessly with it. At the moment it only works (under Windows) if > you use a *nix style build system, such as Cygwin or minGW. Things fall > down badly if you want to use Visual C++. libgdk_pixbuf needs to use > generic handles that can be applied to all file types across all platforms > - it shouldn't be getting brought down by special file handles that only > apply to libTIFF and Visual C++ but nothing else. > > FWIW I've done some experiments today to replace the Windows-centric file > functions with their standard CRT equivalents and the initial signs were > pretty encouraging. How do others feel about using read() and write() > etc, instead of ReadFile() and WriteFile()? I'm not familiar enough with > libTIFF to understand what the ramifications might be. > Can't the existing tif_unix entry points be used, if the application wants to use POSIX file handles? It makes sense to me to have tif_win32 use Win32 file handles; and changing this would break existing code. --Toby > John > P.S - incidentally, the link shown on your home page for subscribing to > this mailing list seems to be broken:- > > http://remotesensing.org/mailman/listinfo/tiff > _______________________________________________ > Tiff mailing list: Tiff@lists.maptools.org > http://lists.maptools.org/mailman/listinfo/tiff > http://www.remotesensing.org/libtiff/ >