As I told before, I’ve been working for a few days on a way to display japanese characters on a web page when you don’t have japanese fonts locally. That involved taking japanese characters from a font file, and turning it into a png image. So I just wrote a small and simple program using libpng and freetype. Which works quite well. But that’s not the point.
During this “development”, I lost 2 or 3 days trying to understand what was wrong with my code that made it work properly from a shell command and fail halfway when wrapped in a php script. I still don’t know what’s wrong with php, but it works well with a perl cgi wrapper, and it still fails if I raise the php memory limit. Interestingly, other libpng tools (such as pngtopnm) fail in the same way. It wouldn’t surprise me if it was libpng’s fault.
The first thing i did to understand why it’d fail, was to run the program under valgrind. And what I saw was not encouraging. A lot of “Conditional jump or move depends on uninitialised value(s)”. I tried different combinations of code, using png_write_image or png_write_row, with large or small buffers…
It turns out libpng can’t write a png file with a width smaller than 148 pixels without those “Conditional jump or move depends on uninitialised value(s)”, which happen, as the stack trace shows, in the zlib, called by png_write_row, but i doubt it to be a zlib issue. You would think it would be a buffer size issue, but if you use a larger buffer for each line you call png_write_row for, you still get the errors.
If anyone has a clue, please leave a comment.
Update: Thanks Mark for your comment, though I don’t get how it can be safe to have such accesses to uninitialized memory. Deuce, the php issue can’t be a timeout issue, the program runs in fractions of a second.
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