ldo at geek-central.gen.new_zealand
Mon Jun 1 12:41:44 CEST 2009
I wrote some code months ago to output 1-bit-per-pixel PNG files from
Python, doing direct calls to libpng using ctypes, because PIL didn't give
me sufficient control over colour tables and pixel depths.
I thought the code was working fine. I left it aside for some months, came
back to it a week or two ago, and found it was crashing. I was creating
CFUNCTYPE objects to do callbacks to my own I/O routines, and with GDB I was
able to narrow down the crashes to the point where libpng was trying to
invoke one of my callbacks.
What had changed? I had switched my OS from Gentoo to Debian Unstable. I
chrooted back to the Gentoo system, tried my script again--still crashed.
Went back to the doc <http://docs.python.org/library/ctypes.html> and tried
the qsort callback example. Worked fine! And then I noticed this little bit:
Important note for callback functions:
Make sure you keep references to CFUNCTYPE objects as long as they are
used from C code. ctypes doesn’t, and if you don’t, they may be garbage
collected, crashing your program when a callback is made.
Yup, that was it. I changed my installation of the callbacks from
ct.CFUNCTYPE(None, ct.c_void_p, ct.c_void_p, ct.c_size_t)(write_data),
cb_write_data = ct.CFUNCTYPE(None, ct.c_void_p, ct.c_void_p, ct.c_size_t)(write_data)
cb_flush_write = ct.CFUNCTYPE(None, ct.c_void_p)(flush_write)
and it works again.
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