Segmentation fault of ImageMagick + Python ctypes

Question

I am using ImageMagick library with Python ctypes. I wrote a following simple code, but it crashes with segmentation fault (KERN_INVALID_ADDRESS) in Mac:

from ctypes import *
from ctypes.util import find_library

lib = CDLL(find_library('MagickWand'))
lib.MagickWandGenesis()
wand = lib.NewMagickWand()
lib.MagickReadImage(wand, 'mona-lisa.jpg')
lib.DestroyMagickWand(wand)
lib.MagickWandTerminus()

It works well in Linux and Windows both, but craches only in Mac OS X Lion. I built ImageMagick in various ways (official binary package, Homebrew, traditional ./configure && make), but it crashed for every trial.

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x00000000009a7638
0x000000010149a8d1 in MagickReadImage ()

Not only for MagickReadImage() function, IsMagickWand() function also crashes. I only guess NewMagickWand() returns a wrong pointer, or ctypes in Mac handles pointers incorrectly, but I’m not sure.

What’s wrong in this situation?

minhee · Accepted Answer · 2011-10-27 08:48:54Z

1

I changed the following code:

lib.MagickReadImage(wand, 'mona-lisa.jpg')

to:

f2 = lib.MagickReadImage
f2.argtypes = [c_void_p, c_char_p]
f2(wand, 'mona-lisa.jpg')

So, it works well.

answered Oct 27, 2011 at 8:48

minhee

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David Heffernan Over a year ago

In other words this is basically a 64 bit pointer issue. I just didn't get them all.

David Heffernan · Accepted Answer · 2011-10-27 07:08:20Z

0

Most likely this is a 32/64 bit issue. Is the Mac version the only 64 bit process that you've tested? Or perhaps you got lucky in the Windows and Linux versions in that they happen to return pointers of the form 0x00000000xxxxxxxx.

wand = lib.NewMagickWand()

NewMagickWand returns a pointer but you have not told ctypes to expect a pointer. As it stands ctypes defaults to a 32 bit integer for the return value. Add this line before you call NewMagickWand.

lib.NewMagickWand.restype = c_void_p

This tells ctypes that NewMagickWand returns a pointer.

answered Oct 27, 2011 at 7:08

David Heffernan

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1 Comment

David Heffernan Over a year ago

As an aside, why not use the ready made Python interfaces to ImageMagick: imagemagick.org/script/api.php

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