QT, ctypes dll and SEG Faults

[sapsi]

Hello,
Below is a small class using ctypes and libspectre to read a
postscript file.
My program is a PyQT 4.4 application and when the user clicks on a
entry in a QTableWidget, i run

PostScriptImage ( filename_as_con tained_in_click ed_tableWidgetI tem )
However on i get a segfault while trying document_load.
Surprisingly, before i run sys.exit(app.ex ec_()) (i.e before the app
starts) if run PostScriptImage (command_line_s pecified_ps_fil e) it
works!

I know the ctypes so file works with QT since i wrote an application
using it, but i hadn't separated the ctypes stuff into separate class
(i.e the spectre code was in the widget method).

Any ideas why the crash?

Regards
Saptarshi
--Python Code--
class PostScriptImage :
def __init__(self,f ilename):
print "Doc New"
self.document=l ibspec.spectre_ document_new()
print "Load Doc" , filename
#############cr ashed in the following
line########### ##############
libspec.spectre _document_load( self.document,f ilename)
print "Done load doc"
if libspec.spectre _document_statu s(self.document ):
return False
self.scale=[1.0,1.0]
self.quicksetup ()

def quicksetup(self ):
print "RC"
rc=libspec.spec tre_render_cont ext_new()
print "Get 0th Page"
page=libspec.sp ectre_document_ get_page (self.document, 0)
if libspec.spectre _document_statu s(self.document ):
raise Exception("Spec tre:Setup Document Error")
w= c_int()
h= c_int()
print "Page Size"
libspec.spectre _page_get_size( page, byref(w),byref( h))
self.initialSiz e=(h.value*1.0, w.value*1.0)
self.initialAsp ect=float(h.val ue)/float(w.value)

self.npages=lib spec.spectre_do cument_get_n_pa ges(self.docume nt)

[sapsi]

Re: QT, ctypes dll and SEG Faults

On Aug 16, 11:37 pm, sapsi <saptarshi.g... @gmail.comwrote :

Hello,
Below is a small class using ctypes and libspectre to read a
postscript file.
My program is a PyQT 4.4 application  and when the user clicks on a
entry in a QTableWidget, i run
>
PostScriptImage ( filename_as_con tained_in_click ed_tableWidgetI tem )
However on i get a segfault while trying document_load.
Surprisingly, before i run sys.exit(app.ex ec_()) (i.e before the app
starts) if run PostScriptImage (command_line_s pecified_ps_fil e) it
works!
>
I know the ctypes so file works with QT since i wrote an application
using it, but i hadn't separated the ctypes stuff into separate class
(i.e the spectre code was in the widget method).
>
Any ideas why the crash?
>
Regards
Saptarshi
--Python Code--
class PostScriptImage :
    def __init__(self,f ilename):
        print "Doc New"
        self.document=l ibspec.spectre_ document_new()
        print "Load Doc" , filename
        #############cr ashed in the following
line########### ##############
        libspec.spectre _document_load( self.document,f ilename)
        print "Done load doc"
        if libspec.spectre _document_statu s(self.document ):
            return False
        self.scale=[1.0,1.0]
        self.quicksetup ()
>
    def quicksetup(self ):
        print "RC"
        rc=libspec.spec tre_render_cont ext_new()
        print "Get 0th Page"
        page=libspec.sp ectre_document_ get_page (self.document, 0)
        if libspec.spectre _document_statu s(self.document ):
            raise Exception("Spec tre:Setup Document Error")
        w= c_int()
        h= c_int()
        print "Page Size"
        libspec.spectre _page_get_size( page, byref(w),byref( h))
        self.initialSiz e=(h.value*1.0, w.value*1.0)
        self.initialAsp ect=float(h.val ue)/float(w.value)
>
self.npages=lib spec.spectre_do cument_get_n_pa ges(self.docume nt)

To answer my own question, partially, i found out if i replace
filename with a hard coded value it doesn't crash.
Now why is that? Does python lose the reference? Should i store
filename as attributed of the object?
Regards
Saptarshi

[Matthieu Brucher]

Re: QT, ctypes dll and SEG Faults

Hi,

If the crash occurs in __init__, why do you give us more code ?

The crash can occur because you pass a char* to your library. This
char* is not managed by your library but by Python. This means that
when the constructor is finished, the string is freed and the char* is
not valid anymore.

This is perhaps what is causing your error. I can't say more because
your code sample is too long and not well explained (what is happening
and what should happen ?)

Matthieu

2008/8/17 sapsi <saptarshi.guha @gmail.com>:

On Aug 16, 11:37 pm, sapsi <saptarshi.g... @gmail.comwrote :

>Hello,
>Below is a small class using ctypes and libspectre to read a
>postscript file.
>My program is a PyQT 4.4 application and when the user clicks on a
>entry in a QTableWidget, i run
>>
>PostScriptImag e( filename_as_con tained_in_click ed_tableWidgetI tem )
>However on i get a segfault while trying document_load.
>Surprisingly , before i run sys.exit(app.ex ec_()) (i.e before the app
>starts) if run PostScriptImage (command_line_s pecified_ps_fil e) it
>works!
>>
>I know the ctypes so file works with QT since i wrote an application
>using it, but i hadn't separated the ctypes stuff into separate class
>(i.e the spectre code was in the widget method).
>>
>Any ideas why the crash?
>>
>Regards
>Saptarshi
>--Python Code--
>class PostScriptImage :
> def __init__(self,f ilename):
> print "Doc New"
> self.document=l ibspec.spectre_ document_new()
> print "Load Doc" , filename
> #############cr ashed in the following
>line########## ###############
> libspec.spectre _document_load( self.document,f ilename)
> print "Done load doc"
> if libspec.spectre _document_statu s(self.document ):
> return False
> self.scale=[1.0,1.0]
> self.quicksetup ()
>>
> def quicksetup(self ):
> print "RC"
> rc=libspec.spec tre_render_cont ext_new()
> print "Get 0th Page"
> page=libspec.sp ectre_document_ get_page (self.document, 0)
> if libspec.spectre _document_statu s(self.document ):
> raise Exception("Spec tre:Setup Document Error")
> w= c_int()
> h= c_int()
> print "Page Size"
> libspec.spectre _page_get_size( page, byref(w),byref( h))
> self.initialSiz e=(h.value*1.0, w.value*1.0)
> self.initialAsp ect=float(h.val ue)/float(w.value)
>>
>self.npages=li bspec.spectre_d ocument_get_n_p ages(self.docum ent)

>
To answer my own question, partially, i found out if i replace
filename with a hard coded value it doesn't crash.
Now why is that? Does python lose the reference? Should i store
filename as attributed of the object?
Regards
Saptarshi
--

http://mail.python.org/mailman/listinfo/python-list

>

--
French PhD student
Website : http://matthieu-brucher.developpez.com/
Blogs : http://matt.eifelle.com and http://blog.developpez.com/?blog=92
LinkedIn : http://www.linkedin.com/in/matthieubrucher

[Wojtek Walczak]

Re: QT, ctypes dll and SEG Faults

On Sat, 16 Aug 2008 21:17:25 -0700 (PDT), sapsi wrote:

>Below is a small class using ctypes and libspectre to read a
>postscript file.
>My program is a PyQT 4.4 application  and when the user clicks on a
>entry in a QTableWidget, i run
>>
>PostScriptImag e( filename_as_con tained_in_click ed_tableWidgetI tem )
>However on i get a segfault while trying document_load.
>Surprisingly , before i run sys.exit(app.ex ec_()) (i.e before the app
>starts) if run PostScriptImage (command_line_s pecified_ps_fil e) it
>works!

To answer my own question, partially, i found out if i replace
filename with a hard coded value it doesn't crash.
Now why is that? Does python lose the reference? Should i store
filename as attributed of the object?

Are you sure that the hardcoded value is _exactly_ the same
as the one passed to __init__? Try it in __init__ to make sure:

assert filename == 'your_hardcoded _value'

And why won't you try to store the filename as an attribute?
(even though I doubt it could help).

--
Regards,
Wojtek Walczak,

http://www.stud.umk.pl/~wojtekwa/