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<TITLE>Python Strings and PyMem_Malloc/Free</TITLE>
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<P><FONT SIZE=2>Hi,</FONT>
</P>
<P><FONT SIZE=2>I fear I may have been operating under a false assumption with regards to creating Python strings. This assumption goes like this:</FONT></P>
<P><FONT SIZE=2>1) Whatever memory you PyMem_Malloc you should always PyMem_Free.</FONT>
<BR><FONT SIZE=2>2) For strings you malloc and need to turn into python strings assign them to a python variable in Pyrex then PyMem_Free the pointer to the string before you return the python object.</FONT></P>
<P><FONT SIZE=2>3) This shouldn't present a problem because PyString_FromString copies the string to an internal representation (this is my assumption).</FONT></P>
<P><FONT SIZE=2>I believe the last statement may be completely erroneous :). Here is some code based upon this assumption:</FONT>
</P>
<P><FONT SIZE=2>cdef class Interface:</FONT>
<BR><FONT SIZE=2> cdef TRK_HANDLE trkHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_ASSOC_HANDLE trkAssociationHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_RECORD_HANDLE trkRecordHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_NOTE_HANDLE trkNoteHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_ATTFILE_HANDLE trkAttFileHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_IMPORT_HANDLE trkImportHandle</FONT>
<BR><FONT SIZE=2> cdef TRK_EXPORT_HANDLE trkExportHandle</FONT>
</P>
<P><FONT SIZE=2> .</FONT>
<BR><FONT SIZE=2> .</FONT>
<BR><FONT SIZE=2> .</FONT>
</P>
<P><FONT SIZE=2> def GetNote(self):</FONT>
<BR><FONT SIZE=2> cdef TRK_UINT bufferSize, remaining, max, titleSize, result</FONT>
<BR><FONT SIZE=2> cdef LPSTR buffer, title, temp_buffer</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> result = TrkGetNoteDataLength(self.trkNoteHandle, &bufferSize)</FONT>
<BR><FONT SIZE=2> if result != TRK_SUCCESS:</FONT>
<BR><FONT SIZE=2> raise ExtensionExceptions.TrackerError, result</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> buffer = <LPSTR>PyMem_Malloc(sizeof(char)*bufferSize)</FONT>
<BR><FONT SIZE=2> if buffer == NULL:</FONT>
<BR><FONT SIZE=2> raise Exception, "Could not malloc %d bytes!!" % bufferSize</FONT>
<BR><FONT SIZE=2> temp_buffer = <LPSTR>PyMem_Malloc(sizeof(char)*bufferSize)</FONT>
<BR><FONT SIZE=2> if temp_buffer == NULL:</FONT>
<BR><FONT SIZE=2> raise Exception, "Could not malloc %d bytes!!" % bufferSize</FONT>
<BR><FONT SIZE=2> memset(buffer, 0x00, bufferSize)</FONT>
<BR><FONT SIZE=2> memset(temp_buffer, 0x00, bufferSize)</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> if bufferSize > 10000:</FONT>
<BR><FONT SIZE=2> max = 10000</FONT>
<BR><FONT SIZE=2> else:</FONT>
<BR><FONT SIZE=2> max = bufferSize</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> remaining = bufferSize</FONT>
<BR><FONT SIZE=2> pyNote = ''</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> while remaining != 0:</FONT>
<BR><FONT SIZE=2> result = TrkGetNoteData(self.trkNoteHandle, max, temp_buffer, &remaining)</FONT>
<BR><FONT SIZE=2> if result != TRK_SUCCESS and result != TRK_E_DATA_TRUNCATED:</FONT>
<BR><FONT SIZE=2> PyMem_Free(buffer)</FONT>
<BR><FONT SIZE=2> PyMem_Free(temp_buffer)</FONT>
<BR><FONT SIZE=2> raise ExtensionExceptions.TrackerError, result</FONT>
<BR><FONT SIZE=2> strcat(buffer, temp_buffer)</FONT>
<BR><FONT SIZE=2> memset(temp_buffer, 0x00, bufferSize)</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> titleSize = 200</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> title = <LPSTR>PyMem_Malloc(sizeof(char)*titleSize)</FONT>
<BR><FONT SIZE=2> if title == NULL:</FONT>
<BR><FONT SIZE=2> raise Exception, "Could not malloc %d bytes!!" % titleSize</FONT>
<BR><FONT SIZE=2> memset(title, 0x00, titleSize)</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> result = TrkGetNoteTitle(self.trkNoteHandle, titleSize, title)</FONT>
<BR><FONT SIZE=2> if result != TRK_SUCCESS:</FONT>
<BR><FONT SIZE=2> PyMem_Free(buffer)</FONT>
<BR><FONT SIZE=2> PyMem_Free(temp_buffer)</FONT>
<BR><FONT SIZE=2> PyMem_Free(title)</FONT>
<BR><FONT SIZE=2> raise ExtensionExceptions.TrackerError, result</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> pyTitle = title</FONT>
<BR><FONT SIZE=2> pyNote = buffer</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> # it is this code that appears to be problematic</FONT>
<BR><FONT SIZE=2> PyMem_Free(buffer)</FONT>
<BR><FONT SIZE=2> PyMem_Free(temp_buffer)</FONT>
<BR><FONT SIZE=2> PyMem_Free(title)</FONT>
<BR><FONT SIZE=2> </FONT>
<BR><FONT SIZE=2> return (pyTitle,pyNote)</FONT>
</P>
<P><FONT SIZE=2>As the code stands calling this function _sometimes_ crashes the Python interpreter (usually the 3rd or 5th call) with an access violation (mem address 0x0000000 which would imply a null pointer reference). If I comment out the final three calls to PyMem_Free then the problem does not arise.</FONT></P>
<P><FONT SIZE=2>If my assumption is wrong does the same hold true for structures? I have similar malloc/free calls for structures which save data from them and then free the structure memory before returning.</FONT></P>
<P><FONT SIZE=2>Any information is appreciated,</FONT>
</P>
<P><FONT SIZE=2>Grant M.</FONT>
</P>
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