[Interest] Access Violation using QFile and QTextStream

[Thomas McGuire]

Hi,

In the FileParser constructor, you create a QFile object on the stack. This 
means that the QFile object will get destroyed when the constructor is done.
Then you create a QTextStream and pass that a pointer to the QFile. This 
pointer will however point to invalid memory after the constructor has 
finished, as the QFile will have been destroyed.
All further accesses to the QTextStream then internally try to use the invalid 
pointer to the already destroyed QFile, which will cause a access violation.
A solution here would be to make the QFile a member variable as well, so that 
it has a longer lifetime than just in the constructor.

Regards,
Thomas

On Friday 14 September 2012 13:44:00 Heidler, Kirstin (GE Oil & Gas) wrote:
> Here is what I do:
> 
> I open a File with QFile. Next I create a QTextStream for that file. This
> TextStream is saved in a member to my FileParsing-class.
> 
> While still in the constructor everything works fine. I can access the
> TextStream, print it's device's size etc.
> 
> The problem arises once my TestCase calls a function to read a line.
> 
> In that function of my File-Parsing class nothing seems to be working. I
> want to use seek but that does not work. Printing the size also no longer
> works.
> 
> When I run the application normally I don't get an error at all. The only
> thing that happens is, that only one testcase is executed (the default
> one, that should always pass).
> 
> The testcase runner however does not notify me, that there was an error.
> 
> When I debug the application I get a write access violation. (Code:
> 0xc0000005) at: 0x8, flags = 0x0;
> 
> This sounds to me like the memory was somehow freed??? I don't understand
> why...
> 
> 
> 
> Here are some code examples:
> 
> The testfunction that does the calls:
> 
> void FileParserTest::testNameReadCorrectly(){
> 
>      FileParser parser = FileParser::FileParser(
> 
>                 "C:/Users/108015407/Documents/DatenSolidAxle/Seite
> A/GE_TDGS_3.5_S56°.GE.20°-80°.ERA.600.2012.2.zeu.Takt 1.txt");
> 
>     parser.readName();
> 
>     QString name = "GE_TDGS_3.5_S56°.GE.20°-80°.ERA.600.2012.2.zeu";
> 
>     QCOMPARE(parser.m_summary.getName(), name);
> 
> }
> 
> 
> 
> Relevant header of the parser:
> 
> #ifndef FILEPARSER_H
> #define FILEPARSER_H
> #include "scanSummary.h"
> #include <iostream>
> #include <string>
> #include <QFile>
> #include <QString>
> #include <QTextStream>
> class FileParser
> {
> public:
>     FileParser(char* fileName);
>     ~FileParser();
>     ScanSummary m_summary;
>     void readName();
> 
> protected:
>     QTextStream *m_file;
> };
> #endif // FILEPARSER_H
> 
> 
> 
> 
> The relevant functions:
> 
> FileParser::FileParser(char *fileName){
> 
>     QFile file;
> 
>     file.setFileName(fileName);
> 
>     if(!file.exists()){
> 
>         std::cout << "File not exist" << std::endl;
> 
>     }
> 
>     bool succ = file.open(QIODevice::ReadWrite);
> 
>     std::cout << succ << std::endl;
> 
>     if(!succ){
> 
>         std::cout << "Not Good!" << std::endl;
> 
>     }
> 
>     m_file = new QTextStream(&file);
> 
>     std::cout << m_file->device()->size() << std::endl;
> 
> }
> 
> FileParser::~FileParser(){
> 
>     m_file->device()->close();
> 
> }
> 
> 
> 
> void FileParser::readName(){
> 
>     if(!m_file->device()){
> 
>         std::cout << "Alarm..." << std::endl;
> 
>     }
> 
>     std::cout << "I got here: 4.05" << std::endl; //gets to this mark but
> not the next
> 
>     //std::cout << m_file->device()->size() << std::endl; //these two lines
> don't work either
> 
>     //std::cout << m_file->pos() << std::endl;
> 
>     bool succ = m_file->seek(0);
> 
>     std::cout << succ << std::endl;
> 
>     std::cout << "I got here: 4.1" << std::endl;
> 
>     QString line;
> 
>     std::cout << "I got here: 4.2" << std::endl;
> 
>     line = m_file->readLine();
> 
>     std::cout << "I got here: 4.3" << std::endl;
> 
>     m_summary.setName(line);
> 
>     std::cout << "I got here: 4.4" << std::endl;
> 
> }

-- 
** Qt Developer Conference: http://qtconference.kdab.com/ **

Thomas McGuire | thomas.mcguire at kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel. Germany +49-30-521325470, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 3637 bytes
Desc: not available
URL: <http://lists.qt-project.org/pipermail/interest/attachments/20120914/dc0a2e27/attachment.bin>