[Interest] Access Violation using QFile and QTextStream

[Nikos Chantziaras]

Now you're leaking memory every time that function is called.


On 14/09/12 15:42, Heidler, Kirstin (GE Oil & Gas) wrote:
> Hi,
>
> thank you, I also found a different solution. I am not sure why it is working, but it does.
> I simply  create the file object as a pointer to the file object like that:
>     QFile *file;
>      file = new QFile(fileName);
>      file->open(QIODevice::ReadOnly);
>
> Thank you for the hint. :)
> Kirstin
>
> -----Ursprüngliche Nachricht-----
> Von: interest-bounces+kirstin.heidler=ge.com at qt-project.org [mailto:interest-bounces+kirstin.heidler=ge.com at qt-project.org] Im Auftrag von Thomas McGuire
> Gesendet: Freitag, 14. September 2012 13:56
> An: interest at qt-project.org
> Betreff: Re: [Interest] Access Violation using QFile and QTextStream
>
> 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;
>>
>> }