Creating temporary text file in qt

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I'm trying to create a temporary text file in QT and then delete it at the end of the program. I have two threads(T1, T2). T1 need to write the text in TEMP file and and T2 will read it continuously. I need to keep tracking the file pointer as well. I haven't had much luck with Google.

Could you tell me how to go for it?

*NOTE, i'm not a professional, so please bear the pain of making it as simple as possible

What I have tried:

I had done this all with writing the text file and than reading it later. But this is creating problem now solution for the problem is generate the Temporary file. But dont know how to write text into TEMP file and how to fetch text.

Posted 2-Mar-16 21:59pm

XamBEE

Updated 2-Mar-16 23:55pm

v2

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Jochen Arndt · Answer 1 · 2016-03-02T23:55:00

Solution 1

When multiple threads should access the same object you must use locking. For your case of a temporary file I would create a class that encapsulates all the necessary operations (including deleting the file in the destructor).

Example (using C library FILE object and QMutex Class | Qt Core 5.5[^] lock object):

C++

// TempFile.h
#include <QMutex>
#include <stdio.h>

class TempFile
{
public:
    TempFile();
    ~TempFile();
    bool Open(const QString& fileName);
    void Close();
    int Read(char * data, int size);
    int Write(const char * data, int size);
protected:
    QString name;
    FILE *file;
    QMutex mutex;
    long pos;
};

C++

// TempFile.cpp
#include "TempFile.h"
TempFile::TempFile()
{
    file = NULL;
    pos = 0;
}

TempFile::~TempFile()
{
    bool isOpen = file != NULL;
    Close();
    if (isOpen)
        unlink(QString::toLocal8Bit(name).data());
}

bool TempFile::Open(const QString& fileName)
{
    file = fopen(QString::toLocal8Bit(fileName).data(), "w+");
    name = fileName;
    return file != NULL;
}

void TempFile::Close()
{
    if (file)
        fclose(file);
    file = NULL;
    pos = 0;
}

int TempFile::Read(char * data, int size)
{
    mutex.lock();
    // Seek to read position
    fseek(file, pos, SEEK_SET);
    int read = fread(data, 1, size, file);
    if (read > 0)
        pos += read;
    mutex.unlock();
    return read;
}

int TempFile::Write(const char * data, int size)
{
    mutex.lock();
    // Seek to write position (end)
    fseek(file, 0, SEEK_END);
    int written = fwrite(data, 1, size, file);
    mutex.unlock();
    return written;
}

Posted 2-Mar-16 23:55pm

Jochen Arndt

Updated 8-Mar-16 5:18am

v4

Comments

XamBEE 3-Mar-16 6:09am

Thanks for your help. I have one question that what about QTemporaryFile class?? Do you think it would difficult with QTemporaryFile class?

Jochen Arndt 3-Mar-16 6:11am

If it fits your needs you may use it but should derive your own class which implements the read and write functions with locking as shown in my solution.

XamBEE 3-Mar-16 7:59am

would be nice if you can guide that how to read data from Temporary file using QTemporaryFile. How one can get the address/access of the Temporary file??
If data is e.g
aa, bbb, 123, d,
bb, ddd, 456, e,
i want to read data Qoma(,) separately

Jochen Arndt 3-Mar-16 8:18am

QTemporaryFile inherits QFile which inherits QIODevice.
So you can use the corresponding QFile and/or QIODevice functions:
fopen -> open
fclose -> close
fread -> readData
fwrite -> writeData
fseek -> seek

But there is no seek() option to set the position to the end of file. So you must track the write position yourself similar to the read position (initialise with zero upon opening and increment by the number of bytes written with each writing).

XamBEE 8-Mar-16 3:56am

is this the correct way to write into the tamp file??
//from Thread-1 class i want to write into the Temp file.
const char* Cardata[] = {"Blue car", "brown", "1996"};
bool fileopned= Tempfile::open(file);
if(fileopned==true){
Tempfile::write(Cardata,size);
}
Tempfile::close();

Jochen Arndt 8-Mar-16 4:33am

No. Your data is an array of string pointers. You must pass each string with it's size (length):
for (int i = 0; i < 3; i++)
write(Cardata[i], strlen(Cardata[i]));

XamBEE 8-Mar-16 4:49am

OK, and file name would be auto generated??
file = tmpfile();
while reading this file from second thread T2. how one can get the file name(in Thread T2)??

Writing Tempfile(f1)from Thread T1
Reading Tempfile(f1)from Thread T2

Jochen Arndt 8-Mar-16 5:47am

If you want to use QTemporyrFile you should read the documentation.
It clearly states that the file name is generated and that it is unique.
There is also a member function to get the name.
But why you need the name?
Just call your implementations of the write and read functions that provides the locking.

All you have to do is:
- Derive a class from QTemporaryFile.
- Add a QMutex member.
- Add your write and read functions using locking as described in my example code.
- Create an instance of that derived class (e.g. as member of your MainWindow).
- Make the instance available to your other thread (e.g. by passing a pointer to the MainWindow)
- Call the read and write functions of that instance.

XamBEE 8-Mar-16 7:08am

What would be the parameters of read(). While calling from MainWindow?
SEEK_SET would be the the last position to write?

Jochen Arndt 8-Mar-16 7:21am

int TempFile::Read(char * data, int size)

A pointer to a char buffer and the size of that buffer. You stated in your question that you want to read continuously. If the size is large enough, my example code will read up to the current end of the file. The next call to read will then start at the last position (the previously end of file) to get new data that has been written meanwhile. If no data has been written meanwhile, the read function return zero.

SEEK_SET is used to set the file pointer to the position where reading should start (initially at zero and later just behind the data that has been already read).

Writing occurs always at the end of file (using SEEK_END). When using QTemporaryFile you must track the write position yourself by using another member variable because QTemporaryFile does not support seeking to the end of file by a flag. That is call seek with the current write position member variable, write the data, and add the number of bytes written to the write position member variable.

XamBEE 8-Mar-16 8:01am

This is how i am trying to read from MainWindow thread T1?

//Thread T1 open the file read 100 Bytes and close the file. Than Thread-2 open and write new data.
//buffer of 100 bytes
char readBuffer[100];
bool fileopned= Tempfile::open(file);
if(fileopned==true){
//read as many as 100 Bytes from Temp file
Tempfile::Read(readBuffer, 100);
}
Tempfile::close();

Jochen Arndt 8-Mar-16 8:19am

Why do you open and close the file with each read and write?
Open it once after the temp file class has been created.
My example will close und delete the file when the instance is deleted.

If you really want to open and close it with each access, the open and close calls must be also performed when locked. And you must take care when opening for writing: You must open for creation with the first call and for appending with following calls.

XamBEE 8-Mar-16 9:39am

error:: Expected class name before ´{´ token
in following line -> class TempFile : public {

Jochen Arndt 8-Mar-16 9:43am

Remove ": public".
Seems to be slipped there by copy and paste.

Or use "class TempFile : public QTemporaryFile" if you want to derive from that.

XamBEE 8-Mar-16 10:06am

I am trying to execute your example class. it gives me following errors.
1: Invalid use of destructor '~TempFile' as type

Jochen Arndt 8-Mar-16 11:18am

I have type that in from scratch.
The error is in the CPP file here:

~TempFile::TempFile()

It must be

TempFile::~TempFile()

XamBEE 9-Mar-16 7:04am

No matching function for call to 'QString::toLocal8Bit(const QString&)'
Error Line :: file = fopen(QString::toLocal8Bit(fileName).data(), "w+");

Jochen Arndt 9-Mar-16 7:08am

As already noted, I write it from scratch.

file = fopen(fileName.toLocal8Bit().data(), "w+");

Similar with the other call to toLocal8Bit().