Convert QFile to FILE*

Question

Is there another way to convet QFile to File? Different than this:

QFile myFile("goforward.raw");
int FileDescriptor = myFile.handle();
FILE* fh = fdopen(FileDescriptor, "rb");

Why do you need this? Can't you simply use the `QFile` for file operations? — Etienne de Martel, Feb 27 '12 at 13:24
Because I use this function: png_init_io(png_ptr, fh); where the second param is File *. Thanks And yes work with code there but I like to use the only library QFile. — Jjreina, Feb 27 '12 at 13:29
This code is the way to do it. So explain why it doesn't work. — Joe, Feb 27 '12 at 13:31
Qt also supports reading and writing PNG files directly from QFile, with QImage::load/save or QImageReader/QImageWriter (or just QPixmap if there is only one PNG image per file). — alexisdm, Feb 27 '12 at 14:24

score 13 · Answer 1 · edited Sep 16 '16 at 09:09

We had very strange problems with our application and finally traced it to the QFile/fdopen issue:

void foo(QString filename)
{
    QFile qf(filename);
    qf.open(QIODevice::ReadOnly);
    int fd = qf.handle();
    FILE* f = fdopen(fd, "rb");
    // do some stuff with f
    fclose(f); // !!! undefined behaviour !!!
}

The problem with this code is that fclose(f) is called before the QFile object is destroyed, which is the wrong order: QTBUG-20372

...so either destroy the QFile object before calling fclose() or duplicate the file descriptor returned by QFile::handle():

void foo(QString filename)
{
    QFile qf(filename);
    qf.open(QIODevice::ReadOnly);
    int fd = qf.handle();
    FILE* f = fdopen(dup(fd), "rb"); // !!! use dup()
    // do some stuff with f
    fclose(f); // correct
}

P.S.: Those strange problems with our app showed up only on very few systems by a 10 second delay between a return statement at the end of a function and the actual return from that function. It was really weird. So this is an example of an "undefined behaviour" manifested in the real world :o)

What happens if you close `qf` before `f`? Can you continue using `f` after that? — Super-intelligent Shade, Jul 30 '17 at 21:42

score 7 · Accepted Answer · answered Feb 27 '12 at 13:30

I think you already know that you have the various open, read, etc. methods in QFile. That said, if the file is not opened, then the handle method returns an error.

QFile myFile("goforward.raw");
myFile.open(QIODevice::ReadOnly);
int fileHandle = myFile.handle();

After that, you might reopen it with:

FILE* fh = fdopen(fileHandle, "rb");

score -1 · Answer 3 · edited Dec 24 '13 at 13:29

-1

If you have the file name, why don't you simply use

QFile *file = fopen(filename.toLocal8Bit().data(), "rb");

?

Isn't it way simpler than creating the QFile, opening it, retrieving the handle, etc.?

And it is pretty bug-free...

edited Dec 24 '13 at 13:29

andrewsi

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answered Dec 24 '13 at 13:12

Edgar Diniz

1

This doesn't compile: "error: cannot convert ‘FILE* {aka _IO_FILE*}’ to ‘QFile*’ in initialization" – Étienne Sep 16 '16 at 11:22

Convert QFile to FILE*

3 Answers3

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