Loading image concurrently



  • Hi,

    I'm trying to load images from a directory. The amount of images can be a couple thousands, so I thought about doing it concurrently and save some IO blocking time. The trick it that I need to wait until all of the images are loaded. The code I've written gets into a deadlock sometime, and almost always when debugging, and I don't really understand why.

    Here's what I have -

         for (int i=0; i <filesToDisp.size(); ++i) {
            QString extenstion = filesToDisp[i].split('.').last();
            if (isImage(extenstion)) {
                QString name = filesToDisp[i];
                if (name.contains("/")) {
                    name = path + "/" + name.split("/")[1];
                } else {
                    name = path + "/" + filesToDisp[i];
                }
                QFuture<QPair<QImage, QString>> f =
                        QtConcurrent::run(this, &imageManager::getImageByName, name, filesToDisp[i]);
                fs.append(f);
            }
        }
    
        // Waiting for all run to finish
        bool flag = true;
        while (flag) {
            flag = false;
            for (auto& f : fs) {
                if (!f.isFinished()) {
                    flag = true;
                } else {
                    QPair<QImage, QString> p = f.result();
                    _imageQueue.enqueue(new IEimage(new QImage(p.first), p.second, getFreeId()));
                    fs.removeOne(f);
                }
            }
        }
    

    getImageByName returns a pair of image and string.

    Any ideas?


  • Qt Champions 2018

    @Jonathan-Levanon said in Loading image concurrently:

    fs.removeOne(f);

    You're modifying the container while looping over it. Better use iterators + erase(iterator) and simply check if the container is empty as abort condition instead the flag stuff.



  • Such a rookie mistake :) Thanks, it works!



  • @Jonathan-Levanon said in Loading image concurrently:

    Such a rookie mistake :) Thanks, it works!

    But you found it NOW and not have it create errors that are hard to detect later. It would have been much worse if you somehow "made it work".


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