Qt threads for an desktop interface - c++

Question

I am developing a Qt interface for a 3D printer. When I cilck the Print button (the printer starts printing) the interface crashes. I am using this code:
*future= QtConcurrent::run(Imprimir,filename.toUtf8().data());
What can I use to solve it?? What types of threads can I use????
I need to use the interface while the printer is printing (it may take several minutes).
Thank you for advance.
Edit:
Imprimir function:
int Imprimir(char *fich)
{
char *aux = new char;
FILE *f;
f=fopen(fich, "r");
while(!feof(f)){
fgets(aux, 200, f);
Enviar(aux);
while(!seguir_imprimiendo);
}
Sleep(7000);
return 0;
}

Answer 1

It's making life harder than necessary by not using QFile. When you use QFile, you don't have to deal with silly things like passing C-string filenames around. You're likely to do it wrong, since who's to guarantee that the platform expects them to be encoded in UTF-8. The whole point of Qt is that it helps you avoid such issues. They are taken care of, the code is tested on multiple platforms to ensure that the behavior is correct in each case.
By not using QByteArray and QFile, you're liable to commit silly mistakes like your C-classic bug of allocating a single character buffer and then pretending that it's 200 characters long.
I see no reason to sleep in that method. It also makes no sense to wait for the continue flag seguir_imprimiendo to change, since Enviar runs in the same thread. It should block until the data is sent.
I presume that you've made Enviar run its code through QtConcurrent::run, too. This is unnecessary and leads to a deadlock. Think of what happens if a free thread can never be available while Imprimir is running. It's valid for the pool Imprimir runs on to be limited to just one thread. You can't simply pretend that it can't happen.
bool Imprimir(const QString & fileName)
{
QFile src(fileName);
if (! src.open(QIODevice::ReadOnly)) return false;
QByteArray chunk;
do {
chunk.resize(4096);
qint64 read = src.read(chunk.data(), chunk.size());
if (read < 0) return false;
if (read == 0) break; //we're done
chunk.resize(read);
if (!Enviar(chunk)) return false;
} while (! src.atEnd());
return true;
}
bool Enviar(const QByteArray & data)
{
...
return true; // if successful
}

Answer 2

Assuming there's no problem with Imprimir, the issue is probably with filename.toUtf8().data(). The data pointer you get from this function is only valid while filename is in-scope. When filename goes out of scope, the data may be deleted and any code accessing the data will crash.
You should change the Imprimir function to accept a QString parameter instead of char* to be safe.
If you can't change the Imprimir function (because it's in another library, for example), then you will have to wrap it in your own function which accepts a QString. If you're using C++11, you can use a lambda expression to do the job:
QtConcurrent::run([](QString filename) {
Imprimir(filename.toUtf8().data());
}, filename);
If not, you will have to write a separate ImprimirWrapper(QString filename) function and invoke it using QtConcurrent::run.