how to read stdin to end in qt?

Question

I have a qt-app which can be invoked with:

cat bla.bin  | myapp

Whats the easiest way to read the entire input (stdin) into a QByteArray on Win,Mac and Linux?

I tired several things, but none of them seems to work (on windows):

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
    QByteArray content;

    //---Test 1: hangs forever, reads 0
    while(!std::cin.eof()) {
        char arr[1024];
        int s = std::cin.readsome(arr,sizeof(arr));
        content.append(arr,s);
    }

    //---Test 2: Runs into timeout
    QFile in;
    if(!in.open(stdin,QFile::ReadOnly|QFile::Unbuffered)) {
        qDebug() << in.errorString();
    }
    while (in.waitForReadyRead(1000)) {
        content+=in.readAll();
    }
    in.close();

    return app.exec();
}

Am I having a Event-Loop Problem or shouldn't it work without?

Answer 1

The primary problem of actually reading from stdin stems from using readsome. readsome is generally not used to read from files (including stdin). Readsome is generally used for binary data on asynchronous sources. Technically speaking eof doesn't get set with readsome. read is different in that regard as it will set eof accordingly. There is an SO question/answer here that may be of interest. If you are supporting Linux and Windows and reading stdin, you have to be aware that on Windows stdin isn't opened in binary mode (neither is stdout). On Windows you would have to use _setmode on stdin. One way to do this is with #ifdefs using Q_OS_WIN32. Using QFile doesn't resolve this issue.

In the code you are trying to create it doesn't appear you are interested in actually having an event loop. You can still use QT objects like QByteArray without an event loop. In your code you read data in from stdin (cin) and then you executed return app.exec(); which put your console application into a loop waiting for events. You didn't add any events to the QT Event queue prior to app.exec(); so effectively the only thing you can do is end your application with control-c. If no event loop is needed then code like this should suffice:

#include <QCoreApplication>
#include <iostream>

#ifdef Q_OS_WIN32
#include <fcntl.h>
#include <io.h>
#endif

int main()
{
    QByteArray content;

#ifdef Q_OS_WIN32
    _setmode(_fileno(stdin), _O_BINARY);
#endif

    while(!std::cin.eof()) {
        char arr[1024];
        std::cin.read(arr,sizeof(arr));
        int s = std::cin.gcount();
        content.append(arr,s);
    }
}

Notice how we used a QByteArray but didn't have a QCoreApplication app(argc, argv); and a call to app.exec();