With Qt, how to check if stdin is empty? - qt

Question

I have a Qt program that processes stdin data like this:
QTextStream qtin(stdin);
QString stdindata = qtin.readAll();
QByteArray ba;
ba = stdindata.toUtf8();
QJsonDocument exJSONDoc(QJsonDocument::fromJson(ba));
QJsonObject extRoot;
extRoot = exJSONDoc.object();
QStringList keys;
keys = extRoot.keys();
for (int n=0; n <= keys.count()-1; n++)
{
qDebug() << extRoot.value(keys[n]).toString();
}
It works when I call my program like this:
myprogram < ./data.json
But if I call it without any "<" it hangs in qtin.readAll().
How can I check with Qt if the stdin is empty?

Answer 1

(I am assuming a Linux -or at least POSIX- operating system)
QTextStream qtin(stdin);
QString stdindata = qtin.readAll();
This would read stdin till end-of-file is reached. So works with a redirected input like
myprogram < ./data.json
But if I call it without any "<" it hangs ...
But then (that is, if you run myprogram alone) stdin is not empty. It is the same as your shell's stdin. and your program, being the foreground job, is waiting for input on the terminal you are typing (see also tty(4)). Try (in that case) typing some input on the terminal (which you could end with Ctrl D to make an end-of-file condition). Read about job control and the tty demystified and see also termios(3).
Perhaps you could detect that situation with e.g. isatty(3) on STDIN_FILENO. But that won't detect a pipe(7) like
tail -55 somefile | myprogram
You need to define what an empty stdin is for you. I have no idea what that means to you, and I would instead think of myprogram < /dev/null (see null(4)) as the way to get an empty stdin.
Perhaps you should design myprogram so that some program
option (perhaps --ignore-stdin) is avoiding any read from stdin.

Answer 2

Problem here is readAll. See documentation:
Reads the entire content of the stream, and returns it as a QString.
Avoid this function when working on large files, as it will consume a
significant amount of memory.
So it reads stdin until it encounters end of file and since stdin is associated with console you have to signal end of file. Usually it is Ctrl-D and press enter.
It is more probable you what to read stdin line by line.
To alow user text editing console transfers data to standard input of the application only line by line. This was designed like this ages ago when computer had only a printer as user interface (no screen).
Now question is how to read JSon form stdin console connected with console without end of file information?
I would use some SAX parser, but this would be to complicated for you.
So is there another way to detect end of JSon?
You can try this approach (this is basic idea, not final solution, so it has couple shortcomings):
QFile file(stdin);
QByteArray data = file.peak(largeNumber);
QJsonParseError error;
QJSonDocument doc = QJSonDocument::fromJson(data, &error);
while (!doc.isValid() && JSonNotTerminatedError(error.error))
{
// TODO: wait for new data - it would be best to use readyRead signal
doc = QJSonDocument::fromJson(data, &error);
}
Where JSonNotTerminatedError returns true for respective QJsonParseError::ParseError values (see linked documentation) which are related with unterminated JSon data.
Now I see QFile doesn't have required constructor, but main concept should be clear. Read data from stdin and check if it is a valid JSon document.