Why does reading from STDOUT work? - qt

Question

I encountered a curious case, where reading from STDOUT works, when running a program in terminal. The question is, why and how? Let's start with code:
#include <QCoreApplication>
#include <QSocketNotifier>
#include <QDebug>
#include <QByteArray>
#include <unistd.h>
#include <errno.h>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
const int fd_arg = (a.arguments().length()>=2) ? a.arguments().at(1).toInt() : 0;
qDebug() << "reading from fd" << fd_arg;
QSocketNotifier n(fd_arg, QSocketNotifier::Read);
QObject::connect(&n, &QSocketNotifier::activated, [](int fd) {
char buf[1024];
auto len = ::read(fd, buf, sizeof buf);
if (len < 0) { qDebug() << "fd" << fd << "read error:" << errno; qApp->quit(); }
else if (len == 0) { qDebug() << "fd" << fd << "done"; qApp->quit(); }
else {
QByteArray data(buf, len);
qDebug() << "fd" << fd << "data" << data.length() << data.trimmed();
}
});
return a.exec();
}
Here's qmake .pro file for convenience, if someone wants to test above code:
QT += core
QT -= gui
CONFIG += c++11
TARGET = stdoutreadtest
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app
SOURCES += main.cpp
And here's output of 4 executions:
$ ./stdoutreadtest 0 # input from keyboard, ^D ends, works as expected
reading from fd 0
typtyptyp
fd 0 data 10 "typtyptyp"
fd 0 done
$ echo pipe | ./stdoutreadtest 0 # input from pipe, works as expected
reading from fd 0
fd 0 data 5 "pipe"
fd 0 done
$ ./stdoutreadtest 1 # input from keyboard, ^D ends, works!?
reading from fd 1
typtyp
fd 1 data 7 "typtyp"
fd 1 done
$ echo pipe | ./stdoutreadtest 1 # input from pipe, still reads keyboard!?
reading from fd 1
typtyp
fd 1 data 7 "typtyp"
fd 1 done
So, the question is, what is going on, why do the last two runs above actually read what is typed on terminal?
I also tried looking at QSocketNotifier sources here leading to here, but didn't really gain any insight.

Answer 1

There is no different between fd 0,1,2, all of the three fds is pointing to terminal if not redirected, they are strictly IDENTICAL!
Programs usually use 0 for input, 1 for output, 2 for error, but all of them can be different ways.
Eg, for less, ordinary usage:
prog | less
Now input of less is redirected to the prog, and less can not read any user input from stdin, so how does less get user input like scroll up/down or page up/down ?
Sure less can read user input from stdout, which is exactly what less does.
So you can use these fds wisely when you know how bash handle these fds.