QProcess::readAllStandardOutput gives flaky outputs - qt

Question

I am trying to create a GUI in Qt4 for my tcl based tool. In order to populate widgets I need to execute some tcl commands. I read about QProcess and I am invoking tcl scripts using QProcess and then grabbing their output from stdout.
Suppose I execute 3 commands in tcl then when I query stdout I believe I should see 3 outputs corresponding to each of the three commands, however this is not happening consistently. Behavior is flaky.
As you can see in the main.cpp I am executing multiple commands using runTclCommand() function and in the end executing getData() function to read stdout.
main.cpp:
#include <QApplication>
#include <QProcess>
#include <QDebug>
#include "Tclsh.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QByteArray out;
Tclsh *tcl = new Tclsh;
tcl->startTclsh();
tcl->runTclCommand("set ::tcl_interactive 1\n");
tcl->runTclCommand("set a 23\n");
tcl->runTclCommand("puts $a\n");
tcl->runTclCommand("set a 40\n");
tcl->runTclCommand("puts $a\n");
// out = idl->getData();
out = tcl->getData();
}
Tclsh.cpp:
#include <QProcessEnvironment>
#include <QProcess>
#include <QDebug>
#include "Tclsh.h"
void Tclsh::startTclsh() {
QString program = "/usr/bin/tclsh8.4";
this->setProcessChannelMode(QProcess::MergedChannels);
this->start(program);
if ( !this->waitForStarted()) {
qDebug()<<"ERROR Starting tclsh";
}
return;
}
void Tclsh::runTclCommand(const char *cmd) {
qDebug()<<"CMD:"<<cmd;
this->write(cmd);
if (!this->waitForBytesWritten()) {
qDebug()<<"Error in writing data";
}
}
QByteArray Tclsh::getData() {
if (!this->waitForReadyRead()) {
qDebug()<<"Error in reading stdout: Ready read signal is not emitted";
}
QByteArray data = this->readAllStandardOutput();
qDebug()<<"DATA:"<<data;
return data;
}
However, sometime I get the following output:
CMD: set ::tcl_interactive 1
CMD: set a 23
CMD: puts $a
CMD: set a 40
CMD: puts $a
DATA: "1
% 23
% 23
% "
And sometimes this:
CMD: set ::tcl_interactive 1
CMD: set a 23
CMD: puts $a
CMD: set a 40
CMD: puts $a
DATA: "1
"
I do not understand why this is happening. I would really appreciate if someone can point me to the error in my approach here.
Thanks,
Newbie
Edit: After some more research, here are my thoughts
According to Qt manual, readyRead signal will be emitted whenever new data is available (as specified by #Frank Osterfeld also, thanks!). It will not wait for complete output data to be available (which is justified since it does not know when will that happen). Hence my approach is not good. What I can do is something like this:
start the process -> wait for process to finish -> read stdout
This will ensure that flaky behavior does not arise as process is already finished when I am reading hence no new data can come.
However, in this proposed approach I am not clear about one thing: Does stdout is specific to a process? I mean can it happen that process which was supposed to read stdout output from process1, can get other stdout data from some other process which happen to write stdout at the same time as process1?
Thanks,
Newbie

Answer 1

I am closing this question. Reading from a channel more than once does not seem to a be a good idea. Instead what I do now is write what I want to write in one go --> close the channel for writing --> then read it back. In that way I get consistent output.