QTProcess gets stuck wile running grep

sambatyon

I have the following small C++ Program, which I am trying to run on windows 7, grep is part of mingw and the minwg powered version of Qt:

@
#include <QStringList>
#include <QProcess>
#include <iostream>

const int64_t kBuffSize = 2048;

int main(int argc, char *argv[]) {
QProcess grep;
QStringList params;
params << "-e" << "".*pas'"" << ""C:\Path To File\file.dpr"";//argv[1];
grep.start("C:\MinGW\msys\1.0\bin\grep.exe", params);
grep.setReadChannel(QProcess::StandardOutput);
if (!grep.waitForFinished(60000)) {
// program always exits here
if (grep.state() == QProcess::Running)
grep.kill();
return 1;
}
std::cout << "ready to read" << std::endl;
char buffer[kBuffSize];
while (grep.readLine(buffer, kBuffSize) > 0) {
std::cout << buffer;
}
if (grep.state() == QProcess::Running)
grep.kill();
return 0;
}
@

The problem is grep never returns anything and the output of the program is always @QProcess: Destroyed while process still running@

andre

QProcess is an asynch beast, like any QIODevice. Almost any real Qt program needs at least a QCoreApplication and a running eventloop. You have neither.

sambatyon

I also tried:

@
#include <QStringList>
#include <QProcess>
#include <QCoreApplication>
#include <iostream>

const int64_t kBuffSize = 2048;

int main(int argc, char *argv[]) {
QCoreApplication app(argc, argv);
QProcess grep;
QStringList params;
params << "-e" << "".*pas'"" << ""C:\Path To File\file.dpr"";//argv[1];
grep.start("C:\MinGW\msys\1.0\bin\grep.exe", params);
grep.setReadChannel(QProcess::StandardOutput);
if (!grep.waitForFinished(60000)) {
// program always exits here
if (grep.state() == QProcess::Running)
grep.kill();
return 1;
}
std::cout << "ready to read" << std::endl;
char buffer[kBuffSize];
while (grep.readLine(buffer, kBuffSize) > 0) {
std::cout << buffer;
}
if (grep.state() == QProcess::Running)
grep.kill();
return 0;
}
@

sambatyon

As it seems, the problem is in grep. But I cannot figure out why

andre

You create the application object, but you do not start the event loop... Note that I'm not a 100% sure that that is the problem.

goetz

Don't double quote your arguments. With your args here:

@
params << "-e" << "".*pas'"" << ""C:\Path To File\file.dpr"";//argv[1];
@

you search for lines that start with a quote (") followed by some characters followed by literal 'pas' followed by apostrophe (') followed by quote ("). I doubt that this is what you want.

Also, your file path is "C:\Path To File\file.dpr" (including the quotes), which does not exist, as no path can start with a quote.

What you probably want is:

@
params << "-e" << ".*pas'" << "C:\Path To File\file.dpr";
@

Also, you can use forward slashes for the path to the exe, Qt handles this for you.

To see what's going wrong, show the error string:

@
if (!grep.waitForFinished(60000)) {
qDebug() << grep.errorString();
// program always exits here
if (grep.state() == QProcess::Running)
grep.kill();
return 1;
}
@

goetz

[quote author="Andre" date="1298645066"]You create the application object, but you do not start the event loop... Note that I'm not a 100% sure that that is the problem.[/quote]

At least on Mac OS X (and the other Unices) it works without a Q(Core)Application object. QProcess should start it's own event loop.

giesbert

QProcess does not need an event loop unless you don't use queued signals :-):

"QProcess docs":http://doc.qt.nokia.com/latest/qprocess.html :

waitForStarted and waitForFinished:

bq. This function can operate without an event loop.

but some points:

setReadChannel after start? What if the process finished before that is executed?
readLine might not return anything if your process does not output new lines.

sambatyon

ok guys, for the interested, here is the solution of the problem. It seems I had to call first wait for started i got the solution "Here":http://stackoverflow.com/questions/5116663/qprocess-is-stuck