Cannot get stdout from QProcess

[johnny_sparx]

I am having trouble to retrieve the console output from an external program. Below, I modified the program so that it is similar to what I have in my program - but with the external program changed to "dir /w".

QProcess System_Call(this);
QStringList Parameters;
 
Parameters<<"/w";
 
System_Call.start("dir",Parameters);
System_Call.waitForStarted();
QString X;
X = System_Call.readAll();
System_Call.waitForFinished();
Parameters.clear();

QProcess System_Call(this);
QStringList Parameters;

Parameters<<"/w";

System_Call.start("dir",Parameters);
System_Call.waitForStarted();
QString X;
X = System_Call.readAll();
System_Call.waitForFinished();
Parameters.clear();

To copy to clipboard, switch view to plain text mode 

I cannot seem to retrieve the output from this program... what am I doing wrong?

[zlatko]

What about readAllStandardOutput() ?

[wysota]

I think you should read after the process has finished.

[zlatko]

I think you should read after the process has finished.

but if app using flush(stdout) first method can be possible?

[jacek]

but if app using flush(stdout) first method can be possible?

No, because you still might try to read the data before the process has written it.

[jpn]

Hmm.. this works fine on Linux (Ubuntu, Qt 4.1.1, gcc 4.0.2) but not in Windows (XP+SP2, Qt4.1.1, VS2003).

QProcess proc;
proc.start(QString("dir"));
proc.waitForFinished(-1);
qDebug() << proc.readAll();

QProcess proc;
proc.start(QString("dir"));
proc.waitForFinished(-1);
qDebug() << proc.readAll();

To copy to clipboard, switch view to plain text mode 

Edit: means that it outputs correct info in linux, and nothing in windows..

[seneca]

Hmm.. this works fine on Linux (Ubuntu, Qt 4.1.1, gcc 4.0.2) but not in Windows (XP+SP2, Qt4.1.1, VS2003).

QProcess proc;
proc.start(QString("dir"));
proc.waitForFinished(-1);
qDebug() << proc.readAll();

QProcess proc;
proc.start(QString("dir"));
proc.waitForFinished(-1);
qDebug() << proc.readAll();

To copy to clipboard, switch view to plain text mode 

Edit: means that it outputs correct info in linux, and nothing in windows..

Is the Qt application built as console, or are you checking the dDebug output with the debugger? Otherwise you wont see any qDebug output on windows.

[jpn]

Is the Qt application built as console, or are you checking the dDebug output with the debugger? Otherwise you wont see any qDebug output on windows.

With a debugger... I can see qDebug() outputs fine, that is not the problem.

[wysota]

Reading all output at once, even after the application is finished is not a very good idea. For short outputs it is fine, but if you expect a large amount of data to come through that pipe, you should use a signal/slot mechanism to read the data as it comes in. Why? Because it may happen that you fill the pipe buffer between those two processes and the child process will never finish, because it'll be waiting until some of the buffer is read so that it can write all the remaining data. So you should use readyReadStdout() and family.

[jpn]

My point is, that I never get any output on windows with the above code.
Even when I set the working dir so that the output is expected to be extremely short.

[wysota]

My point is, that I never get any output on windows with the above code.
Even when I set the working dir so that the output is expected to be extremely short.

Because there is no "dir" executable on Windows. "dir" is a command.com (or cmd.exe) command, so you have to run it from the shell. There are threads on this forum that cover the topic, maybe you should take a look at them.

[jpn]

Because there is no "dir" executable on Windows. "dir" is a command.com (or cmd.exe) command, so you have to run it from the shell. There are threads on this forum that cover the topic, maybe you should take a look at them.

Ahh, now I get the point..