why can't QProcess read all output?

[wysota]

@Wysota: I didn't said you are wrong, and you are right about line-buffer in *nix, but this isn't a behavior that you can assume it's true (especially if you want multi-platform)

Can you point a platform that doesn't perform a flush on newline?

[squidge]

I just use QProcess start the program.and read the process.I didn't know how to request su first.if you can tell me how to request su first.

Check out PAM.

Can you point a platform that doesn't perform a flush on newline?

What would also be interesting is that if you open a file as binary, you don't (or at least, I would expect you didn't) get flushing on newline. Do you get fliushing on endl even though the file is binary? I know using endl on a binary file is a bit strange, but it would be interesting to find out. Unless of course endl on a binary file causes a run time error?

[Zlatomir]

The OS might flush the stream with new line or might not, we can't know for sure, it might look that in 1000 cases flushes the stream and in one case, when we assume it does... the OS make us a surprise... (i saw some people surprised by this issue)

I was talking from the C/C++ programmer point of view, for us there is no guarantee that the OS will flush on new line (\n), but in case the programmer isn't "happy" with the OS flushes, he can do it when he want it.

[Raul]

[QUOTE=fatjuicymole;146411]Check out PAM.


what the "PAM" reffers to?

[squidge]

http://en.wikipedia.org/wiki/Pluggab...cation_Modules

[Raul]

Then it seems like you can't get the same behaviour from your Qt app without first requesting su?

I think my problem occurs without first requesting su.and i have studied the PAM.but i can't solve my problem yet,the main reason is that i don't know how to request su first.

[squidge]

Well, you can always give root access to your program permanently, if you don't want to request it whilst running:

chown root your_app
chmod a+xs your_app

[Raul]

Well, you can always give root access to your program permanently, if you don't want to request it whilst running:

chown root your_app
chmod a+xs your_app

the result is the same as before.

[ChrisW67]

I have knew that the second line end with a \n.and it should be the second possibility,but,before I start the program,I set the process like this:

inprocess.setReadChannelMode(QProcess::MergedChannels);

inprocess.setReadChannelMode(QProcess::MergedChannels);

To copy to clipboard, switch view to plain text mode 

and in the connect, I write like this:

connect(&inprocess,SIGNAL(readyReadStandardOutput()),this,SLOT(iupdateTextEdit()));
connect(&inprocess,SIGNAL(readyReadStandardError()),this,SLOT(iupdateTextEdit()));

connect(&inprocess,SIGNAL(readyReadStandardOutput()),this,SLOT(iupdateTextEdit()));
connect(&inprocess,SIGNAL(readyReadStandardError()),this,SLOT(iupdateTextEdit()));

To copy to clipboard, switch view to plain text mode 

the following show the iupdateTextEdit() fuction

void mainWindow::iupdateTextEdit() 
{
    QByteArray newData=inprocess.readAllStandardOutput();
    QString text=textEdit->toPlainText()+QString::fromLocal8Bit(newData);
    textEdit->setPlainText(text);
}

void mainWindow::iupdateTextEdit() 
{
    QByteArray newData=inprocess.readAllStandardOutput();
    QString text=textEdit->toPlainText()+QString::fromLocal8Bit(newData);
    textEdit->setPlainText(text);
}

To copy to clipboard, switch view to plain text mode 

even so,it can't show the second line in the textEdit.

We cannot see where, but you are allocating "inprocess" on the stack from the look of this. Are you sure that inprocess is not going out-of-scope at some inconvenient point in time? This might explain the "sometime works" behaviour.

[Raul]

We cannot see where, but you are allocating "inprocess" on the stack from the look of this. Are you sure that inprocess is not going out-of-scope at some inconvenient point in time? This might explain the "sometime works" behaviour.

I'm not sure.but in Qt,it almost declare a QProcess object .and hardly ever use "new" .what's more,the output only has two lines if it output all results. so i think it isn't like what you say.

[ChrisW67]

The amount of output you are expecting is irrelevant if the QProcess instance is being destroyed before you want it to be. It's also irrelevant how someone else chooses to create QProcess because it is your code with the problem, not theirs. The first example in the QProcess docs uses the heap for a long-running process, and the second uses a local (stack) variable to run a short-lived process entirely entirely within the same scope.

[wysota]

What would also be interesting is that if you open a file as binary, you don't (or at least, I would expect you didn't) get flushing on newline. Do you get fliushing on endl even though the file is binary? I know using endl on a binary file is a bit strange, but it would be interesting to find out. Unless of course endl on a binary file causes a run time error?

I would expect a binary stream to be buffered as well. Remember TCP is a binary stream and it is flushed on newline (there are no "modes" for creating a socket).

[marc_sturm]

If anyone is still interested, here is a possible workaround for the buffer problem:
https://sites.google.com/site/marcst...output-channel