QT: How to get free port number?

Teufel · 04-06-2017, 02:59 PM

Hi!
I have to redirect sopcast (sp-sc-auth) stream to any free port.
How do I get a free port number using Qt?
Thanks

dugan · 04-06-2017, 04:43 PM

Use Port 0

Teufel · 04-07-2017, 02:43 AM

I missed some details, sorry.
The thing is I starting few instances of my sopcast frontend (written in Qt4) simultaneously. Every instance needs free port to redirect stream. Right now I set a port number manually for every program instance, but I'd like to have it done automatically.

NevemTeve · 04-07-2017, 08:29 AM

Before calling bind(2), set port number to zero so the kernel will pick up a port-number for you.

rtmistler · 04-07-2017, 08:50 AM

Quote:

Originally Posted by Teufel

I missed some details, sorry.

Yes, I noticed this as well.

Please post the code you are working with where you have these questions. And consider also dugan's suggestion; however since I have no idea if you are opening TCP or UDP, or acting as a client or server, or if you are talking about a Serial Port, I really can't offer much in the way of better advice than to input 0.

Teufel · 04-07-2017, 09:24 AM

As I wrote above, I built frontend, which starts command-line sopcast tool: sp-sc-auth.
This snippet of code starts sp-sc-auth with appropriate parameters:

Code:

void Sop::connectClicked(){
    QString command = "/usr/bin/sp-sc-auth " + txtUrl->text() + "  3908 8908";
    
    stopped = FALSE;
    process = new QProcess;
    connect (process, SIGNAL(readyReadStandardOutput()), this, SLOT(processOutput())); 
    connect (process, SIGNAL(readyReadStandardError()), this, SLOT(processOutput()));  
    connect (process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(processRestart(int, QProcess::ExitStatus)));
    
    process->start (command);

}

it reads stream url from txtUrl and starts process like this:
/usr/bin/sp-sc-auth sop://broker.sopcast.com:3912/123456 3908 8908
There is no any problem.
However when I starting a second instance of my frontend (while the first is running), and trying to connect to some different stream (with different url), sp-sc-auth fails because 3908 and 8908 ports already in use. So I need some different free port for second instance.
The simplest way to get alternative port number is qrand routine. It works, but I'd like to have more sane solution than use random ports.

rtmistler · 04-07-2017, 09:49 AM

Thank you for posting the code, it clarifies that this has nothing to do with Qt programming.

I'll let someone who knows how to drive the sc-sp-auth program answer regarding how you obtain any free port, but the suggestion to try zero seems to be a valid assumption.

Teufel · 04-07-2017, 11:00 AM

OK, attempt to use zero allocates free port from kernel pool.
How to allocate port and get allocated port number?
Any Qt example?

NevemTeve · 04-07-2017, 11:17 AM

No real solution on a multi-tasking systems: you check in time t1 that port n1 is free; you start your sub-server; the sub-server tries bind(2) in time t2, and it might find that port n1 is already in use: someone else bound to it between t1 and t2.