Getting the last background pid

Fabio Paolini · 03-31-2012, 08:53 PM

Hi, I know that it is possible to get the last background pid using the $! command, however when I use it in my QT application I got a wrong result. Below follows an example where this procedure works

Code:

[linux@localhost ~]$ (sleep 6 && echo "A test") &
[1] 3312
[linux@localhost ~]$ echo $!
3312
[linux@localhost ~]$ A test

Now my Qt application, where I got a wrong result:

Code:

./dbsync &>/dev/null &
[1] 5047
[linux@localhost comm5]$ echo $!
5047

but using the ps command it is possible to verify that the pid related to the right process is not the 5047 but 5050.
Generally it is the process that comes with the command

Code:

echo $!

added of two.

I use QCoreApplication in my program. I do not know if it has something to do with this error.

Any advice is welcome

suicidaleggroll · 03-31-2012, 10:49 PM

Quote:

but using the ps command it is possible to verify that the pid related to the right process is not the 5047 but 5050.

What is the "right process"? Is it not dbsync? That's the process with a PID of 5047. If it's another process you need (one that is being called by dbsync), that will have a different PID.

Fabio Paolini · 03-31-2012, 11:38 PM

The dbsync is the right process.
This time a better log is shown below. Probably de program calls another one, but I cannot see how it is done.
Below is the log

Code:

[linux@localhost ~]$ /home/linux/comm5/dbsync  &> /dev/null  &
[1] 5313
[linux@localhost ~]$ ps aux|grep dbsync |grep -v grep
linux     5316 12.3  1.0 119768 11072 ?        Sl   01:26   0:00 /home/linux/comm5/dbsync
[1]+  concluded              /home/linux/comm5/dbsync &>/dev/null
[linux@localhost ~]$

See that the code that would return from "echo $!" is 5313, put the code that comes from a call to the process is 5316.
At this time the 5313 does not exist any more.

Below the part of the program where it is started

Code:

class Service : public QtService<QCoreApplication>
{
public:
	Service( int argc, char** argv ) : QtService<QCoreApplication>(argc, argv, "Program")
	{
		setServiceDescription(("Name of the program"));
		setServiceFlags(QtServiceBase::Default);
	}

protected:
	void start()
	{
		
		quint16 listenPort = Settings::instance()->value("server/listen", xxxx).toUInt();
		if ( !listener.listen( QHostAddress::Any, listenPort ) )
			applog.out(QString("Error to listen port %1").arg(listenPort), Logfile::ERROR );

	}

This code is called server instead of dbsync, but it has the same problem.

Fabio Paolini · 04-26-2015, 02:44 PM

Hi, I had already given up this problem, but recently I've got a similar problem and so I returned to think about it. I got some advances although I am far from really solving all my doubts in this issue. It follows below some considerations.

The main method to be studied is

Code:

QtServiceBase::exec()

that we may see details here
http://doc.qt.digia.com/solutions/4/...base.html#exec

When this method is called without arguments it calls the method

Code:

 
char* env = getenv("QTSERVICE_RUN")

If

Code:

getenv(char*)

returns a null pointer then the method

Code:

QProcess::startDetached(const QString)

is called which create a new process and the first one is killed. On the other hand if

Code:

getenv(char*)

does not return a null pointer then the method

Code:

int QtServiceBasePrivate::run(bool asService, const QStringList &argList)

is called and the service realy starts.

If someone could explains a bit better how the QtService class works or point some link I would thanks.
Nowadays I am using a Debian distribution.