QProcess, Cannot Create Pipe - c++

Question

I am running a QProcess in a timer slot at 1 Hz. The process is designed to evoke a Linux command and parse it's output.
The problem is this: after the program runs for about 20 minutes, I get this error:
QProcessPrivate::createPipe: Cannot create pipe 0x104c0a8: Too many open files
QSocketNotifier: Invalid socket specified
Ideally, this program would run for the entire uptime of the system, which may be days or weeks.
I think I've been careful with process control by reading the examples, but maybe I missed something. I've used examples from the Qt website, and they use the same code that I've written, but those were designed for a single use, not thousands. Here is a minimum example:
class UsageStatistics : public QObject {
Q_OBJECT
public:
UsageStatistics() : process(new QProcess) {
timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(getMemoryUsage()));
timer->start(1000); // one second
}
virtual ~UsageStatistics() {}
public slots:
void getMemoryUsage() {
process->start("/usr/bin/free");
if (!process->waitForFinished()) {
// error processing
}
QByteArray result = process->realAll();
// parse result
// edit, I added these
process->closeReadChannel(QProcess::StandardOutput);
process->closeReadChannel(QProcess::StandardError);
process->closeWriteChannel();
process->close();
}
}
I've also tried manually deleting the process pointer at the end of the function, and then new at the beginning. It was worth a try, I suppose.
Free beer for whoever answers this :)

Answer 1

QProcess is derived from QIODevice, so I would say calling close() should close the file handle and solve you problem.

Answer 2

I cannot see the issue, however one thing that concerns me is a possible invocation overlap in getMemoryUsage() where it's invoked before the previous run has finished.
How about restructuring this so that a new QProcess object is used within getMemoryUsage() (on the stack, not new'd), rather than being an instance variable of the top-level class? This would ensure clean-up (with the QProcess object going out-of-scope) and would avoid any possible invocation overlap.
Alternatively, rather than invoking /usr/bin/free as a process and parsing its output, why not read /proc/meminfo directly yourself? This will be much more efficient.

Answer 3

First I had the same situation with you. I got the same results.
I think that QProcess can not handle opened pipes correctly.
Then, instead of QProcess, I have decided to use popen() + QFile().
class UsageStatistics : public QObject {
Q_OBJECT
public:
UsageStatistics(){
timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(getMemoryUsage()));
timer->start(1000); // one second
}
virtual ~UsageStatistics() {}
private:
QFile freePipe;
FILE *in;
public slots:
void getMemoryUsage() {
if(!(in = popen("/usr/bin/free", "r"))){
qDebug() << "UsageStatistics::getMemoryUsage() <<" << "Can not execute free command.";
return;
}
freePipe.open(in, QIODevice::ReadOnly);
connect(&freePipe, SIGNAL(readyRead()), this, SLOT(parseResult()) );
// OR waitForReadyRead() and parse here.
}
void parseResult(){
// Parse your stuff
freePipe.close();
pclose(in); // You can also use exit code by diving by 256.
}
}

Answer 4

tl;dr:
This occurs because your application wants to use more resources than allowed by the system-wide resource limitations. You might be able to solve it by using the command specified in [2] if you have a huge application, but it is probably caused by a programming error.
Long:
I just solved a similar problem myself. I use a QThread for the logging of exit codes of QProcesses. The QThread uses curl to connect to a FTP server uploads the log. Since I am testing the software I didn't connect the FTP server and curl_easy_perform apparently waits for a connection. As such, my resource limit was reached and I got this error. After a while my program starts complaining, which was the main indicator to figure out what was going wrong.
[..]
QProcessPrivate::createPipe: Cannot create pipe 0x7fbda8002f28: Too many open files
QProcessPrivate::createPipe: Cannot create pipe 0x7fbdb0003128: Too many open files
QProcessPrivate::createPipe: Cannot create pipe 0x7fbdb4003128: Too many open files
QProcessPrivate::createPipe: Cannot create pipe 0x7fbdb4003128: Too many open files
[...]
curl_easy_perform() failed for curl_easy_perform() failed for disk.log
[...]
I've tested this by connecting the machine to the FTP server after this error transpired. That solved my problem.
Read:
[1] https://linux.die.net/man/3/ulimit
[2] https://ss64.com/bash/ulimit.html
[3] https://bbs.archlinux.org/viewtopic.php?id=234915