I am writing C++ class (SerialPort) in order to use it in 2 threads (one object for each with shared descriptor). I want to send data in first thread and receive data in second. It works as expected. But I want to do it right. And I want my app resist heavy trafic (in the cases Linux output buffer is full very often). So I use pselect in SerialPort::receive() and it works as expected. But when I use it in SerialPort::send() it does not block when output buffer is full!!! I am able to detect this situation with ioctl, but I am unable to wait for some free space in this buffer. So active waiting loop is happen witch simply hangs the app!!! Note: I wrote about Linux serial port output buffer, not my app serial output buffer (I alocate 1MB for it, and it works as expected). Bellow I paste functions SerialPort::receive() and SerialPort::send() and pease give me some hints how to avoid active waiting loop in cases serial port otuput buffer is full...
thanks in advance and best regards Jacek
ps1: Here is my code (error messages are in polish due to previous developers customs - it is not mine decision, it is just required by translation tools in order to keep translation process consistent):
void SerialPort::receive()
{
// http://www.linuxprogrammingblog.com/code-examples/using-pselect-to-avoid-a-signal-race
// https://www.cmrr.umn.edu/~strupp/serial.html
if(mPortDescriptor < 0)
throw Exception(tr("Zły deskryptor pliku: %1").arg(mPortDescriptor).toUtf8().data());
struct sigaction lSigAction;
memset (&lSigAction, 0, sizeof(lSigAction));
lSigAction.sa_handler = gSignTermHandler;
// This server should shut down on SIGTERM.
if(sigaction(SIGTERM, &lSigAction, 0))
throw Exception(tr("Błąd sigaction! %1, %2").arg(errno).arg(strerror(errno)));
sigset_t lMask;
sigemptyset (&lMask);
sigaddset (&lMask, SIGTERM);
sigset_t orig_mask;
if(sigprocmask(SIG_BLOCK, &lMask, &orig_mask) < 0)
throw Exception(tr("Błąd sigprocmask! %1, %2").arg(errno).arg(strerror(errno)));
struct timespec timeout;
timeout.tv_sec = 1;
timeout.tv_nsec = 0;
fd_set lFileDescriptorSet;
/* BANG! we can get SIGTERM at this point, but it will be
* delivered while we are in pselect(), because now
* we block SIGTERM.
*/
FD_ZERO(&lFileDescriptorSet);
FD_SET(mPortDescriptor, &lFileDescriptorSet);
while(true)
{
int lRes = pselect(mPortDescriptor + 1, &lFileDescriptorSet, NULL, NULL, &timeout, &orig_mask);
if(mEnd)
return;
if((lRes < 0) && (errno != EINTR))
throw Exception(tr("Bład pselect! %1, %2").arg(errno).arg(strerror(errno)));
if(lRes == 0) // timeout
continue;
if(gExitRequest)
throw Exception(tr("Przechwycono sygnał SIGTERM! Koniec odbierania danych."));
if(!FD_ISSET(mPortDescriptor, &lFileDescriptorSet))
continue;
int lSize = 0;
ioctl(mPortDescriptor, FIONREAD, &lSize);
if(lSize <= 0)
continue;
QByteArray lResult;
lResult.resize(lSize);
int lReaded(0), lPartial(0);
while(lReaded < lSize)
{
if(mEnd)
return;
lPartial = ::read(mPortDescriptor, lResult.data(), lSize);
if(lPartial > 0)
lReaded += lPartial;
else if(lPartial < 0)
{
qCritical() << tr("Uwaga! Błąd czytania portu szeregowego! Kod błędu: %1, wiadomość: %2").arg(errno).arg(strerror(errno));
break;
}
}
emit read(lResult);
}
}
void SerialPort::send()
{
// http://www.linuxprogrammingblog.com/code-examples/using-pselect-to-avoid-a-signal-race
// https://www.cmrr.umn.edu/~strupp/serial.html
if(mPortDescriptor < 0)
throw Exception(tr("Zły deskryptor pliku: %1").arg(mPortDescriptor).toUtf8().data());
struct sigaction lSigAction;
memset (&lSigAction, 0, sizeof(lSigAction));
lSigAction.sa_handler = gSignTermHandler;
// This server should shut down on SIGTERM.
if(sigaction(SIGTERM, &lSigAction, 0))
throw Exception(tr("Błąd sigaction! %1, %2").arg(errno).arg(strerror(errno)));
sigset_t lMask;
sigemptyset(&lMask);
sigaddset(&lMask, SIGTERM);
sigset_t orig_mask;
if(sigprocmask(SIG_BLOCK, &lMask, &orig_mask) < 0)
throw Exception(tr("Błąd sigprocmask! %1, %2").arg(errno).arg(strerror(errno)));
struct timespec timeout;
timeout.tv_sec = 1;
timeout.tv_nsec = 0;
fd_set lFileDescriptorSet;
/* BANG! we can get SIGTERM at this point, but it will be
* delivered while we are in pselect(), because now
* we block SIGTERM.
*/
FD_ZERO(&lFileDescriptorSet);
FD_SET(mPortDescriptor, &lFileDescriptorSet);
int lNumberToAquire(0);
while(true)
{
lNumberToAquire = 0;
if(mBufferEnd > mBufferStart)
lNumberToAquire = mBufferEnd - mBufferStart;
else if(mBufferEnd < mBufferStart)
lNumberToAquire = mSendBuffer.size() - mBufferStart;
if(lNumberToAquire <= 0)
lNumberToAquire = 1;
int lFreeSpaceSize = 0;
ioctl(mPortDescriptor, TIOCOUTQ, &lFreeSpaceSize);
if(lFreeSpaceSize <= 0) // we must wait for serial port
{
int lRes = pselect(mPortDescriptor + 1, NULL, &lFileDescriptorSet, NULL, &timeout, &orig_mask);
if(mEnd)
return;
if((lRes < 0) && (errno != EINTR))
throw Exception(tr("Bład pselect! Kod błedu: %1, komunikat: %2").arg(errno).arg(strerror(errno)));
if(lRes == 0) // timeout
continue;
if(gExitRequest)
throw Exception(tr("Przechwycono sygnał SIGTERM! Koniec wysyłania danych."));
if(!FD_ISSET(mPortDescriptor, &lFileDescriptorSet))
continue;
}
ioctl(mPortDescriptor, TIOCOUTQ, &lFreeSpaceSize);
if(lFreeSpaceSize <= 0)
{
qWarning() << tr("%1 Uwaga: Brak miejsca w bufoerze wyjściowym portu szeregowego!").arg(QTime::currentTime().toString("hh:mm:ss.z"));
continue;
}
lNumberToAquire = qMin(lNumberToAquire, lFreeSpaceSize);
//#ifdef DEBUG
// int lSemaphore(mAvailableData.available());
//#endif
if((lNumberToAquire > 0) && mAvailableData.tryAcquire(lNumberToAquire, 1000))
{
//#ifdef DEBUG
// int lSemaphore2(mAvailableData.available());
//#endif
int lWriten(0), lPartial(0);
while(lWriten < lNumberToAquire)
{
lPartial = ::write(mPortDescriptor, mSendBuffer.data() + mBufferStart, lNumberToAquire);
if(lPartial > 0)
lWriten += lPartial;
else if(lPartial < 0)
{
qCritical() << tr("Błąd zapisu do portu szeregowego!!! Kod błędu: %1, Wiadomość błędu: %2").arg(errno).arg(strerror(errno));
break;
}
}
// qDebug() << QTime::currentTime().toString("hh:mm:ss.z") << "Write to the serial port.";
mBufferStart = (mBufferStart + lNumberToAquire) % mSendBufferSize;
mFreeSpace.release(lNumberToAquire);
}
if(mEnd)
return;
}
}
EDIT:
I found solution!
I have bug with pselect usage: I do not reinitialize fd_set lFileDescriptorSet; after first usage. It need to be reinitialized before each call:
FD_ZERO(&lFileDescriptorSet);
FD_SET(mPortDescriptor, &lFileDescriptorSet);
According to this site: [How can I determine the amount of write/output buffer space left on a linux serial port? Bogdan-Stefan Mirea wrote:
The serial port is a character device not a block device. It has no buffer
But some other sources claims Linux has 4KB serial port buffer though. Follow his advice I worte function with loop with pselect call (waiting for serial port write readines), and write only 1 byte per iteration. And it works!!!
