Resource temporarily unavailable - qt

Question

Consider this thread, it acts like a timer, send some packet to serial:
void PlCThead::run()
{
while(1)
{
const char str[]={UPDATE_PACKET};
QByteArray built;
built.append(0x02);
built.append(0x05);
built.append(0x03);
emit requestForWriteAndReceive(built);
msleep(100);
}
}
emit works fine, it goes inside the slot, there, it writes only 78 or char x to serial instead of a packet of 3 bytes.
bool RS::rs_ThreadPlcDataAqustn(QByteArray byteArray)
{
QByteArray rd15Bytes;
char *data = byteArray.data();
int len = byteArray.length();
if(!rs_serialWrite(data, len))
{
qDebug() << "Failure:( rs_dataqustn: rs_plcWrite(data, len)";
emit plc_port_dscntd();
return false;
}
}
bool RS::rs_serialWrite(char* buff, size_t length)
{
int tries;
int len;
tries = 0;
QByteArray built((char*)buff, length);
qDebug() << built.toHex();
len = write(fd, buff, length);
qDebug() << len;
qDebug() << strerror(errno);
return true;
}
this is how fd created:
fd = open(portPath, O_RDWR | O_NOCTTY | O_NDELAY | O_NONBLOCK, S_IWUSR | S_IRUSR | S_IXUSR);
this is how the thread created in mainwindow:
rs_plc->rs_plcOpenPort((char *)"/dev/ttyS0"); /*/dev/ttyS3*/
PlCThead *thread = new PlCThead();
connect(thread, SIGNAL(requestForWriteAndReceive(QByteArray)), rs_plc, SLOT(rs_ThreadPlcDataAqustn(QByteArray )));
thread->start();
rs_plc is a private member of MainWindow.
strerror returns back this warning:
> Resource temporarily unavailable
any ideas? this code works fine with timers, it has been checked and tested accurately, but now i need to add this thread instead of the timer. Thanks

Answer 1

Your question isn't complete enough for a full diagnosis since you aren't showing how fd is created, how the threads are set up (which you say is part of the problem), etc.
But... your resource temporarily unavailable line is a big hint. The write() function isn't succeeding to write everything because it's returning an error (probably EAGAIN or EWOULDBLOCK). The fd file descriptor is attached to something that either has a small buffer, no buffer, or a buffer which is already full. And it's full, and it's the applications job to not send it anything else until it can handle it. A common thing to do there is to sleep and then try the write again if the error code is EAGAIN or EWOULDBLOCK.
But, you said it's returning 3, which actually indicates "no error" too. And if that's the case then the error string won't be referring to write itself, and something else set errno previously. (which could have been write itself in the past).
In short, if this is getting called more than once (likely) you probably need to watch out for writing too fast (and it looks like a serial buffer, which definitely falls into the category of easy-to-fill-the-buffer).
In short: if it's not writing all the bytes to the fd than you want, it's because it can't handle more than that.
This likely has absolutely nothing to do with qt by the way. It's all about the write() call.