How to recieve data on serial port synchronously? - c

Question

I have the following set up: a computer that runs Linux (TCL) and device that sends packets of fixed length (10 bytes) with fixed frequency (100 Hz).
On computer, i have that serial port open, and i am trying to read incoming data. However, instead of recieving it in 10-byte chunks all the time, lesser amounts of data are being read, and i am forced to reassemble them back.
My goal is to send a responce for each packet once it arrives, preserving intervals between packets as much as possible.
This is how i open the serial port:
int fd = open(device_name, O_RDWR | O_NOCTTY | O_NONBLOCK);
if(fd == -1)
{
LOGPRINT("Failed to open terminal file.\nError #%d (%s)\n",
errno, strerror(errno));
return fd;
}
LOGPRINT("Setting terminal attributes\n");
struct termios config;
struct termios *pterm = &config;
// set baud rate
LOGPRINT("Setting baud rate to %d.\n", baud_rates[baud_rate_index]);
// change to raw mode
LOGPRINT("Setting terminal to raw mode\n");
pterm->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR
| IGNCR | ICRNL | IXON);
pterm->c_oflag &= OPOST;
pterm->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
cfsetispeed(pterm, baud_bits[pconfig->baud_rate_index]);
cfsetospeed(pterm, baud_bits[pconfig->baud_rate_index]);
LOGPRINT("Set character size, parity and stop bit flags.\n");
// set character size
pterm->c_cflag &= ~CSIZE;
pterm->c_cflag |= csize_flag;
// set parity
pterm->c_cflag &= ~(PARENB | PARODD);
pterm->c_cflag |= parity_flag;
// set stopbits
pterm->c_cflag &= ~CSTOPB;
pterm->c_cflag |= stopbits_flag;
// enable reading; ignore control lines
pterm->c_cflag |= CREAD | CLOCAL;
// disable flow control
pterm->c_cc[VMIN] = 1;
pterm->c_cc[VTIME] = 0;
LOGPRINT("Flush terminal.\n");
// flush terminal
tcflush(fd, TCIOFLUSH);
LOGPRINT("Apply parameters.\n");
return WRAPSYSCALL(tcsetattr(fd, TCSANOW, pterm));
Additionally i set the following on file descriptor with fnctl:
int flags = fcntl(fd, F_GETFL);
fcntl(fd, F_SETFL, flags | FASYNC | O_DIRECT);
I am reading from file descriptor in a cycle with select() call on file descriptor set containing fd, then reading all bytes avaiable (requesting number of bytes much bigger then 10).
What should i change to ensure incoming data are processed as they come properly, in timely manner?

Answer 1

If your program does nothing else than waiting for bytes and answering, then you can try using blocking file operations. Remove O_NONBLOCK flag from your open function call, and after setting the port just do a read(fd, buffer, 10). The function will return after it reads exactly 10 bytes (or an error happens or signal arrives).
However, bear in mind that you read operation can start in the middle of packet transmission, so for example you can get last x bytes of n-th packet and first 10-x of packet n+1.