Epoll reads wrong bytes - c

Question

I'm newbie in linux system programming, so be kind please.
I have to read data from serial port (/dev/ttyX) with baudrate 921600 in 8N1 mode with no parity via RS-422.
stty output signals that this baudrate is supported.
So I've decided to call epoll.
Problem is that epoll returns wrong bytes.
Since I have specified messages format I'm trying to debug by verifying incoming data by hands. So I've drawn this:
All messages have 2 bytes crc on the tail.
11 b5 means message start. Message should be 36 bytes in length.
72 b5 is another message start marker. 112 bytes len.
73 b5 is message marker too. 36 bytes.
Please find blue underline: this is good message.
Tiny red + fat red is a bad one.
It is 37 bytes len. I have one extra byte and crc mismatch.
Next good one goes (green).
And next bad one. It is 114 bytes instead of 112 bytes and crc mismatch of course.
Here is my code:
... All necessary includes
#define SERIAL_BAUD 921600
#define MAXEVENTS 1024
int openSerial()
{
struct termios options;
int fd;
if ((fd = open("/dev/ttyUSB0", O_RDWR)) == -1)
{
return -1;
}
if (tcgetattr(fd, &options) < 0)
{
printf("Unable to get options with tcgetattr\n");
return -1;
}
if (cfsetispeed(&options, SERIAL_BAUD) < 0)
{
printf("Unable to set input speed with cfsetispeed\n");
return -1;
}
if (cfsetospeed(&options, SERIAL_BAUD) < 0)
{
printf("Unable to set output speed with cfsetispeed\n");
return -1;
}
cfmakeraw(&options);
//options.c_cflag |= SERIAL_BAUD; // Set Baudrate first time
options.c_cflag |= (CLOCAL | CREAD);
options.c_cflag &= ~CRTSCTS;
options.c_cflag &= ~ECHO; // Disable echoing of input characters
options.c_cflag &= ~ECHOE;
// set to 8N1
options.c_cflag &= ~PARENB; // no parity
options.c_cflag &= ~CSTOPB; // 1 stop bit
options.c_cflag &= ~CSIZE; // Mask the character size bits
options.c_cflag |= CS8; // 8 data bits
options.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
options.c_oflag = 0;
options.c_cc[VTIME] = 2; // inter-character timer
options.c_cc[VMIN] = 1; // blocking read until
if (tcflush(fd, TCIFLUSH) < 0)
{
printf("Unable to flush fd with tcflush\n");
return -1;
}
if (tcsetattr(fd, TCSANOW, &options) != 0)
{
printf("Unable to set options with tcsetattr\n");
return -1;
}
return fd;
}
int main(void)
{
int fd;
int efd;
struct epoll_event event;
struct epoll_event* events;
int length;
unsigned char buff[512];
if ((fd = openSerial()) < 0)
{
printf("Exiting because of openSerial failure\n");
return 1;
}
efd = epoll_create1(0);
event.data.fd = fd;
event.events = EPOLLIN;
if (epoll_ctl(efd, EPOLL_CTL_ADD, fd, &event) < 0)
{
printf("Epoll_ctl error occured\n");
return 1;
}
events = (epoll_event*) calloc(MAXEVENTS, sizeof(event));
for(;;)
{
int n = epoll_wait(efd, events, MAXEVENTS, 5000);
if (n < 0)
{
// No ready descriptors, so wait a bit longer
continue;
}
if(events[0].events & EPOLLIN)
{
length = read(events[0].data.fd, buff, sizeof(buff) / 2);
if(length > 0)
{
printf("\n------MESSAGE START-------\n");
for (int i = 0 ; i < length ; ++i)
{
if (i && i % 16 == 0)
{
printf("\n");
}
printf("%02x ", buff[i]);
}
printf("\n------MESSAGE FINISH-------\n");
}
}
else if(events[0].events & EPOLLOUT)
{
// TODO Write here to serial
}
else if(events[0].events & EPOLLHUP || events[0].events & EPOLLERR)
{
printf("Error occured on serial port\n");
}
else
{
printf("No data whthin 5 seconds.\n");
}
}
free(events);
close(fd);
return 0;
}

Answer 1

Your problem is that read() returns bytes that fail to meet your expectations. I see no reason to believe that epoll has anything to do with that.
I also see no reason to suppose that read() is delivering different bytes than the system received from device /dev/ttyUSB0. If these are different than you expected then I'm inclined to believe that either the device is malfunctioning or your expectations are incorrect (or at least incomplete).