Reading from serial port fails - linux

Question

I have the following C program:
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
int main()
{
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);
if(fd < 0)
{
perror("Could not open device");
}
printf("Device opened\n");
struct termios options;
tcgetattr(fd, &options);
cfmakeraw(&options);
cfsetispeed(&options, B19200);
cfsetospeed(&options, B19200);
tcsetattr(fd, TCSANOW, &options);
char txpacket[] = {0x23, 0x06, 0x00, 0x00, 0xdd, 0xf9};
ssize_t written = write(fd, txpacket, sizeof(txpacket));
printf("Written %d bytes\n", written);
printf("Starting to wait for target to respond\n");
while(1)
{
fd_set readset;
FD_ZERO(&readset);
FD_SET(fd, &readset);
int nCount = select(fd + 1, &readset, NULL, NULL, NULL);
if(nCount > 0)
{
if(FD_ISSET(fd, &readset))
{
int i;
char buffer[128];
ssize_t bytesread = read(fd, buffer, sizeof(buffer));
printf("Received %d bytes\n", bytesread);
for(i = 0; i < bytesread; i++)
{
printf(" %02x", buffer[i]);
}
}
}
}
}
This program opens the serial device /dev/ttyS0, writes a sequence of data to it and starts listening for a response. I get the following output:
Device opened
Written 6 bytes
Starting to wait for target to respond
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
...
And the application consumes 100% CPU. I'm not able to receive any data, even though the target hardware actually transmits it.
What is wrong?

Answer 1

read() returning 0 indicates the end-of-file condition. You should check for that and break out of the loop if it occurs.
As to what's causing that - end-of-file on a serial port indicates it has detected a hangup, meaning that the DCD line has been dropped.
You can set the CLOCAL flag in options.c_cflag to ignore the modem control lines, if your device doesn't set them properly.

Answer 2

You should try without the O_NONBLOCK flag. in raw mode, if the settings of c_cc[VMIN] and c_cc[VTIME] is 0, the serial port behave like this (according to man cfmakeraw) :
If data is available, read returns
immediately, with the lesser of the
number of bytes available, or the
number of bytes requested. If no data
is available, read returns 0
So what you should try is :
options->c_cc[VMIN]=1;