How to detect USB disconnect while blocking reading? - c

Question

I am quite new to this stuff, so be aware. I want to read a single char from an arduino. Therefore I have enabled the serial communication on the arduino with a baud rate of 9600 (more than enough for my needs). This following code works - sometimes. Sometimes I read the correct data and sometimes I get some invaid characters. Also there is the problem that if the device is initialized but then the USB is disconnected (for example just pull the cable), this program does not terminate. What am I doing wrong?
int main() {
int fd = open("/dev/ArduinoDeviceName", O_RDWR | O_NOCTTY | O_NDELAY);
if(fd < 0) {
printf("Failed to connect to device!\n");
return EXIT_FAILURE;
}
struct termios SerialPortSettings;
tcgetattr(fd, &SerialPortSettings);
cfsetispeed(&SerialPortSettings,B9600);
cfsetospeed(&SerialPortSettings,B9600);
if((tcsetattr(fd,TCSANOW,&SerialPortSettings)) != 0) {
printf("Error while setting attributes \n");
return EXIT_FAILURE;
}
printf("Init done!\n");
printf("read: _%c_", read_next_char(fd));
close(fd);
return EXIT_SUCCESS;
}
EDIT 1:
Now with some additional settings I could achieve to read the correct data values, see code below:
char read_next_char(int fd) {
char buffer[1] = {0};
fcntl(fd, F_SETFL);
switch (read(fd, &buffer, sizeof(buffer))) {
case 1: printf("Read bytes correct!\n"); break;
case 0: printf("Reached EOF\n"); break;
case -1: printf("Error occured while reading!\n"); break;
default: break;
};
return buffer[0];
}
int main() {
int fd = open("/dev/tty.usbmodem14101", O_RDWR | O_NOCTTY | O_NDELAY);
if(fd < 0) {
printf("Failed to connect to device!\n");
return EXIT_FAILURE;
}
struct termios SerialPortSettings;
tcgetattr(fd, &SerialPortSettings);
cfsetispeed(&SerialPortSettings,B9600);
cfsetospeed(&SerialPortSettings,B9600);
SerialPortSettings.c_iflag=0;
SerialPortSettings.c_oflag=0;
SerialPortSettings.c_cflag=CS8|CREAD|CLOCAL;
SerialPortSettings.c_lflag=0;
SerialPortSettings.c_cc[VMIN]=1;
SerialPortSettings.c_cc[VTIME]=5;
if((tcsetattr(fd,TCSANOW,&SerialPortSettings)) != 0) {// Set the attributes to the termios structure
printf("Error while setting attributes \n");
return EXIT_FAILURE;
}
printf("Init done!\n");
char read;
while((read = read_next_char(fd)) != 0) {
printf("Correctly read: _%c_", read);
}
close(fd);
return EXIT_SUCCESS;
}
The only thing that sometimes can be annoying is, that the arduino sends some data via serial in a loop with some delay. If the computer connects there are always some data sent, before the arduino resets. How can I achieve that the arduino / serial port gets reset before I read any data?