Read UART continuously without blocking host application - c

Question

I have been searching around and reading the documentation, but I am at the point where I need a little help. The use case is relatively simple, but the implementation is where I am stuck.
On one end is a wireless Xbee radio. This radio transmits a signal every 300ms. The payload of this signal consists of button states and joystick positions.
On the receiving end is a Linux based SBC running Ubuntu with an Xbee attached. For the sake of this post, I have combined the following into one single file and added a main for readability:
This code is based off the Serial programming Guide
#include <stdio.h>
#include <stdlib.h>
#include <linux/ioctl.h>
#include <fcntl.h>
#include <termios.h>
#include <sys/signal.h>
#include <sys/types.h>
#include <unistd.h>
#include "XbeeReceiver.h"
#define FALSE 0
#define TRUE 1
volatile int STOP = FALSE;
void signal_handler_IO(int status);
int wait_flag = TRUE;
int fileDescriptor = -1;
/* Constants */
const char *DEFAULT_PORT = "/dev/ttyTHS1";
const int DEFAULT_BAUD = B115200;
/* Prototypes */
struct N64_DTO queryController(int fd);
void init(const char *port, const int baud);
void setTermConfig(int fd, const int baud);
void signal_handler_IO(int status);
int openUART(const char *port);
void init(const char *port, const int baud) {
fileDescriptor = openUART(port ? port : DEFAULT_PORT);
setTermConfig(fileDescriptor, baud ? baud : DEFAULT_BAUD);
while (STOP == FALSE) {
if (wait_flag == FALSE) {
struct N64_DTO controller = queryController(fileDescriptor);
printf("\n\n\n Y Joystick:: %i \n", (signed char) controller.y);
printf("X Joystick:: %i \n", (signed char) controller.x);
/* I am supposed to set STOP to TRUE for this loop to stop, however I always want to read from the controller,
which is why this is now blocking */
wait_flag = TRUE;
}
}
}
struct N64_DTO queryController(int fd) {
struct N64_DTO controller;
int byteCount = read(fd, (char *) &controller, sizeof(struct N64_DTO));
if (byteCount == -1) {
printf("I'll handle this error later on");
exit(1);
}
return controller;
}
void signal_handler_IO(int status) {
printf("received SIGIO signal.\n");
wait_flag = FALSE;
}
void setTermConfig(int fd, const int baud) {
struct sigaction saio;
saio.sa_handler = signal_handler_IO;
sigemptyset(&saio.sa_mask);
sigaddset(&saio.sa_mask, SIGINT);
saio.sa_flags = 0;
saio.sa_restorer = NULL;
sigaction(SIGIO, &saio, NULL);
fcntl(fd, F_SETOWN, getpid());
fcntl(fd, F_SETFL, FASYNC);
struct termios toptions;
tcgetattr(fd, &toptions);
cfsetspeed(&toptions, baud);
/* man termios for individual definitions */
toptions.c_cflag &= ~PARENB;
toptions.c_cflag &= ~CSTOPB;
toptions.c_cflag &= ~CSIZE;
toptions.c_cflag |= CS8;
toptions.c_cflag &= ~CRTSCTS;
toptions.c_cflag |= CREAD | CLOCAL;
toptions.c_iflag &= ~(IXON | IXOFF | IXANY);
toptions.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
toptions.c_oflag &= ~OPOST;
toptions.c_cc[VMIN] = sizeof(struct N64_DTO);
toptions.c_cc[VTIME] = 0;
tcsetattr(fd, TCSANOW, &toptions);
tcflush(fd, TCIFLUSH);
}
int openUART(const char *port) {
return open(port, O_RDWR | O_NOCTTY | O_NONBLOCK);
}
int main(int argc, char *argv[]) {
int i;
init("/dev/ttyTHS1", 115200);
for (i = 0; i < 10; i++) {
printf("This loop never prints..");
}
return 0;
}
Although it needs more refactoring, this code works and gives me the expected output. However, now that I have the output, I would like to do something with it, i.e send as input to a motor controller etc. The problem is, due to the frequency of reads(300ms) the program is ultimately blocking any other routines from executing.
I went back to the drawing board a few weeks ago and started down a few different paths. I explored threading, signal handlers, leveraging poll/select API, and the possibility of creating a dedicates kernel module. Last week, for a different reason, I put the controller up on a scope and setup triggers for the rising edge of the signal. That’s when it hit me. What I want is an event driven model where the OS notifies me when a signal is detected on a particular serial port. I figured I would reach out and see if someone has experience in this area that would be willing to steer me in the right direction. I have been at this issue for a week and still have not made any progress. In the end, the goal is to continuously receive serial data from the port without blocking other parts of my applications(driving motors, moving servos).