How do you poll data from a device in C++ without blocking? - c++

Question

I have a device connected via USB, which is outputting a stream of characters separated by a newline, every few seconds. I want to repeatedly poll this device for input without blocking on the event that no such input exists.
I am trying to use the poll() function as recommended on other questions, however, it is not working, and I am having trouble determining where I am going wrong.
My device is at /dev/TTYACM0 and when I run cat /dev/TTYACM) I can see the stream of data.
This is my current program, however it just causes a segmentation fault:
#include <iostream>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <poll.h>
#include <unistd.h>
int main (int argc, char* argv[]){
char buf[1024];
int fd = open("/dev/ttyACM0", O_RDONLY | O_NONBLOCK);
int ret;
if (fd < 0){
std::cout << "ERROR" << fd << std::endl;
}
int i;
struct pollfd *pfds;
while(1){
pfds->fd = 0;
pfds->events = POLLIN;
ret = poll(pfds, 1, 0);
if (ret > 0) {
if (pfds->revents & POLLWRBAND)
{
i = read(0, buf, 1024);
if (!i){
printf("stdin closed\n");
return 0;
}
std::cout << i << std::endl;
}
}
}
return 0;
}
UPDATE:
I fixed the code, here is the working version
#include <poll.h>
#include <iostream>
#include <cstring>
#include <fcntl.h>
#include <unistd.h>
int main(){
char readBuf[5];
char c;
std::string buf;
int counter = 0;
int ret;
int i;
struct pollfd mypoll;
int fd = open("/dev/ttyACM0", O_RDONLY | O_NONBLOCK);
//init mypoll
memset(&mypoll, 0, sizeof(mypoll));
mypoll.fd = fd;
mypoll.events = POLLIN;
while(1){
// poll (pointer to pollfd, number of polls, timeout)
// noneg = successful poll
// 0 = systemcall timeout (no data)
// -1, errno set to indicate error
ret = poll(&mypoll, 1, 0);
if (ret == 1){
int bytes_read = read(fd, readBuf, sizeof(readBuf));
for (i = 0; readBuf[i] != '\n' && i < bytes_read; i++){
buf.push_back(readBuf[i]);
}
std::cout << "#" << buf << "#" << std::endl;
buf = "";
}
}
printf("done\n");
close(fd);
return 0;
}