C++ handle keypress in a multithread program in Linux - c++

Question

I'am writing a program which capturing videos from 4 cameras simultaneously, so I have 4 threads to control each camera. In each thread I want it to continue capturing until I hit a key and that key corresponds to 'q' or something.
For keypress handle I search the Internet and found method like this:
#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>
int kbhit(int key) {
int ch;
int old_file_status;
struct termios old_term_attr;
struct termios new_term_attr;
tcgetattr(STDIN_FILENO, &old_term_attr);
new_term_attr = old_term_attr;
new_term_attr.c_lflag &= ~(ICANON | ECHO);
tcsetattr(STDIN_FILENO, TCSANOW, &new_term_attr);
old_file_status = fcntl(STDIN_FILENO, F_GETFL, 0);
fcntl(STDIN_FILENO, F_SETFL, old_file_status | O_NONBLOCK);
ch = getchar();
tcsetattr(STDIN_FILENO, TCSANOW, &old_term_attr);
fcntl(STDIN_FILENO, F_SETFL, old_file_status);
if(ch == c)
return 1;
return 0;
}
And in my VideoCapture class I have code like this (not complete):
static void *capureVideo(void *para) {
// Some code...
while(!(kbhit('q') {
// Read frame...
}
}
void creatThread() {
if (pthread_create(&threadID, NULL, capureVideo, this) != 0) {
perror("thread create faild");
exit(EXIT_FAILURE);
}
}
When the program runs, once I hit the key 'q' 4 times the program did quits and control is given back to the shell. But in some certain circumstances (I don't exactly know, it doesn't happen every time) it lead to a problem, that is when I then go and type commands into the shell, the characters I type don't show up. When I press enter the commands are submitted.
I search this problem and found this: https://askubuntu.com/a/172747, which indicates that my terminal attributes were not reset properly. But in the keypress handle code I noticed that this two lines of code
tcsetattr(STDIN_FILENO, TCSANOW, &old_term_attr);
fcntl(STDIN_FILENO, F_SETFL, old_file_status);
did reset the terminal attributes. So I wonder if it's relevant to the multithread. I'm new to mutlithread programming and couldn't solve it myself, so could someone help me? Any suggestions are highly appreciated.

Answer 1

You have multiple threads trying to change parameters of the (static) terminal at the same time:
tcgetattr(STDIN_FILENO, &old_term_attr);
new_term_attr = old_term_attr;
new_term_attr.c_lflag &= ~(ICANON | ECHO);
tcsetattr(STDIN_FILENO, TCSANOW, &new_term_attr);
Without this being locked, your terminal attributes are completely random.
The fixes are to either protect this with a mutex, or if you spawn another thread to read the keyboard and set a flag for when 'q' has been pressed; which your other threads can read, you can do something like
(pardon the psudo code)
bool shouldRun = true
void captureThreadMain {
while (shouldRun) {
captureFrame();
}
}
void keyboardPressMain {
while (getKey('q'));
shouldRun = false;
}
This will mean that you only need to press 'q' once to stop all your frame collecting threads.