No-Waiting Command for receiving input [duplicate] - c++

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C/C++: Capture characters from standard input without waiting for enter to be pressed
char ch;
think I want to get a character. I know two different ways.
1- using cin command in iostream header file.
cin >> ch;
It will wait for user to type something, then user MUST press enter to send input into ch variable.
2- getch() command in conio.h header file.
ch = _getch();
It will wait for user to type something, as soon as typing a character it will be saved in ch variable and user DOES NOT need to press enter. BTW this code will stop program and will wait for an input.
Now I want to write a command which does not need pressing enter and it does not stop program for pressing something. Just imagine I delay program for 1 seconds, if user presses something it will save it into ch variable, if not nothing, program will continue and it won't stop for pressing something. It is like a picker command, if there is something it will pick it up, if not it will continue.
Hope I'm clear. So how to do it?

Answer 1

On Windows, you can do this:
CreateFile using the filename "CONIN$", which will give you a Win32 handle to the console.
WaitForSingleObject, passing the console handle and a timeout.
If the wait succeeds, use ReadConsoleInput to determine what even happened.
If the wait fails, the timeout occurred.
If you're just polling in a loop performing some other action, then you can use PeekConsoleInput, which checks to see if any events are in the input queue, and always returns immediately.
On Unix, a similar approach will work. Just note that:
It's not necessary to open a file, since stdin is always file descriptor 0.
Use select or poll to test for activity on the input with a timeout.
Final difference: On Windows, mouse activity is captured the same way. On Unix, you'll usually be reading keyboard input from a tty, and mouse stuff is totally separate.

Answer 2

First we need a function to turn on and off nonblocking input:
void nonblock(const bool state){
struct termios ttystate;
//get the terminal state
tcgetattr(STDIN_FILENO, &ttystate);
if (state){
//turn off canonical mode
ttystate.c_lflag &= ~ICANON;
//minimum of number input read.
ttystate.c_cc[VMIN] = 1;
}
else{
//turn on canonical mode
ttystate.c_lflag |= ICANON;
}
//set the terminal attributes.
tcsetattr(STDIN_FILENO, TCSANOW, &ttystate);
}
Now we need a function to test and see if a key was pressed:
int keypress(void){
struct timeval tv;
fd_set fds;
tv.tv_sec = 0;
tv.tv_usec = 0;
FD_ZERO(&fds);
FD_SET(STDIN_FILENO, &fds);
select(STDIN_FILENO+1, &fds, NULL, NULL, &tv);
return FD_ISSET(STDIN_FILENO, &fds);
}
We are going to be checking for two things in parallel. Has the user pressed a key, or has time run out?
Here is a function to change a boolean value after a specified number of seconds:
void SleepForNumberOfSeconds(const int & numberofSeconds,bool & timesUp){
timespec delay = {numberofSeconds,0};
timespec delayrem;
nanosleep(&delay, &delayrem);
timesUp = true;
return;
}
Here is the main function you will be able to call:
void WaitForTimeoutOrInterrupt(int const& numberofSeconds){
bool timesUp = false;
std::thread t(SleepForNumberOfSeconds, numberofSeconds, std::ref(timesUp));
nonblock(1);
while (!timesUp && !keypress()){
}
if (t.joinable()){
t.detach();
}
nonblock(0);
return;
}
Here is the code to test out.
compiled with:
g++ -std=c++0x -o rand rand.cpp -lpthread
on:
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
This is just one solution, and it may not work for you.
Consider looking into ncurses as well.