Non-blocking console input C++ - c++

Question

I'm looking for a (multiplatform) way to do non-blocking console input for my C++ program, so I can handle user commands while the program continually runs. The program will also be outputting information at the same time.
What's the best/easiest way to do this? I have no problem using external libraries like boost, as long as they use a permissive license.

Answer 1

Example using C++11:
#include <iostream>
#include <future>
#include <thread>
#include <chrono>
static std::string getAnswer()
{
std::string answer;
std::cin >> answer;
return answer;
}
int main()
{
std::chrono::seconds timeout(5);
std::cout << "Do you even lift?" << std::endl << std::flush;
std::string answer = "maybe"; //default to maybe
std::future<std::string> future = std::async(getAnswer);
if (future.wait_for(timeout) == std::future_status::ready)
answer = future.get();
std::cout << "the answer was: " << answer << std::endl;
exit(0);
}
online compiler: https://rextester.com/GLAZ31262

Answer 2

I would do this by creating separate a thread which calls normal blocking IO functions and pass it a callback function which it would call when it got input. Are you sure you need to do what you said you want to do?
As for outputting information at the same time, what would happen if the user was in the middle of typing some input and you printed something?

Answer 3

I've done this on QNX4.5 that doesn't support threads or Boost by using select. You basically pass select STDIN as the file descriptor to use and select will return when a new line is entered. I've added a simplified example loop below. It's platform independent, at least for Unix like systems. Not sure about Windows though.
while (!g_quit)
{
//we want to receive data from stdin so add these file
//descriptors to the file descriptor set. These also have to be reset
//within the loop since select modifies the sets.
FD_ZERO(&read_fds);
FD_SET(STDIN_FILENO, &read_fds);
result = select(sfd + 1, &read_fds, NULL, NULL, NULL);
if (result == -1 && errno != EINTR)
{
cerr << "Error in select: " << strerror(errno) << "\n";
break;
}
else if (result == -1 && errno == EINTR)
{
//we've received and interrupt - handle this
....
}
else
{
if (FD_ISSET(STDIN_FILENO, &read_fds))
{
process_cmd(sfd);
}
}
}

Answer 4

There is one easy way:
char buffer[512];
int point = 0;
...
while (_kbhit()) {
char cur = _getch();
if (point > 511) point = 511;
std::cout << cur;
if (cur != 13) buffer[point++] = cur;
else{
buffer[point] = '\0';
point = 0;
//Run(buffer);
}
}
No block, all in 1 thread. As for me, this works.

Answer 5

Non-blocking console input C++ ?
Ans: do console IO on a background thread and provide a means of communicating between threads.
Here's a complete (but simplistic) test program that implements async io by deferring the io to a background thread.
the program will wait for you to enter strings (terminate with newline) on the console and then perform a 10-second operation with that string.
you can enter another string while the operation is in progress.
enter 'quit' to get the program to stop on the next cycle.
#include <iostream>
#include <memory>
#include <string>
#include <future>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <deque>
int main()
{
std::mutex m;
std::condition_variable cv;
std::string new_string;
bool error = false;
auto io_thread = std::thread([&]{
std::string s;
while(!error && std::getline(std::cin, s, '\n'))
{
auto lock = std::unique_lock<std::mutex>(m);
new_string = std::move(s);
if (new_string == "quit") {
error = true;
}
lock.unlock();
cv.notify_all();
}
auto lock = std::unique_lock<std::mutex>(m);
error = true;
lock.unlock();
cv.notify_all();
});
auto current_string = std::string();
for ( ;; )
{
auto lock = std::unique_lock<std::mutex>(m);
cv.wait(lock, [&] { return error || (current_string != new_string); });
if (error)
{
break;
}
current_string = new_string;
lock.unlock();
// now use the string that arrived from our non-blocking stream
std::cout << "new string: " << current_string;
std::cout.flush();
for (int i = 0 ; i < 10 ; ++i) {
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << " " << i;
std::cout.flush();
}
std::cout << ". done. next?\n";
std::cout.flush();
}
io_thread.join();
return 0;
}
sample test run:
$ ./async.cpp
first
new string: first 0 1las 2t 3
4 5 6 7 8 9. done. next?
new string: last 0 1 2 3 4 5 6 7 8quit 9. done. next?

Answer 6

ncurses can be a good candidate.

Answer 7

The StdinDataIO class of the BSD-licensed MUSCLE networking library supports non-blocking reads from stdin under Windows, MacOS/X, and Linux/Unix ... you could use that (or just examine the code as an example of how it can be done) if you want.

Answer 8

You can use the tinycon library to do this. Just spawn a tinycon object in a new thread, and you are pretty much done. You can define the trigger method to fire off whatever you'd like when enter is pressed.
You can find it here:
https://sourceforge.net/projects/tinycon/
Also, the license is BSD, so it will be the most permissive for your needs.

Answer 9

libuv is a cross-platform C library for asynchronous I/O. It uses an event loop to do things like read from standard input without blocking the thread. libuv is what powers Node.JS and others.

Answer 10

In a sense, this answer is incomplete. But yet, I think it can be useful even for people who have different platforms or circumstances, giving the idea, what to look for in their platform.
As I just wrote some scripting engine integration into an SDL2 main event loop (which is supposed to read lines from stdin if there are lines to be read), here is how I did it (on linux (debian bullseye 64 bit)). See below.
But even if you are not on linux, but on some other posix system, you can use the equivalent platform APIs of your platform. For example, you can use kqueue on FreeBSD. Or you can consider using libevent for a bit more portable approach (still will not really work on Windows).
This approach might also work on Windows if you do some special fiddling with the rather new-ish ConPTY. In traditional windows console applications, the problem is, that stdin is not a real file handle and as such, passing it to libevent or using IOCP (IO completion ports) on it will not work as expected.
But, this approach should also work on posix systems, if there is redirection at play. As long as there is a file handle available.
So how does it work?
Use epoll_wait() to detect if there is data available on stdin. While consoles can be configured in all sorts of ways, typically, they operate on a line by line basis (should also apply for ssh etc.).
Use your favorite getline() function to read the line from stdin. Which will work, because you know, there is data and it will not block (unless your console is not defaulting to line by line handling).
Rince and repeat.
#include <unistd.h>
#include <sys/epoll.h>
#include <iostream>
#include <string>
#include <array>
using EpollEvent_t = struct epoll_event;
int main(int argc, const char* argv[]) {
//
// create epoll instance
//
int epollfd = epoll_create1(0);
if (epollfd < 0) {
std::cout << "epoll_create1(0) failed!" << std::endl;
return -1;
}
//
// associate stdin with epoll
//
EpollEvent_t ev;
ev.data.ptr = nullptr;
ev.data.fd = STDIN_FILENO; // from unistd.h
ev.data.u32 = UINT32_C(0);
ev.data.u64 = UINT64_C(0);
ev.events = EPOLLIN;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, STDIN_FILENO, &ev) < 0) {
std::cout
<< "epoll_ctl(epollfd, EPOLL_CTL_ADD, fdin, &ev) failed."
<< std::endl;
return -1;
}
//
// do non-blocking line processing in your free running
// main loop
//
std::array<EpollEvent_t,1> events;
bool running = true;
while (running) {
int waitret = epoll_wait(epollfd,
events.data(),
events.size(),
0); // 0 is the "timeout" we want
if (waitret < 0) {
std::cout << "epoll_wait() failed." << std::endl;
running = false;
}
if (0 < waitret) { // there is data on stdin!
std::string line;
std::getline(std::cin, line);
std::cout
<< "line read: [" << line << "]" << std::endl;
if (line == "quit")
running = false;
}
// ... Do what you usually do in your main loop ...
}
//
// cleanup of epoll etc.
//
close(epollfd);
return 0;
}

Answer 11

You could do:
#include <thread>
#include <chrono>
#include <string>
#include <iostream>
int main() {
std::cout << "Type exit to quit." << std::endl;
// initialize other std::thread handlers here
std::string input;
while (input != "exit") {
std::getline(std::cin, input);
std::this_thread::sleep_for(std::chrono::milliseconds(500));
}
std::cout << "Cleaning up and quitting" << std::endl;
return 0;
};

Answer 12

A simple answer with thread/future and reading a single char at a time (you can replace getchar with cin as required)
Timeout is set to zero and a new future is created every time the previous call is completed.
Like cin, getchar requires that the user hits the RETURN key to end the function call.
#include <chrono>
#include <cstdio>
#include <future>
#include <iostream>
#include <thread>
static char get_usr_in()
{
return std::getchar();
}
int main()
{
std::chrono::seconds timeout(0);
std::future<char> future = std::async(std::launch::async, get_usr_in);
char ch = '!';
while(ch!='q') {
if(future.wait_for(timeout) == std::future_status::ready) {
ch = future.get();
if(ch!='q') {
future = std::async(std::launch::async, get_usr_in);
}
if(ch >= '!' && ch <'~')
std::cout << "ch:" << ch << std::endl;
}
std::cout << "." << std::endl;
}
exit(0);
}

Answer 13

Why not use promises?
#include <iostream>
#include <istream>
#include <thread>
#include <future>
#include <chrono>
void UIThread(std::chrono::duration<int> timeout) {
std::promise<bool> p;
std::thread uiWorker([&p]() {
bool running = true;
while(running) {
std::string input;
std::cin >> input;
if(input == "quit") {
p.set_value(true);
running = false;
}
}
});
auto future = p.get_future();
if (future.wait_for(timeout) != std::future_status::ready) {
std::cout << "UI thread timed out" << std::endl;
uiWorker.detach();
return;
}
uiWorker.join();
}
int main()
{
std::thread uiThread(UIThread, std::chrono::seconds(3));
std::cout << "Waiting for UI thread to complete" << std::endl;
uiThread.join();
}
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