Possible to retrieve input from user and running another process? - c++

Question

Is it possible to use getline(cin,buffer); at the top of my program, then have a "animated menu" still running below it?
For example (very basic):
#include <iostream>
#include <string>
using namespace std;
int stringLen=0;
string buffer;
getline(cin, buffer);
for (int i = 0; i < kMaxWait;i++)
{
printf("counter waiting for user input %d",i);
if (1 >= buffer.length())
break;
}
Would I have to fork that loop somehow so it would keep counting and display the counter until the user enters something??

Answer 1

One possible answer, given in the comments, is to use threads. But it's not necessary, there's a way to do this without threads.
Make stdin a non-blocking file descriptor.
Wait for stdin to become readable, via poll()/select(), in the meantime do your animation, etc...
Make stdin a blocking file descriptor, again.
Use std::getline().
There are also some ancillary issues to consider, such as the buffering that comes from std::streambuf, so before doing all that, check if there's already something to read from std::cin, first.

Answer 2

This is something I used sometime ago. It's quite rudimentary, but you can get the gist of the process - using poll. It returns true if there is input, and puts it in str, false otherwise. So, you can put this in your loop somewhere, and take action when there is input.
bool polled_input(std::string& str)
{
struct pollfd fd_user_in;
fd_user_in.fd = STDIN_FILENO;
fd_user_in.events = POLLIN;
fd_user_in.revents = 0;
int rv = poll(&fd_user_in, 1, 0);
if (rv == -1) {/* error */}
else if (rv == 0) return false;
else if (fd_user_in.revents & POLLIN)
{
char buffer[MAX_BUFF_SIZE];
int rc = read(STDIN_FILENO, buffer, MAX_BUFF_SIZE-1);
if (rc >= 0)
{
buffer[rc]='\0';
str = std::string(buffer);
return true;
}
else {/* error */}
}
else {/* error */}
}

Answer 3

select is meant for this, multiplexed, blocking I/O. It can be done without a poll I think:
#include <iostream>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char **arg)
{
const int time_in_secs = 10;
const int buffer_size = 1024;
fd_set readfds;
FD_ZERO(&readfds);
FD_SET(STDIN_FILENO, &readfds);
struct timeval tv;
tv.tv_sec = time_in_secs;
tv.tv_usec = 0;
int ret = select(STDIN_FILENO + 1, &readfds, NULL, NULL, &tv);
if (!ret)
{
std::cout << "Timeout\n";
exit(1);
}
char buf[buffer_size];
if (FD_ISSET(STDIN_FILENO, &readfds))
{
int len = read(STDIN_FILENO, buf, buffer_size);
buf[len] = '\0';
}
std::cout << "You typed: " << buf << "\n";
return 0;
}