I have create a simple application which using C++ and produce a executable file.
I would like to ensure the process cannot be start twice. How to enforce a process/service is start once only ?
Thanks.
What operating system? On Windows, the typical way to do this is to create a named Mutex, because the OS will give you an ERROR_ALREADY_EXISTS if some other process already created a mutex with that name, and the OS will ensure that the mutex is released when the process ends (even if it crashes or is killed).
Write a temporary file and use it as a lock.
Edit: To answer the comment: If you are on a Unix system, write a file /tmp/my_application_lock_file. If it already exists, stop your program with an appropriate message. On exit of the creator of the file, delete it.
#include <sys/stat.h>
#include <errno.h>
#include <unistd.h>
#include <iostream>
#include <fstream>
int main (void)
{
struct stat file_info;
// test for lock file
if (stat("/tmp/my_application_lock", &file_info) == 0) {
std::cout << "My application is already running, will abort now..." << std::endl;
return -1;
} else {
// create lock file
std::ofstream out;
out.open("/tmp/my_application_lock");
if (!out) {
std::cout << "Could not create lock file!" << std::endl;
return -1;
}
out << "locked" << std::endl;
out.close();
// do some work
std::string s;
std::cin >> s;
// remove lock file
errno = 0;
if (unlink("/tmp/my_application_lock"))
std::cout << "Error: " << strerror(errno) << std::endl;
}
return 0;
}
Related
On Linux, I am trying to detect a bluetooth controller being connected and start reading from it. I know there's SDL to do that, but I just wanted to learn how to do it specifically on Linux. So I'm using the inotify api to wait for the file /dev/input/js0 to show up. But when I detect the file I cannot open it. I have the following c++ code:
#include <iostream>
#include <fstream>
#include <sys/inotify.h>
#include <unistd.h>
#include <linux/joystick.h>
#include <string.h>
constexpr int NAME_MAX = 16;
int main(int argc, char **argv) {
std::string path = std::string(argv[1]);
std::string directory = path.substr(0, path.find_last_of("/"));
std::string file = path.substr(path.find_last_of("/") + 1);
std::cout << "Directory is " << directory << ", file is " << file << std::endl;
int fd = inotify_init();
if (inotify_add_watch(fd, directory.c_str(), IN_CREATE) < 0) {
std::cout << "Could not watch: " << file << std::endl;
return -1;
}
else
std::cout << "Watching: " << file << std::endl;
char buffer[sizeof(struct inotify_event) + NAME_MAX + 1];
while (true) {
if (read(fd, buffer, sizeof(buffer)) < 0) {
std::cout << "Error reading event" << std::endl;
break;
}
struct inotify_event &event = (struct inotify_event &) buffer;
std::cout << event.name << std::endl;
if ((strcmp(event.name, file.c_str()) == 0) && (event.mask & IN_CREATE)) {
std::cout << "File has been created" << std::endl;
close(fd);
break;
}
}
std::fstream file_stream(file, std::fstream::in);
std::cout << file_stream.is_open() << std::endl;
}
If I run it to detect a regular file, it works, it waits for the file creation event, and when trying to open it with a std::fstream, is_open returns true. But if I run it to detect /dev/input/js0, even when the event comes and the file is detected, opening the fstream does not work, as is_open returns false. Is inotify appropriate to detect device files? If not, what would be the right way to do so?
According to inotify(7)
Inotify reports only events that a user-space program triggers
through the filesystem API. As a result, it does not catch
remote events that occur on network filesystems. (Applications
must fall back to polling the filesystem to catch such events.)
Furthermore, various pseudo-filesystems such as /proc, /sys, and
/dev/pts are not monitorable with inotify.
I would say that /dev/input/ also falls into this bucket.
I wonder if udev could be used: you should get info about the device using udevinfo -a -p /dev/input/js0, but also see what events connecting the peripheral generates using udevadm monitor --environment --udev.
Edit: if you successfuly get an inotify event but can't read the file:
Did you try reading the file with another simpler program when the BT device is already connected?
Is there a difference between fstream::open and open from <cstdio>?
Have you checked the permissions on the device? Also what does cat /dev/input/js0 produces?
I am using boost::process to call an external program - the external program reads input via stdin, and writes to stdout and stderr. The external program is as follows (expects a single argument - the path to a file for debugging)
#include <fstream>
#include <iostream>
#include <stdexcept>
#include <string>
#include <vector>
int main(int argc, char** argv)
{
try
{
if (argc != 2)
{
throw std::logic_error("Expected two arguments");
}
std::ofstream ofs(argv[1]);
std::vector<std::string> someTestInput;
ofs << "Starting program..." << std::endl;
// Read from cin
{
ofs << "Reading from cin..." << std::endl;
std::string input;
while (std::getline(std::cin, input))
{
ofs << "Received from cin: " << input << std::endl;
someTestInput.emplace_back(input);
}
ofs << "Finished receiving from cin..." << std::endl;
}
// Error if nothing has been input
if (someTestInput.empty())
{
throw std::logic_error("Expected some input and received nothing...");
}
ofs << "Writing to cout..." << std::endl;
// Write to cout
for (const auto& output : someTestInput)
{
std::cout << output << '\n';
}
ofs << "Finished!\n";
}
catch (std::exception& e)
{
std::cerr << "Error caught: " << e.what() << '\n';
return 1;
}
return 0;
}
The caller expects 2+ arguments, one of which is the path to the external program, and the rest are passed on as arguments to the external program.
It hangs while waiting for the process to exit, and it seems like the external program is waiting for an EOF from stdin.
#include <memory>
#include <vector>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/process.hpp>
int main(int argc, char** argv)
{
try
{
if (argc < 2)
{
throw std::logic_error("Expecting at least 2 arguments...");
}
std::vector<std::string> args;
for (int i = 1; i < argc; ++i)
{
args.emplace_back(argv[i]);
}
std::cout << "Creating stdout, stderr pipes...\n";
// Create pipes for stdout, stderr
boost::process::pipe pstdout = boost::process::create_pipe();
boost::process::pipe pstderr = boost::process::create_pipe();
std::cout << "Mapping pipes to sources...\n";
// Map pipe source from stdout and stderr to sources
boost::iostreams::file_descriptor_source sourcestdout(pstdout.source, boost::iostreams::close_handle);
boost::iostreams::file_descriptor_source sourcestderr(pstderr.source, boost::iostreams::close_handle);
std::cout << "Setting up streams for the sources...\n";
// And set up streams for the sources
boost::iostreams::stream<boost::iostreams::file_descriptor_source> istdout(sourcestdout);
boost::iostreams::stream<boost::iostreams::file_descriptor_source> istderr(sourcestderr);
std::unique_ptr<boost::process::child> p;
// Want to check for process result, but also need to ensure stdin handle is closed properly,
// so place everything in separate scope
{
std::cout << "Mapping pipes to sinks...\n";
// Map pipe sink from stdout and stderr to sinks
boost::iostreams::file_descriptor_sink sinkstdout(pstdout.sink, boost::iostreams::close_handle);
boost::iostreams::file_descriptor_sink sinkstderr(pstderr.sink, boost::iostreams::close_handle);
std::cout << "Creating stdin pipe, mapping to source and sink...\n";
boost::process::pipe pstdin = boost::process::create_pipe();
// For stdin, map pipe to source and sink as before - want it to close on exiting this scope
boost::iostreams::file_descriptor_sink sinkstdin(pstdin.sink, boost::iostreams::close_handle);
boost::iostreams::file_descriptor_source sourcestdin(pstdin.source, boost::iostreams::close_handle);
boost::iostreams::stream<boost::iostreams::file_descriptor_sink> ostdin(sinkstdin);
std::cout << "Calling process... \n";
// Call process
p = std::unique_ptr<boost::process::child>(new boost::process::child(boost::process::execute(
boost::process::initializers::set_args(args),
boost::process::initializers::throw_on_error(),
boost::process::initializers::bind_stdout(sinkstdout),
boost::process::initializers::bind_stderr(sinkstderr),
boost::process::initializers::bind_stdin(sourcestdin)
)));
std::cout << "Sending test data...\n";
// Send some test data to cin - comment out the below to test for error case
ostdin << "Test Input 1\n";
ostdin << "Some\n";
ostdin << "Useful\n";
ostdin << "Data\n";
std::cout << "Test data sent, exiting scope...\n";
}
std::cout << "Check if process has exited...\n";
// Check if process has exited OK - if not, report errors
if (boost::process::wait_for_exit(*p))
{
std::cout << "Has not exited OK, reporting problems...\n";
// Gather output from stderr
std::string error;
while (std::getline(istderr, error))
{
std::cout << "Error: " << error << '\n';
}
throw std::logic_error("Problem executing TestProgram...");
}
std::cout << "Exited OK, here is output from the callee...\n";
// Gather the output
std::string output;
while (std::getline(istdout, output))
{
std::cout << output << '\n';
}
}
catch (std::exception& e)
{
std::cerr << "Error: " << e.what() << '\n';
return 1;
}
}
I was under the impression that placing my stdin pipe and related sources/sinks within a scope will guarantee they're closed, and therefore send the EOF.
The same code works perfectly under Windows (VS2013, boost_1_53).
I am using boost_1_53, boost-process 0.5, gcc 4.8.2.
That does not happen, because there's still a pipe handle open in the child process; that is only closed on posix if you set it explicitly (on windows it is done automatically). So you'd need to add something like that:
#if defined (BOOST_POSIX_API)
fcntl(pstdout.sink, F_SETFD, FD_CLOEXEC);
fcntl(pstderr.sink, F_SETFD, FD_CLOEXEC);
#endif
I would however recommend to use boost.asio and wait asynchronously for the exit of the subprocess and close the pipes there.
Just FYI: I've worked on boost-process 0.6 which has a different interface but makes the asio stuff much easier. This will hopefully be in review in October/November, so it might become an official boost library soon. It's currently in beta so you might want to check that one out.
What I want to do :
1.Create and open for writing in.fifo by process1
2.Open in.fifo for reading in process2
3.Write from cin to in.fifo by process1 line
4.Read and cout line by process2
5.When input "exit" to cin (process2), it closed file in.fifo, delete it and exit
6.process2 exit, because in.fifo has no writer
In my programs process2 doesn't exit. In c it works with read,write when O_NONBLOCK is clear , but I want to do it in c++
write.cpp:
#include <stdlib.h>
#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main(){
std::ofstream fifo;
fifo.open("/home/fedor/projects/fifo2/in",ios::out);
if(! fifo.is_open() ){
std::cout << " error : cannot open file " << std :: endl;
return 1;
}
std::cout << " file open " << std :: endl;
std::string line;
while (line.compare("exit") != 0 ){
std::getline(cin, line);
fifo << line << endl;
/* do stuff with line */
}
fifo.close();
remove("/home/fedor/projects/fifo2/in");
return 0;
}
read.cpp:
#include <stdlib.h>
#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main(){
std::ifstream fifo;
fifo.open("/home/fedor/projects/fifo2/in",ifstream::in);
if(! fifo.is_open() ){
std::cout << " error : cannot open file " << std :: endl;
return 1;
}
std::cout << " file open " << std :: endl;
std::string line;
bool done = false;
while (!done)
{
while (std::getline(fifo, line))
{
cout << line << endl;
/* do stuff with line */
}
if (fifo.eof())
{
fifo.clear(); // Clear the EOF bit to enable further reading
}
else
{
done = true;
}
}
return 0;
}
I can't find where I can read about blocking read by streams like http://linux.die.net/man/3/read about blocking read
If process2 closed if input "exit", like process1 is that a life lock ? (Is it blocking on read, or just trying and trying to read )
There is no way to do what you want using C++ standard library, because in C++ there is no notion of processes and file sharing. You have to use OS-specific APIs, which most likely are C (like open()) but in theory they can be C++.
Your reader appears to explicitly ignore an eof, instead clearing the end of file condition, and continuing. You're saying that your reader isn't exiting. Of course it's not, you're explicitly clearing EOF and continuing. It's going to exit only on an error reading from the FIFO, which is going to be a fairly unusual event.
If you want to exit on an EOF, you'll have to explicitly do that. Or, if you want to terminate on receiving the exit message, your writer isn't going to send it. When the writer itself receives a typed "exit", it terminates without writing it to the fifo (and your reader needs to check for that).
I think I must be assuming something from the name boost::interprocess that is not true.
The documents repeat that named_mutex is global here.
I am unable to make it work though. Two copies of the same executable should be run at the same time, and I expect that a named mutex in a library named boost::interprocess might actually BLOCK sometimes. It doesn't. It also doesn't prevent data file corruption in the code below.
Here's some code from the boost docs:
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <fstream>
#include <iostream>
#include <cstdio>
int main ()
{
using namespace boost::interprocess;
try{
struct file_remove
{
file_remove() { std::remove("file_name"); }
~file_remove(){ std::remove("file_name"); }
} file_remover;
struct mutex_remove
{
mutex_remove() { named_mutex::remove("fstream_named_mutex"); }
~mutex_remove(){ named_mutex::remove("fstream_named_mutex"); }
} remover;
//Open or create the named mutex
named_mutex mutex(open_or_create, "fstream_named_mutex");
std::ofstream file("file_name");
for(int i = 0; i < 10; ++i){
//Do some operations...
//Write to file atomically
scoped_lock<named_mutex> lock(mutex);
file << "Process name, ";
file << "This is iteration #" << i;
file << std::endl;
}
}
catch(interprocess_exception &ex){
std::cout << ex.what() << std::endl;
return 1;
}
return 0;
Here's what I did to it so I could prove to myself the mutex was doing something:
#include <windows.h>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <iostream>
#include <iterator>
#include <algorithm>
#include <fstream>
#include <iostream>
#include <cstdio>
int main (int argc, char *argv[])
{
srand((unsigned) time(NULL));
using namespace boost::interprocess;
try{
/*
struct file_remove
{
file_remove() { std::remove("file_name"); }
~file_remove(){ std::remove("file_name"); }
} file_remover;
*/
struct mutex_remove
{
mutex_remove() { named_mutex::remove("fstream_named_mutex"); }
~mutex_remove(){ named_mutex::remove("fstream_named_mutex"); }
} remover;
//Open or create the named mutex
named_mutex mutex(open_or_create, "fstream_named_mutex");
std::ofstream file("file_name");
for(int i = 0; i < 100; ++i){
//Do some operations...
//Write to file atomically
DWORD n1,n2;
n1 = GetTickCount();
scoped_lock<named_mutex> lock(mutex);
n2 = GetTickCount();
std::cout << "took " << (n2-n1) << " msec to acquire mutex";
int randomtime = rand()%10;
if (randomtime<1)
randomtime = 1;
Sleep(randomtime*100);
std::cout << " ... writing...\n";
if (argc>1)
file << argv[1];
else
file << "SOMETHING";
file << " This is iteration #" << i;
file << std::endl;
file.flush(); // added in case this explains the corruption, it does not.
}
}
catch(interprocess_exception &ex){
std::cout << "ERROR " << ex.what() << std::endl;
return 1;
}
return 0;
}
Console Output:
took 0 msec to acquire mutex ... writing...
took 0 msec to acquire mutex ... writing...
took 0 msec to acquire mutex ... writing...
took 0 msec to acquire mutex ... writing...
Also, the demo writes to a file, which if you run two copies of the program will be missing some data.
I expect that if I delete file_name and run two copies of the program, I should get interleaved writes to file_name containing 100 rows from each instance.
(Note, that the demo code is clearly not using an ofstream in append mode, instead it simply rewrites the file each time this program runs, so if we wanted a demo to show two processes writing to a file, I'm aware of that reason why it wouldn't work, but what I did expect is for the above code to be a feasible demonstration of mutual exclusion, which it is not. Also calls to a very handy and aptly named ofstream::flush() method could have been included, and weren't.)
Using Boost 1.53 on Visual C++ 2008
It turns out that Boost is a wonderful library, and it code examples interspersed in the documentation may sometimes be broken. At least the one for boost::interprocess::named_mutex in the docs is not functional on Windows systems.
*Always deleting a mutex as part of the demo code causes the mutex to not function. *
That should be commented in the demo code at the very least. It fails to pass the "principle of least amazement", although I wondered why it was there, I thought it must be idiomatic and necessary, it's idiotic and unnecessary, in actual fact. Or if it's necessary it's an example of what Joel Spolsky would call a leaky abstraction. If mutexes are really filesystem points under C:\ProgramData in Windows I sure don't want to know about it, or know that turds get left behind that will break the abstraction if I don't detect that case and clean it up. (Sure smells like posix friendly semantics for mutexes in Boost have caused them to use a posix-style implementation instead of going to Win32 API directly and implementing a simple mutex that has no filesystem turds.)
Here's a working demo:
#include <windows.h>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/lambda/lambda.hpp>
#include <iostream>
#include <iterator>
#include <algorithm>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <fstream>
#include <iostream>
#include <cstdio>
#include <windows.h>
int main (int argc, char *argv[])
{
srand((unsigned) time(NULL));
using namespace boost::interprocess;
try{
/*
// UNCOMMENT THIS IF YOU WANT TO MAKE THIS DEMO IMPOSSIBLE TO USE TO DEMO ANYTHING
struct file_remove
{
file_remove() { std::remove("file_name"); }
~file_remove(){ std::remove("file_name"); }
} file_remover;
// UNCOMMENT THIS IF YOU WANT TO BREAK THIS DEMO HORRIBLY:
struct mutex_remove
{
mutex_remove() { named_mutex::remove("fstream_named_mutex"); }
~mutex_remove(){ named_mutex::remove("fstream_named_mutex"); }
} remover;
*/
//Open or create the named mutex
named_mutex mutex(open_or_create, "fstream_named_mutex");
std::ofstream file("file_name", std::ios_base::app );
int randomtime = 0;
for(int i = 0; i < 100; ++i){
//Do some operations...
//Write to file atomically
DWORD n1,n2;
n1 = GetTickCount();
{
scoped_lock<named_mutex> lock(mutex);
n2 = GetTickCount();
std::cout << "took " << (n2-n1) << " msec to acquire mutex";
randomtime = rand()%10;
if (randomtime<1)
randomtime = 1;
std::cout << " ... writing...\n";
if (argc>1)
file << argv[1];
else
file << "SOMETHING";
file << "...";
Sleep(randomtime*100);
file << " This is iteration #" << i;
file << std::endl;
file.flush();
}
Sleep(randomtime*100); // let the other guy in.
}
}
catch(interprocess_exception &ex){
std::cout << "ERROR " << ex.what() << std::endl;
return 1;
}
return 0;
}
I would love critques and edits on this answer, so that people will have a working demo of using this named mutex .
To use the demo:
- Build it and run two copies of it. Pass a parameter in so you can see which instance wrote which lines (start myexename ABC and start myexename DEF from a command prompt in windows)
- If it's your second run, delete any stray output named "file_name" if you don't want the second run appended to the first.
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.
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
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?
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);
}
}
}
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.
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?
ncurses can be a good candidate.
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.
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.
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.
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;
}
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;
};
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);
}
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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