I have a multithreaded program in which on thread waits for input through a terminal and the other will get data from the socket. Is there any way to abort first threads cin/scanf to print in console data from second thread.
I think to kill the first thread, print data from second thread then run first thread again. But I'm looking for a better method, something like abort cin then reawoke it.
void thread1(){
cin>>string;
doSomething();
}
void thread2(){
cout<<getSomeData();
}
In usual case, it won't print data till something would be entered from keyboard.
[EDIT]
I found a particular solution, like if it doesn't get input it will interrupt, everything was done in C style.
In any case if you are interested check "Head First C" book, section "Interprocess Communication: It's good to talk".
I know it's bit too late. After a while I was supposed to make cpp multithreaded application again and had the same problem. This time it was done with implementing non blocking input with stdio`s getch() and I think it fits to be the best solution.
I don't know if the following solution is standard enough, but it works on both my Fedora Linux (GCC 10.3) & Windows 10 (MSVC 16.11)
#include <iostream>
#include <csignal>
int main()
{
std::signal(SIGINT, [] (int s)
{
std::signal(s, SIG_DFL);
std::fclose(stdin);
std::cin.setstate(std::ios::badbit); // To differenciate from EOF input
});
std::string input;
std::cout << "Input CTRL+C or EOF now!" << std::endl;
std::getline(std::cin, input);
std::cout << (std::cin.bad() ? "\rinterrupted" : "eof") << std::endl;
}
Don't ask me how to reuse cin from that state now.
Related
I have a task to create a program that does some calculation on a vector of numbers. The vector must contain at least 1 number in it and if it doesn't I have to throw an exception and try again. There is a video example how the code should work here: https://asciinema.org/a/283343
I'm guessing that EOF is being signaled using CTRL+D and that's what causes the exception to be thrown.
If they were using Enter (new line), it would leave a blank line behind.
But in my case, after I press CTRL+D, my program just runs in an infinite loop because the stdin stays in a failed state despite me using cin.clear().
Is there another shortcut similar to CTRL+D that they might be using for this, or is there a way to reopen the stream, or restart the whole application.
The program runs fine on Windows when I use CTRL+Z, but on Linux I just can't get it to work the same.
Example code below:
#include<iostream>
#include<vector>
void enter_elements(std::vector<double>& input_list){
double x;
std::cout << "Enter numbers: " << std::endl;
while(std::cin >> x){
input_list.push_back(x);
}
if(input_list.empty()){
throw std::string("You must enter at least 1 number!");
}
}
int main(){
std::vector<double> input_list;
try {
enter_elements(input_list);
} catch (const std::string& e) {
std::cout << "Error: " << e << std::endl;
std::cin.clear();
enter_elements(input_list);
}
return 0;
}
Can anyone help me with this problem or suggest where could I maybe read more about it?
No.
Once the stream is closed, the stream is closed. That's it.
What I'd do is accept a set of numbers on one line. Your input iteration would end at the end of the line. Then you validate those numbers, and ask for another line if necessary.
You can do that by looping over std::getline instead of using formatted extraction. Then you'd need to parse the line you get.
That's not what the video shows, but I don't know how they achieved that. Maybe you should ask them!
Are you asking how to stop the program?
If so, on linux, you should try CTRL+C instead of CTRL+D or CTRL+Z while your program is running
This is the sort of reason why I hate using cin >> anInt. I much prefer to getLine and parse it myself. Then you can make it do anything and decide how you're going to determine end of list, etc.
Yes, it's more code. But you don't run into these weird problems like this.
My program, which is executed from the command line, looks like this (execute command declared somewhere else):
int commandHandler::handleRequest(...)
{
bool cmdresult = execute(output);
if cmdresult
{
std::cout << output << std::endl;
}
}
The problem:
If you break the ongoing output to cout with ^C, another call to the program will crash at the output to cout, since "cout is locked but the owner is dead".
How do I prevent this in the easiest way? Are there any methods to check if cout is locked before trying to redirect output to that stream, and in that case, unlock it?
If I do a testprogram like this:
int main(void)
{
std::string output = "Superlongstringwouldbeprinted here... ";
for(int i=0;i<40000;i++)
{
output.append("Superlongstringwouldbeprinted here... ");
}
std::cout << output << std::endl;
}
in the "standard" environment, the output is breakable with ^C and I am able to run the program again with output to std::cout. That is, it seems like the implementation of direction to std::cout is flawed in the real time OS I am writing code for?
If you are with C++11, you may use
std::mutex::lock()
std::mutex::try_lock()
std::mutex::unlock()
functions.
However, you are not so lucky, you may use platform/library dependent codes
In Linux you can use POSIX mutex.
Another good idea to use scoped locks.
I made a console applications that accepts commands from two sources:
The actual console -> this is a while(getLine()) loop in a seperate thread.
A websocket server -> this also runs on a seperate thread
If there is a command entered, the command is stored in a vector until another while loop (that runs every 20ms) loops trough all the commands entered in the time passed. If he reads a command, he executes it.
Now, there is a Stop command who stops the application. When entered, the application shuts down as expected. But the problem is: this takes some time, and you can still enter text from the first command source (getline()). Once you type something, the shutdown sequence stops, and waits until you pressed enter.
I terminate the first thread (that contains the getline loop) once the shutdown sequence starts. But that doesn't work...
Any ideas?
Thanks in advance!
getline() is a blocking call, you'll probably have to use something different, if you want to receive messages (i.e. a shut down command) from other threads. You did not mention which library you use for multithreading and how you terminate the console reading thread (it's possible, that your way of stopping the thread still does not force it to exit from getline)
This question seems to have some relevant answers: Peek stdin using pthreads
By the way, you mentioned a vector, which is (if I understood it correctly) accessed from multiple threads. You'll have to take care about proper synchronization (e.g. using a mutex when accessing the vector).
Also, the fact that you have some kind of loop, which "polls" the vector every 20 ms is indicating that you may have some flaws in overall design of your application. Try to get rid of it by using more appropriate means for passing events between threads, such as condition variables.
The problem is that getline is a blocking call and will return just after you press ENTER in case of stdin.
I had a similar problem and solved it as shown below.
I used two file descriptors: one to monitor the stdin and an other to monitor a "self-pipe". The latter acts as a trigger to unlock the select in case some event happens. The former ensures getline is called just once it can read a full line.
#include <future>
#include <string>
#include <iostream>
#include <thread>
#include <unistd.h>
#include <stdio.h>
#include <sys/select.h>
int pipe_fd[2];
auto p = pipe(pipe_fd);
auto stdin_fd = fileno(stdin); // 0
fd_set check_fd;
int main(int argc, char const *argv[])
{
FD_ZERO(&check_fd);
FD_SET(stdin_fd, &check_fd);
FD_SET(pipe_fd[0], &check_fd);
auto t1 = std::async(std::launch::async, [] {
std::this_thread::sleep_for(std::chrono::seconds(10));
uint32_t dummy = 43;
// this will stop the data input
write(pipe_fd[1], &dummy, sizeof(dummy));
});
auto maxfd = pipe_fd[0] > pipe_fd[1] ? pipe_fd[0] : pipe_fd[1];
select(maxfd + 1, &check_fd, nullptr, nullptr, nullptr);
if (FD_ISSET(stdin_fd, &check_fd))
{
std::string input;
std::getline(std::cin, input);
std::cout << "You enterd:" << input << std::endl;
}
if (FD_ISSET(pipe_fd[0], &check_fd))
std::cout << "Event" << std::endl;
return 0;
}
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to stop C++ console application from exiting immediately?
I am a newbie on C++. Following tutorials, my program selfkilled when it finished executing simple commands (like cout ans stuff). I discovered the get.cin() function that avoided that. However, anytime I use 'cin' commands to insert variables, the program selfkills just after receiving the input and executing the work. Is there a way to avoid that? thanks a lot!
The reason it quits when your program receives input, even though you are using std::cin.get() is because whenever cin reads input, there's a chance that some junk is left behind; when you call std::cin.get(), you will get that junk.
What you have to do is clear cin of any undesired data, such that std::cin.get() has nothing to read and is required to wait for new input.
...
std::cin.clear();
std::cin.get();
return 0;
A program doesn't "kill itself". It just exits when it has finished doing everything it was supposed to do (i.e. when you return from main).
It is up to you to set up your work environment so that you can see the output of a program. For example, if you are in Windows, you could open your own command line (run cmd) and run your program from there; or instruct your IDE to not close the terminal window after the program exits.
Your program doesn't kill itself after execution, it just ends it.
Simple example:
#include <iostream>
int main( int argc, const char* argv[] )
{
std::cout << "Hello, World" << std::endl;
return 0; // End of execution
}
In that example a small window opens then close very fast because the logic of the code says so, However in the next example:
#include <iostream>
int main( int argc, const char* argv[])
{
std::cout <<"Hello, World!" << std::endl;
std::cin.get();
return 0;
}
Your application will still be showing in the screen until you press enter key 'Return key' then it will exit.
In case you are using Windows Operating System, consider the next example:
#include <iostream>
int main( int argc, const char* argv[])
{
std::cout << "Hello, World!" << std::endl;
system("PAUSE");
return 0;
}
Please note that system("PAUSE") is in Windows only and won't run on other operating systems.
One more thing worth mentioning here, there are a lot of methods to use other that these, but I wrote the most common ones.
In some windowing systems, a console window is created when your program executes. When your program finishes, this console window disappears.
I always recommend the "pause" pattern to newbies:
cout << "Paused. Press ENTER to continue.\n");
cin.ignore(10000, '\n'); // Ignore until 100000 characters are entered or a newline is entered.
Sometimes, I make this into a function:
void Pause(void)
{
cout << "Paused. Press ENTER to continue.\n");
cin.ignore(10000, '\n'); // Ignore until 100000 characters are entered or a newline is entered.
}
Hope this helps,
std::cin.get() work well and its usage is pretty easy but it expect user to press return.
I used to end my program using ESC, so it won't work for me, so I use this
#ifdef _WIN32
std::system( "pause" );
#else
std::system( "read -n1 -r -p \"Press any key to continue...\"" );
#endif
It would print "Press any key to continue..." and continue execution with pressing any key so I can use my lovely ESC
One of the first things I learned in C++ was that
#include <iostream>
int main()
{
std::cout<<"Hello, World!\n";
return 0;
}
would simply appear and disappear extremely quickly without pause. To prevent this, I had to go to notepad, and save
helloworld.exe
pause
ase
helloworld.bat
This got tedious when I needed to create a bunch of small test programs, and eventually I simply put while(true); at the end on most of my test programs, just so I could see the results. Is there a better wait function I can use?
you can require the user to hit enter before closing the program... something like this works.
#include <iostream>
int main()
{
std::cout << "Hello, World\n";
std::cin.ignore();
return 0;
}
The cin reads in user input, and the .ignore() function of cin tells the program to just ignore the input. The program will continue once the user hits enter.
Link
Please note that the code above was tested on Code::Blocks 12.11 and Visual Studio 2012
on Windows 7.
For forcing your programme stop or wait, you have several options :
sleep(unsigned int)
The value has to be a positive integer in millisecond.
That means that if you want your programme wait for 2 seconds, enter 2000.
Here's an example :
#include <iostream> //for using cout
#include <stdlib.h> //for using the function sleep
using namespace std; //for using cout
int main(void)
{
cout << "test" << endl;
sleep(5000); //make the programme waiting for 5 seconds
cout << "test" << endl;
sleep(2000); // wait for 2 seconds before closing
return 0;
}
If you wait too long, that probably means the parameter is in seconds. So change it to this:
sleep(5);
For those who get error message or problem using sleep try to replace it by _sleep or Sleep especially on Code::Bloks.
And if you still getting problems, try to add of one this library on the beginning of the code.
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <dos.h>
#include <windows.h>
system("PAUSE")
A simple "Hello world" programme on windows console application would probably close before you can see anything. That the case where you can use system("Pause").
#include <iostream>
using namespace std;
int main(void)
{
cout << "Hello world!" << endl;
system("PAUSE");
return 0;
}
If you get the message "error: 'system' was not declared in this scope" just add
the following line at the biggining of the code :
#include <cstdlib>
cin.ignore()
The same result can be reached by using cin.ignore() :
#include <iostream>
using namespace std;
int main(void)
{
cout << "Hello world!" << endl;
cin.ignore();
return 0;
}
cin.get()
example :
#include <iostream>
using namespace std;
int main(void)
{
cout << "Hello world!" << endl;
cin.get();
return 0;
}
getch()
Just don't forget to add the library conio.h :
#include <iostream>
#include <conio.h> //for using the function getch()
using namespace std;
int main(void)
{
cout << "Hello world!" << endl;
getch();
return 0;
}
You can have message telling you to use _getch() insted of getch
Lots of people have suggested POSIX sleep, Windows Sleep, Windows system("pause"), C++ cin.get()… there's even a DOS getch() in there, from roughly the late 1920s.
Please don't do any of these.
None of these solutions would pass code review in my team. That means, if you submitted this code for inclusion in our products, your commit would be blocked and you would be told to go and find another solution. (One might argue that things aren't so serious when you're just a hobbyist playing around, but I propose that developing good habits in your pet projects is what will make you a valued professional in a business organisation, and keep you hired.)
Keeping the console window open so you can read the output of your program is not the responsibility of your program! When you add a wait/sleep/block to the end of your program, you are violating the single responsibility principle, creating a massive abstraction leak, and obliterating the re-usability/chainability of your program. It no longer takes input and gives output — it blocks for transient usage reasons. This is very non-good.
Instead, you should configure your environment to keep the prompt open after your program has finished its work. Your Batch script wrapper is a good approach! I can see how it would be annoying to have to keep manually updating, and you can't invoke it from your IDE. You could make the script take the path to the program to execute as a parameter, and configure your IDE to invoke it instead of your program directly.
An interim, quick-start approach would be to change your IDE's run command from cmd.exe <myprogram> or <myprogram>, to cmd.exe /K <myprogram>. The /K switch to cmd.exe makes the prompt stay open after the program at the given path has terminated. This is going to be slightly more annoying than your Batch script solution, because now you have to type exit or click on the red 'X' when you're done reading your program's output, rather than just smacking the space bar.
I assume usage of an IDE, because otherwise you're already invoking from a command prompt, and this would not be a problem in the first place. Furthermore, I assume the use of Windows (based on detail given in the question), but this answer applies to any platform… which is, incidentally, half the point.
The appearance and disappearance of a window for displaying text is a feature of how you are running the program, not of C++.
Run in a persistent command line environment, or include windowing support in your program, or use sleep or wait on input as shown in other answers.
the equivalent to the batch program would be
#include<iostream>
int main()
{
std::cout<<"Hello, World!\n";
std::cin.get();
return 0;
}
The additional line does exactly what PAUSE does, waits for a single character input
There is a C++11 way of doing it. It is quite simple, and I believe it is portable. Of course, as Lightness Races in Orbit pointed out, you should not do this in order to be able to see an Hello World in your terminal, but there exist some good reason to use a wait function. Without further ado,
#include <chrono> // std::chrono::microseconds
#include <thread> // std::this_thread::sleep_for
std::this_thread::sleep_for(std::chrono::microseconds{});
More details are available here. See also sleep_until.
Actually, contrary to the other answers, I believe that OP's solution is the one that is most elegant.
Here's what you gain by using an external .bat wrapper:
The application obviously waits for user input, so it already does what you want.
You don't clutter the code with awkward calls. Who should wait? main()?
You don't need to deal with cross platform issues - see how many people suggested system("pause") here.
Without this, to test your executable in automatic way in black box testing model, you need to simulate the enter keypress (unless you do things mentioned in the footnote).
Perhaps most importantly - should any user want to run your application through terminal (cmd.exe on Windows platform), they don't want to wait, since they'll see the output anyway. With the .bat wrapper technique, they can decide whether to run the .bat (or .sh) wrapper, or run the executable directly.
Focusing on the last two points - with any other technique, I'd expect the program to offer at least --no-wait switch so that I, as the user, can use the application with all sort of operations such as piping the output, chaining it with other programs etc. These are part of normal CLI workflow, and adding waiting at the end when you're already inside a terminal just gets in the way and destroys user experience.
For these reasons, IMO .bat solution is the nicest here.
What you have can be written easier.
Instead of:
#include<iostream>
int main()
{
std::cout<<"Hello, World!\n";
return 0;
}
write
#include<iostream>
int main()
{
std::cout<<"Hello, World!\n";
system("PAUSE");
return 0;
}
The system function executes anything you give it as if it was written in the command prompt. It suspends execution of your program while the command is executing so you can do anything with it, you can even compile programs from your cpp program.
Syntax:
void sleep(unsigned seconds);
sleep() suspends execution for an interval (seconds).
With a call to sleep, the current program is suspended from execution for the number of seconds specified by the argument seconds. The interval is accurate only to the nearest hundredth of a second or to the accuracy of the operating system clock, whichever is less accurate.
This example should make it clear:
#include <dos.h>
#include <stdio.h>
#include <conio.h>
int main()
{
printf("Message 1\n");
sleep(2); //Parameter in sleep is in seconds
printf("Message 2 a two seconds after Message 1");
return 0;
}
Remember you have to #include dos.h
EDIT:
You could also use winAPI.
VOID WINAPI Sleep(
DWORD dwMilliseconds
);
Sleep Function(Windows)
Just a note,the parameter in the function provided by winapi is in milliseconds ,so the sleep line at the code snippet above would look like this "Sleep(2000);"
getchar() provides a simplistic answer (waits for keyboard input).
Call Sleep(milliseconds) to sleep before exit.
Sleep function (MSDN)
You can use sleep() or usleep().
// Wait 5 seconds
sleep( 5 );
Well, this is an old post but I will just contribute to the question -- someone may find it useful later:
adding 'cin.get();' function just before the return of the main() seems to always stop the program from exiting before printing the results: see sample code below:
int main(){
string fname, lname;
//ask user to enter name first and last name
cout << "Please enter your first name: ";
cin >> fname;
cout << "Please enter your last name: ";
cin >> lname;
cout << "\n\n\n\nyour first name is: " << fname << "\nyour last name is: "
<< lname <<endl;
//stop program from exiting before printing results on the screen
cin.get();
return 0;
}
Before the return statement in you main, insert this code:
system("pause");
This will hold the console until you hit a key.
#include<iostream>
#include<string>
using namespace std;
int main()
{
string s;
cout << "Please enter your first name followed by a newline\n";
cin >> s;
cout << "Hello, " << s << '\n';
system("pause");
return 0; // this return statement isn't necessary
}
The second thing to learn (one would argue that this should be the first) is the command line interface of your OS and compiler/linker flags and switches.