C++: ctrl+d terminates program (ubuntu) - c++

(New to C++)
I have this simple code (simplified for the question):
int main()
{
string currInput;
while (getline(cin, currInput))
{
}
cout << "wont be printed" << std::flush;
return 0;
}
I have been debugging for a while and I probably miss something:
When running it and pressing ctrl+d (after some strings or right away), it does not print the string that is after the while loop. It just stop running. I thought it might be something with flushing so I added that too.
What am I missing?
PS: When running in debug, it mentions something about sighup signal.

So, with a lot of help from #manni and #rici, we found the problem.
It turns out to be a known problem in cLion.
see sending EOF to stdin in Clion IDE
https://intellij-support.jetbrains.com/hc/en-us/community/posts/206849765-How-to-enter-EOF-Ctrl-z-in-Run-console-
https://youtrack.jetbrains.com/issue/IDEA-12514

Your error is the while loop. You should not be using a loop here. What happens is it hits this line and attempts to get input. Regardless of whether you type in anything or not, if you hit CTRL+D it will end the getline. But you have it in a while loop....so it'll hop back to the top of the loop and get another line...then another line....then another line. Welcome to your first infinite loop.
while (getline(cin, currInput))
{
}
The simplest thing would be to do just
getline(cin, currInput);
If you're starting out programming this is probably what you want to do anyway.
If you feel gutsy, read this page: http://www.cplusplus.com/reference/string/string/getline/
You'll notice that getline returns the stream you pass in. Which will evaluate to true as far the loop is concerned.

Related

cmd shell flashes across in VS code [duplicate]

Lately, I've been trying to learn C++ from this website. Unfortunately whenever I try to run one of the code samples, I see that program open for about a half second and then immediately close. Is there a way to stop the program from closing immediately so that I can see the fruits of my effort?
If you are using Visual Studio and you are starting the console application out of the IDE:
pressing CTRL-F5 (start without debugging) will start the application and keep the console window open until you press any key.
Edit: As Charles Bailey rightly points out in a comment below, this won't work if there are characters buffered in stdin, and there's really no good way to work around that. If you're running with a debugger attached, John Dibling's suggested solution is probably the cleanest solution to your problem.
That said, I'll leave this here and maybe someone else will find it useful. I've used it a lot as a quick hack of sorts when writing tests during development.
At the end of your main function, you can call std::getchar();
This will get a single character from stdin, thus giving you the "press any key to continue" sort of behavior (if you actually want a "press any key" message, you'll have to print one yourself).
You need to #include <cstdio> for getchar.
The solution by James works for all Platforms.
Alternatively on Windows you can also add the following just before you return from main function:
system("pause");
This will run the pause command which waits till you press a key and also displays a nice message Press any key to continue . . .
If you are using Microsoft's Visual C++ 2010 Express and run into the issue with CTRL+F5 not working for keeping the console open after the program has terminated, take a look at this MSDN thread.
Likely your IDE is set to close the console after a CTRL+F5 run; in fact, an "Empty Project" in Visual C++ 2010 closes the console by default. To change this, do as the Microsoft Moderator suggested:
Please right click your project name and go to Properties page, please expand Configuration Properties -> Linker -> System, please select Console (/SUBSYSTEM:CONSOLE) in SubSystem dropdown. Because, by default, the Empty project does not specify it.
I usually just put a breakpoint on main()'s closing curly brace. When the end of the program is reached by whatever means the breakpoint will hit and you can ALT-Tab to the console window to view the output.
Why not just run the program from a console ie run the program from cmd.exe if you're using Windows. That way the window stays open after the program finishes.
[EDIT]: When I use KDevelop4 there is a fully fledged instance of Bash (a Linux CLI) running in a tab at the bottom of the IDE. Which is what I use in these sort of circumstances.
Before the end of your code, insert this line:
system("pause");
This will keep 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
}
Call cin.get(); 2 times:
//...
cin.get();
cin.get();
return 0
}
If you run your code from a competent IDE, such as Code::Blocks, the IDE will manage the console it uses to run the code, keeping it open when the application closes. You don't want to add special code to keep the console open, because this will prevent it functioning correctly when you use it for real, outside of the IDE.
I just do this:
//clear buffer, wait for input to close program
std::cin.clear(); std::cin.ignore(INT_MAX, '\n');
std::cin.get();
return 0;
Note: clearing the cin buffer and such is only necessary if you've used cin at some point earlier in your program. Also using std::numeric_limits::max() is probably better then INT_MAX, but it's a bit wordy and usually unnecessary.
Okay I'm guessing you are on Windows using Visual Studio... why? Well because if you are on some sort of Linux OS then you'd probably be running it from the console.
Anyways, you can add crap to the end of your program like others are suggesting, or you can just hit CTRL + F5 (start without debugging) and Visual Studio will leave the console up once complete.
Another option if you want to run the Debug version and not add crap to your code is to open the console window (Start -> Run -> cmd) and navigate to your Debug output directory. Then, just enter the name of your executable and it will run your debug program in the console. You can then use Visual Studio's attach to process or something if you really want to.
Just add the following at the end of your program. It will try to capture some form of user input thus it stops the console from closing automatically.
cin.get();
If you are actually debugging your application in Visual C++, press F5 or the green triangle on the toolbar. If you aren't really debugging it (you have no breakpoints set), press Ctrl+F5 or choose Start Without Debugging on the menus (it's usually on the Debug menu, which I agree is confusing.) It will be a little faster, and more importantly to you, will pause at the end without you having to change your code.
Alternatively, open a command prompt, navigate to the folder where your exe is, and run it by typing its name. That way when it's finished running the command prompt doesn't close and you can see the output. I prefer both of these methods to adding code that stops the app just as its finished.
Add the following lines before any exit() function or before any returns in main():
std::cout << "Paused, press ENTER to continue." << std::endl;
cin.ignore(100000, "\n");
For Visual Studio (and only Visual Studio) the following code snippet gives you a 'wait for keypress to continue' prompt that truly waits for the user to press a new key explicitly, by first flushing the input buffer:
#include <cstdio>
#include <tchar.h>
#include <conio.h>
_tprintf(_T("Press a key to continue "));
while( _kbhit() /* defined in conio.h */ ) _gettch();
_gettch();
Note that this uses the tchar.h macro's to be compatible with multiple 'character sets' (as VC++ calls them).
Use #include "stdafx.h" & system("pause"); just like the code down below.
#include "stdafx.h"
#include <iostream>
using namespace std;
int main()
{
std::cout << "hello programmer!\n\nEnter 2 numbers: ";
int x, y;
std::cin >> x >> y;
int w = x*y;
std::cout <<"\nyour answer is: "<< w << endl;
system("pause");
}
simply
#include <cstdio>
int main(){
// code...
std::getchar();
std::getchar();
return 0;
}
for some reason there is usually 1 character possible to read with getchar already in stdin when you run a program. so the first getchar reads this character, and the second getchar waits for user (your) input before exiting the program. And after a program exits most of terminals, especially on Windows close terminal immediately.
so what we aim to is a simple way of preventing a program from finishing after it outputs everything.
Of course there are more complex and clean ways to solve this, but this is the simplest.
Similar idea to yeh answer, just minimalist alternative.
Create a batch file with the following content:
helloworld.exe
pause
Then use the batch file.
See if your IDE has a checkbox in project setting to keep the window open after the program terminates. If not, use std::cin.get(); to read a character at the end of main function. However, be sure to use only line-based input (std::getline) or to deal with leftover unread characters otherwise (std::ignore until newline) because otherwise the .get() at the end will only read the garbage you left unread earlier.
This seems to work well:
cin.clear();
cin.ignore(2);
If you clear the buffer first it won't be a problem when you read the next one.
For some reason cin.ignore(1) does not work, it has to be 2.
You could always just create a batch file. For example, if your program is called helloworld.exe, some code would be:
#echo off
:1
cls
call helloworld.exe
pause >nul
goto :1
If you are running Windows, then you can do system("pause >nul"); or system("pause");. It executes a console command to pause the program until you press a key. >nul prevents it from saying Press any key to continue....
I'm putting a breakpoint at the last return 0 of the program. It works fine.
I used cin.get() and that is worked but one day I needed to use another cin.get([Array Variable]) before that to grab a ling string with blank character in middle of. so the cin.get() didn't avoid command prompt window from closing. Finally I found Another way:
Press CTRL+F5 to open in an external window and Visual Studio does not have control over it anymore. Just will ask you about closing after final commands run.
I tried putting a getchar() function at the end. But it didn't work. So what I did was add two getchar() functions one after another. I think the first getchar() absorbs the Enter key you press after the last data input. So try adding two getchar() functions instead of one
Instead of pressing the run button, press CTRL and F5 at the same time, it will give you the press any key to continue message. Or type "(warning use this only for testing not actual programs as an antiviruses don't like it!!!!)" at the end of your main function but: (warning use this only for testing not actual programs as an antiviruses don't like it!!!!)
just use cin.ignore() right before return 0; twice
main()
{
//your codes
cin.ignore();
cin.ignore();
return 0;
}
thats all
you can try also doing this
sleep (50000);
cout << "any text" << endl;
This will hold your code for 50000m, then prints message and closes. But please keep in mind that it will not pause forever.
Here's a problem, not so obvious. Somehow I had added a debug breakpoint at the very last line of my program. } Not sure how I did that, perhaps with an erroneous mouse click while jumping between different screens. I'm working in VS Code.
And when I go to debug, the system jumps immediately to that breakpoint. No error message, no interim output, nothing. I'm like, how did the program rush thru all my set breakpoints? This took too long to figure out.
Apparently the system sees that last line breakpoint as a "first" stop. The simple fix? Delete that breakpoint, doh! (insert forehead slap here.)
All you have to do set a variable for x then just type this in before the return 0;
cout<<"\nPress any key and hit enter to end...";
cin>>x;

How Ctrl Z works actually

Just for curious mind. During problem solving many question says,"Input will be terminated by Ctrl+z". I know its "EOF(End Of File)" But...
while(scanf("%d",&a)==1)
{ cout<<"OK"<<endl;}
while(scanf("%d",&a)!=EOF)
{cout<<"OK"<<endl;}
while(cin>>a)
{cout<<"OK"<<endl;}
Above 3 will be terminated by Ctrl+z.
while(scanf("%d",&a))
{cout<<"OK"<<endl;}
It will give OK by pressing Ctrl+z.
and
while(1){cin>>a;
cout<<"OK"<<endl;}
Its a infinte loop.
I want to know how Ctrl+z works on a program termination. What is the reason behind it. Please answer in details.
Ctrl+z does not terminate your program. It also doesn't pause its execution. It's a 0x1A byte that is interpreted by iostream and stdio methods to be EOF (end-of-file). After that character is read from the console, nothing is read further and the method that is reading it returns. In the case of iostream, std::ios::eof() becomes true.
You would notice in your last case that if you structured it as:
while(cin >> a) { ... }
It would exit like the others.

A function in C++ made being skipped?

I have made a function called pause(). Here is the code for it:
void pause(){
std::cin.sync();
std::cin.ignore();
}
I also use a declaration in front of my code:
void pause();
int main(){
}
It worked at first, but now for some reason it just skips the function when I call it...
std::cout << "Random text. Press any key to carry on...";
pause();
EDIT: Unfortunately, when I try it differently, it goes makes me enter something and then I have to press enter. Is there really not a way to make it so that if you press any key (like the ugly system("pause") it immediately carries on? I'll have to make a new thread for this...
This is why it's important to post the smallest complete code you can come up with that compiles and runs and shows the problem. Chances are, though, that there's some input done before calling this function, and it's leaving something in the input buffer. So pause isn't being skipped, but it returns immediately because of the noise in the input.

Getline() Not keeping Application Open?

I'm a first semester C++ student and in class we are building a BMI calculator (Win32 Console Application). I've gotten everything to work just fine, except for one of the instructions, which is wait for user to press enter to close application.
I had success using the system("PAUSE"); statement but in the past I would declare a string variable, like for example, initialize string genVar; and then use getline(cin, genVar); and when the user pressed Enter, the application would close, but it didnt work this time. The application would simply close. It worked just fine with pause, but not with getline().
Is using getline() for this purpose bad practice? Anyone have any clue why it didn't work?
Thank you in advance!
I know this is an old post, but look at the solution which I posted in this question:
C++ Multiple Program Issues (rand, conversion, crashing)
It explains (as chris mentioned in a comment) that getline tends to leave in a new line character in the buffer, which needs to be cleared and reset. Answer above explains it.
std::cin and std::getline do not mix well in general, you usually want to stick to one or the other in a program to avoid input errors and bugs. Your getline probably is grabbing a left over input from as earlier std::cin. I would recommend using this:
std::cout << "Press ENTER to continue...";
std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
return 0;
}
you'll need to include <limits> for this to work correctly.
you can also use the getch() function in the place of std::getline() IMHO

C++ unexpected hangtime

I am trying to write a function in my program that loads a huge text-file of 216,555 words and put them as strings into a set. This works properly, but as expected, it will hang for a few micro seconds while looping through the file. But there is something funky going on with my loop, and it's not behaving properly. Please take the time to read, I am sure there's a valid reason for this, but I have no idea what to search for.
The code, which is working by the way, is this:
ifstream dictionary;
dictionary.open("Dictionary.txt");
if(dictionary.fail())
{
cout<<"Could not find Dictionary.txt. Exiting."<<endl;
exit(0);
}
int i = 0;
int progress = 216555/50;
cout<<"Loading dictionary..."<<endl;
cout<<"< >"<<endl;
cout<<"<";
for(string line; getline(dictionary, line);)
{
usleep(1); //Explanation below (not the hangtime)
i++;
if(i%progress == 0)
cout<<"=";
words.insert(line);
}
The for-loop gets every string from the file, and inserts them in the map.
This is a console-application, and I want the user to see the progress. It's not much of a delay, but I wanted to do it anyway. If you don't understand the code, I'll try to explain.
When the program starts, it first prints out "Loading Dictionary...", and then a "<" and a ">" separated by 50 spaces. Then on the next line: "<" followed by a "=" for every 433 word it loops through (216555/50). The purpose of this is so the user can see the progress. The wanted output half way through the loop would be like this:
Loading dictionary...
< >
<=========================
My problem is:
The correct output is shown, but not at the expected time. It prints out the full progress bar, but that only after it has hanged and are done with the loop. How is that possible? The correct number of '=' are shown, but they all pop out at the same time, AFTER it hangs for some microseconds. I added the usleep(1) to make the hangtime a bit longer, but the same thing happened. The if-statement clearly works, or the '=' would've never showed at all, but it feels like my program is stacking the cout-calls for after the entire loop.
The weirdest thing of all, is that the last cout<<"<"; before the for-loop starts, is also shown at the same time as the rest of its line; after the loop is finished. Why and how?
You never flush the stream, so the output just goes into a buffer.
Change cout<<"="; to cout<<"="<<std::flush;
You need to flush the output stream.
cout << "=" << std::flush;
The program is "stacking the cout-calls". It's called output buffering, and every major operating system does it.
When in interactive mode (as your program is intended to be used), output is buffered by line; that is, it will be forced to the terminal when a newline is seen. You can also have block-buffered (fixed number of bytes between forced outputs; used when piping output) and unbuffered.
C++ provides the std::flush stream modifier to force an output at any point. It can be used like this:
cout << "=" << std::flush;
This will slow down your program a bit; the point of buffering is for efficiency. As you'll only be doing it about 51 times, though, the slowdown should be negligible.