when using freopen() in visual studio c++ system("pause") is not working

Question

I was trying to read from a file in vs17. But here system("pause") isn't working. The console window here just pops up and vanishes. The input.txt file contains only one integer.

#include<iostream>
#include<stdio.h>
#include<cstdio>
#pragma warning(disable:4996)
using namespace std;
int main()
{
    freopen("input.txt", "r", stdin);
    int n;
    cin >> n;
    cout << n << endl;
    system("pause");
   return 0;
}

So is there any way to read from a file and show the output in a console until another input from keyboard is given. Thanks in advance

Using `freopen` to repoint `stdin` limits your options for getting further input from a user via the keyboard. — user4581301, Mar 12 '18 at 06:50
`freopen` would do that and there is no easy way to revert. Why don't you use `std::ifstream fin("input.txt");` and later `fin >> n;`? — Gyapti Jain, Mar 12 '18 at 06:54

Gyapti Jain · Accepted Answer · 2018-03-26T06:20:01.173

You either don't mess with stdin to use system("pause") or restore it after use.

Method 1: Don't mess with `stdin`

#include<iostream>
#include<stdio.h>
#include<cstdio>
#include <fstream> // Include this
#pragma warning(disable:4996)
using namespace std;
int main()
{
    std::ifstream fin("input.txt");  // Open like this
    int n;
    fin >> n;  // cin -> fin
    cout << n << endl;
    system("pause");
   return 0;
}

Using separate stream for reading file keeps console reading isolated.

Method 2: Restore `stdin`

#include <io.h>  
#include <stdlib.h>  
#include <stdio.h>  
#include <iostream>

using std::cin;
using std::cout;

int main( void )  
{  
   int old;  
   FILE *DataFile;  

   old = _dup( 0 );   // "old" now refers to "stdin"   
                      // Note:  file descriptor 0 == "stdin"   
   if( old == -1 )  
   {  
      perror( "_dup( 1 ) failure" );  
      exit( 1 );  
   }  

   if( fopen_s( &DataFile, "input.txt", "r" ) != 0 )  
   {  
      puts( "Can't open file 'data'\n" );  
      exit( 1 );  
   }  

   // stdin now refers to file "data"   
   if( -1 == _dup2( _fileno( DataFile ), 0 ) )  
   {  
      perror( "Can't _dup2 stdin" );  
      exit( 1 );  
   }  
   int n;
   cin >> n;
   cout << n << std::endl;

   _flushall();  
   fclose( DataFile );  

   // Restore original stdin 
   _dup2( old, 0 );  
   _flushall();  
   system( "pause" );  
}

Here you restore original stdin so that console input can be used by the system("pause"). Breaking this to 2 separate functions override_stdin and restore_stdin can be more manageable.

Method 3: Don't use `system("pause")`

You can (optionally compile your test program at console using cl command line compilation tool provided by MSVC and) run the program on command line so that output is not missed when the program exits. Or you can search for some IDE options which keep the console for monitoring the output or you can put a breakpoint on the last line. (May be return 0) which may have its own consequences/issues.

score -1 · Answer 2 · edited Mar 12 '18 at 10:26

Information about system("pause");

As for system("Pause") and it's use: I wouldn't recommend using it in any code base that is designed to be ported or distributed and here is why: Why is it wrong!.

Now if you are using it for your own quick dirty hack to keep windows console open just for testing small functions or classes that is fine, however here is an alternative that will do the same thing that meets the standard as well as being portable.

#include <iostream>
#include <string>

int main() {        
    [code...]

    std::cout << "\nPress any key to quit.\n";
    std::cin.get(); // better than declaring a char.
    return 0;
}

Information about freopen()

Another thing to be aware of is that you are calling freopen() on a text file but you never have any call to close the file after you are done with it; I don't know if freopen() does this for you automatically, but if it doesn't then you should close the file handle before exiting the program and after you have extracted all the needed information from it.

Here is a related Q/A stack: freopen().

For more information on freopen() here is an excellent resource webpage: C: <cstdio> - freopen() & C++: <cstdio> - std::freopen()

My Attempt of running your code.

Now that I was able to test your program. If you are using Visual Studio and running the application from your debugger in debug mode it will automatically close out the application when it is done. You can either run it without the debugger( ctrl + F5 ) or run it from a console or terminal outside of the IDE from the path of the generated executable and the program will run in the manner you are expecting it to.

You can simply use `std::cin.get();` instead of declaring a `char` — Tas, Mar 12 '18 at 06:51
AFAIK, "pause" isn't case sensitive, because Windows isn't case sensitive. e.g. in your command window, having "StArT nOtEpAD" is the same as "start notepad" — Tas, Mar 12 '18 at 06:53

when using freopen() in visual studio c++ system("pause") is not working

2 Answers2

Method 1: Don't mess with stdin

Method 2: Restore stdin

Method 3: Don't use system("pause")

Method 1: Don't mess with `stdin`

Method 2: Restore `stdin`

Method 3: Don't use `system("pause")`