freopen & cout

freopen & cout

[Alex Vinokur]

Hi,

Where has output-to-cout gone in the program below?

Thanks,

========= C++ code : File foo.cpp : BEGIN =========
#include <cstdio>
#include <cassert>
#include <iostream>
#include <fstream>
using namespace std;

#define FILE_NAME "foobar"

int main ()
{
ofstream ofs;

  remove (FILE_NAME);

  ofs.open (FILE_NAME);
  assert (ofs);
  assert (ofs.is_open());

FILE* stdof = freopen (FILE_NAME, "w", stdout);
  assert (stdof);

  cout << "to cout (first time)" << endl;   // Problematic line
  ofs  << "to ofs  (first time)" << endl;
  cout << "to cout (second time)" << endl;  // Problematic line
  ofs  << "to ofs  (second time)" << endl;

  ofs.close();
  assert (!ofs.is_open());

int rc = fclose (stdof);
  assert (!rc);

  return 0;
}
========= C++ code : File foo.cpp : END ===========


========= Compilation & Run : BEGIN =========

$ g++ -v
[---omitted---]
gcc version 3.3.1 (cygming special)

$ g++ -W -Wall foo.cpp

$ a
// No output; Where has output-to-cout gone?

$ cat foobar
to ofs  (first time)
to ofs  (second time)

========= Compilation & Run : END ===========


--
   =====================================
   Alex Vinokur
     mailto:alexvn@connect.to
     http://up.to/alexvn
   =====================================

Re: freopen & cout

[Burak Serdar]

Alex Vinokur wrote:
> Hi,
> 
> Where has output-to-cout gone in the program below?
> 
> Thanks,
> 
> ========= C++ code : File foo.cpp : BEGIN =========
> #include <cstdio>
> #include <cassert>
> #include <iostream>
> #include <fstream>
> using namespace std;
> 
> #define FILE_NAME "foobar"
> 
> int main ()
> {
> ofstream ofs;
> 
>   remove (FILE_NAME);
> 
>   ofs.open (FILE_NAME);
>   assert (ofs);
>   assert (ofs.is_open());
> 
> FILE* stdof = freopen (FILE_NAME, "w", stdout);

This call to freopen closes stdout, therefore you cannot print anything 
using cout after this point.



>   assert (stdof);
> 
>   cout << "to cout (first time)" << endl;   // Problematic line
>   ofs  << "to ofs  (first time)" << endl;
>   cout << "to cout (second time)" << endl;  // Problematic line
>   ofs  << "to ofs  (second time)" << endl;
> 
>   ofs.close();
>   assert (!ofs.is_open());
> 
> int rc = fclose (stdof);
>   assert (!rc);
> 
>   return 0;
> }
> ========= C++ code : File foo.cpp : END ===========
> 
> 
> ========= Compilation & Run : BEGIN =========
> 
> $ g++ -v
> [---omitted---]
> gcc version 3.3.1 (cygming special)
> 
> $ g++ -W -Wall foo.cpp
> 
> $ a
> // No output; Where has output-to-cout gone?
> 
> $ cat foobar
> to ofs  (first time)
> to ofs  (second time)
> 
> ========= Compilation & Run : END ===========
> 
> 
> --
>    =====================================
>    Alex Vinokur
>      mailto:alexvn@connect.to
>      http://up.to/alexvn
>    =====================================
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 

RE: freopen & cout

[lrtaylor]

Alex,

According to MSDN:

"The freopen function closes the file currently associated with stream
and reassigns stream to the file specified by path."

That probably means that the original stdout (to which cout writes) has
been closed, and so your cout output is getting lost in the big void...
I know that you were probably expecting cout to get redirected to the
file, but perhaps this just isn't the right way to do it (due to how
cout is implemented in C++, etc.)...

Good luck,
Lyle

-----Original Message-----
From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
Behalf Of Alex Vinokur

Hi,

Where has output-to-cout gone in the program below?