Redirecting standard output to a namedpiped

Question

Hello,

I know that with freopen() I can redirect the standard output to a file, now question is there anyway to use freopen() and give it a named pipe. This way instead of redirecting to a file I will redirect it to the named pipe.

cheers,

Ehsan 

Answer 1

CODEPROJECT:

The technique of redirecting the input/output of a console process is very sample: The CreateProcess() API through the STARTUPINFO structure enables us to redirect the standard handles of a child console based process. So we can set these handles to either a pipe handle, file handle, or any handle that we can read and write. The detail of this technique has been described clearly in MSDN: HOWTO: Spawn Console Processes with Redirected Standard Handles.

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Answer 2

I would appreciate the answer as well. I am not aware of mechanism that can reroute all console IO. Even for above example it works for C++ conio commands, and I had never heard how to do it for C functions like printf.

I am exploring following option:

CMDLINE:

ProcessThatUsesConsole.exe > MyExeThatReceivesAndManagesConsoleInput.exe

where ">MyExeThatReceivesAndManagesConsoleInput.exe" is added to command line parameters

In case you are not familiar with syntax, I am talking [dir >listing.txt] where you will get output to file listing.txt

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Answer 3

On 6/15/2012 9:06 AM, ehsansad wrote:

I know that with freopen() I can redirect the standard output to a file, now question is there anyway to use freopen() and give it a named pipe. This way instead of redirecting to a file I will redirect it to the named pipe.

Try something like this:

HANDLE pipe = CreateNamedPipe(...);
int stream = _open_osfhandle(intptr_t(pipe), 0);
stdout = _fdopen(stream, "wt");

---

Igor Tandetnik

Answer 4

I have been trying to make this work without any result, this is the code:

#include <windows.h>
#include <process.h>
#include <stdio.h>
#include <stdlib.h>
#include <io.h>


int     Buffer_out;
int     count,i;
int     stream;
int     old;
LPCTSTR pipeName = "\\\\.\\pipe\\ehsan";
HANDLE  hPipe;
FILE    *freOpenstream = NULL;
FILE    *test = NULL;

int main(int argc, char* argv[])
{	
   hPipe = CreateNamedPipe(pipeName, 
                            PIPE_ACCESS_OUTBOUND,
                            PIPE_TYPE_BYTE | PIPE_WAIT,
                            10, 0, sizeof(Buffer_out), 
                            10000, 
                            NULL);    
   if(INVALID_HANDLE_VALUE == hPipe)
   {
     printf("Server Pipe not created\n");
     exit(0);
   }
   else
     printf("Successful in creating server pipe\n");
   
   old = _dup(1);
   if( old == -1 )
   {
      perror( "_dup( 1 ) failure" );
      exit( 1 );
   }
   stream = _open_osfhandle((long)hPipe, 0);
   if(stream != -1)
       test = _fdopen(stream, "wt");   
  
  
   // stdout now refers to file "test" 
   if(_dup2(_fileno( test), 1 ) == -1 )
   {
      perror( "Can't _dup2 stdout" );
      exit( 1 );
   }
   
  /* if((freOpenstream = freopen(test, "w", stdout)) == NULL)
       exit(-1);
   */
    
   // wait of a  connection.
   while (!ConnectNamedPipe(hPipe, (LPOVERLAPPED) NULL)); printf("Client has connected\n");
   
  for(Buffer_out=0;; Buffer_out++)
  {
  // printf("sending %d\n", Buffer_out);
   WriteFile(hPipe,&Buffer_out, sizeof(Buffer_out),(LPDWORD) &count, NULL);
   printf("sending printf to pipe\n");
   Sleep(1000);
  }
 
    printf("press 'c' to quit\n");
    while( toupper(getchar()) != 'C');
    CloseHandle(hPipe);
 return 0;
}

the stdout = _fdopen(stream, "wt"); gave me a lvaue error. My problem here is that the printf("sending to pipe") which I thought would be sent to the pipe is not working, it's not going to the stdout  or to the pipe. 

so any suggestions?

with regards,

ehsan

Answer 5

On 6/19/2012 9:35 AM, ehsansad wrote:

My problem here is that the printf("sending to pipe") which I thought

would be sent to the pipe is not working, it's not going to the stdout or to the pipe.

I bet the output gets buffered by the run-time. There's some magic in the CRT that detects whether the stream is attached to the console (as opposed to, say, being redirected to a file), and automatically disables buffering.

Try calling fflush() after printf, and/or use setvbuf to suppress buffering.

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Igor Tandetnik

Answer 6

 

Hello,

 

For addition, you should use multithread for write and read name pipe. Or you can use a named Pipe client to read. Please check these sample

Multithreaded Pipe Server

http://msdn.microsoft.com/en-us/library/windows/desktop/aa365588(v=vs.85).aspx

Named Pipe Client

http://msdn.microsoft.com/en-us/library/windows/desktop/aa365592(v=vs.85).aspx

 

Best regards,

Jesse

---

Jesse Jiang [MSFT]
MSDN Community Support | Feedback to us

Answer 7

This is the changes to the code:

  // stdout now refers to file "test" 
   if(_dup2(_fileno( test), 1 ) == -1 )
   {
      perror( "Can't _dup2 stdout" );
      exit( 1 );
   }
   setvbuf(stdout, NULL, _IONBF, 0);
   
while (!ConnectNamedPipe(hPipe, (LPOVERLAPPED) NULL));   
  for(Buffer_out=0;; Buffer_out++)
  {
   printf("sending printf to pipe\n");
   fflush(NULL);

  }
 

and its working, the client is receiving the printf() string  in the for loop. NOW another question how safe is this code?

-ehsan

Answer 8

It looks safe because you are not using single buffer or pointer to array. More interesting than server would be client.

I assume that last 6 lines of code are meant to be nonsense in terms it would fill endlessly fill the pipe with test message. They are still safe.

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Answer 9

The client is a standard  client  

#include <windows.h>
#include <process.h>
#include <stdio.h>


HANDLE    hPipe;
const int BUFSIZE = 10;
int Buffer_in;
int       count;

int main(int argc, char* argv[])
{
 
  hPipe = CreateFile("\\\\.\\pipe\\ehsan", // this machine
  GENERIC_READ, 0,
  NULL, OPEN_EXISTING,
  FILE_ATTRIBUTE_NORMAL,
  (HANDLE) NULL);
  if(INVALID_HANDLE_VALUE == hPipe)
    {
     printf("Server Pipe not found\n");
     goto done;
    }
  else
     printf("Successful in finding server pipe\n");
    for (;;)
     {
      ReadFile(hPipe,(LPVOID) &Buffer_in, (DWORD) sizeof(Buffer_in), (LPDWORD) &count, (LPOVERLAPPED) NULL);	     	  	  
	  if (Buffer_in != '\n')
	     printf("%c", Buffer_in);
	  else
		 printf("\n");

     } 

  CloseHandle(hPipe);

return 0;

} 

Answer 10

On 6/20/2012 3:13 AM, ehsansad wrote:

NOW another question how safe is this code?

Safe from what threat? I suppose I don't understand the question.

---

Igor Tandetnik

Answer 11

  ReadFile(hPipe,(LPVOID) &Buffer_in, (DWORD) sizeof(Buffer_in),

is potential threat. Since you are not using any arrays, and you write fixed size that is determined from variable it self,  you can't cause buffer overrun. Your code seems safe from managed vs. native perspective.

  I hope I understood your question correctly.

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Answer 12

I was just thinking if the whole system will collapse or not, based on this  code.

Answer 13

On 6/21/2012 10:05 AM, ehsansad wrote:

I was just thinking if the whole system will collapse or not, based on this  code.

Highly unlikely. The worst that could happen is your process could crash.

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Igor Tandetnik

Answer 14

Thanks for the help.