Re: [Mingw-users] [OT] Reading from Win32 pipes

[Manu <ma...@wa...>]

Alberto Luaces wrote:

> Hello, I'm having problems when using win32 pipes. I'm using them to run 
> programs as 'sed' or 'gawk' and read the results from my program. The code:
> 
> #include <stdio.h>
> #include <windows.h>
> 
> HANDLE OpenPipe(char *,HANDLE *);
> void ClosePipe(void);
> 
> int i=0;
> char buffer[1024];
> 
> int main(){
> DWORD read;
> HANDLE hProcess, hRead;
> 
> hRead=OpenPipe("gawk '/^[0-9\\-]/{print $1,$2}' 
> at020304_No3_n2.txt",&hProcess);
> file://WaitForSingleObject(hProcess,INFINITE); 
> 
> do{
> ReadFile(hRead,buffer,sizeof(buffer)-1,&read,0);
> 
> printf("Size of buffer - %d\n",sizeof(buffer));
> printf("Bytes read - %i\nTimes - %i\n",read,i++);
> 
> buffer[read]=0; /* Zero-terminated string */
> 
> puts(buffer);
> }while (read==sizeof(buffer)-1);
> 
> ClosePipe();
> 
> return 0;
> 
> }
> 
> My OpenPipe() and ClosePipe() functions are implemented with CreatePipe() 
> and CreateProcess(), feeding the pipe with handle to standard output of 
> the child process and returning the read handle and the process handle.
> 
> At first I placed the WaitForSingleObject(hProcess,INFINITE) call, but it 
> never returned, I wonder why. From what I read in the documentation, 
> ReadFile() waits until something is written to the pipe, so I didn't place 
> another WaitForSingleObject(hRead,INFINITE) call.
> 
> Am I doing anything wrong? How can I know if the last read size fits 
> exactly in the buffer or there is more data to read? I tried at first 
> something like this:
> 
> do{
> /*read the pipe*/
> }while(read!=0);
> 
> but ReadFile() never returned as the pipe was empty from the last call.


Well, I can share a bit of my experience, this is how I proceed in
Visual-MinGW:
------
* src/libtemp/process/childprs.cpp (CChildProcess::Run)
 // simple process without IO redirection.
 if (!_redirect){
  
  // wait for this process.
  ::WaitForSingleObject(pi.hProcess, INFINITE);
  ::GetExitCodeProcess(pi.hProcess, &exitCode);
 
 // process with IO redirection.
 }else{
  
  // perform IO.
  for (;;){
   Sleep(100L);
   
   // read std out.
   _outPipe.Read(_outBuffer);
   ProcessStdOut();
   
   // read std error.
   _errPipe.Read(_errBuffer);
   ProcessStdErr();
   
   // still running ?
   ::GetExitCodeProcess(pi.hProcess, &exitCode);
   if (exitCode != STILL_ACTIVE){
    break;
   }else if (_abort){
    ::TerminateProcess(pi.hProcess, 1);
    break;
   }
  }
  
 }
 
-----
* src/libtemp/process/pipe.cpp (CPipe::Read)
 int CPipe::Read(COutBuffer &buffer){
 /* Read all available bytes from the pipe into a buffer, then output
 * each '\n' terminated line. Non-terminated ones remains first in buffer,
 * waiting for more bytes.
 */

 if (!hRead)
  return 0;

 // get the total number of bytes available from the pipe.
 DWORD avail = 0;
 while (_peek(&avail)){
  DWORD bytes_read = 0;
  // seek the buffer, read all bytes, update the buffer's state.
  if (!_read(buffer, avail, &bytes_read))
   return 0;
  
  return 0;
 } // nothing to read.
 return 0;
} 

CPipe::_peek() calls PeekNamedPipe to know if we have
something to read, then CPipe::_read() fills the output buffer
(COutBuffer).

You can freely use the code in /libtemp, it is currently GPL'ed,
but it will be moved into WinUI library soon, then it will be switched
to public domain. As the copyright holder, I give you acknowledgement
to use it in your app, in case it is helpfull.
http://cvs.sourceforge.net/viewcvs.py/visual-mingw/visual-mingw/src/libtemp/

A Google search with "PeekNamedPipe", "ReadFile", etc... will give
you much more inputs.

Enjoy!

Manu.