Question

I run MSDN example for createprocess parent and child but it doesn’t work correctly,
becouse it writes the information to child input (what I’ve checked by printing it to the cmd), but after
printf( “\n->Contents of child process STDOUT:\n\n”);
when it calls

ReadFromPipe();

in the cmd from which the application was called I receive nothing but it waits, waits. and only if I do ctrl+c it ends.
what might be the reason?

#include "stdafx.h"
#include <windows.h> 
#include <tchar.h>
#include <stdio.h> 
#include <strsafe.h>
#include <iostream> //only for cout

#define BUFSIZE 4096 

HANDLE g_hChildStd_IN_Rd = NULL;
HANDLE g_hChildStd_IN_Wr = NULL;
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;

HANDLE g_hInputFile = NULL;

void CreateChildProcess(void); 
void WriteToPipe(void); 
void ReadFromPipe(void); 
void ErrorExit(PTSTR); 

int _tmain(int argc, TCHAR *argv[]) 
{ 
   SECURITY_ATTRIBUTES saAttr; 

   printf("\n->Start of parent execution.\n");

// Set the bInheritHandle flag so pipe handles are inherited. 

   saAttr.nLength = sizeof(SECURITY_ATTRIBUTES); 
   saAttr.bInheritHandle = TRUE; 
   saAttr.lpSecurityDescriptor = NULL; 

// Create a pipe for the child process's STDOUT. 

   if ( ! CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0) ) 
      ErrorExit(TEXT("StdoutRd CreatePipe")); 

// Ensure the read handle to the pipe for STDOUT is not inherited.

   if ( ! SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0) )
      ErrorExit(TEXT("Stdout SetHandleInformation")); 

// Create a pipe for the child process's STDIN. 

   if (! CreatePipe(&g_hChildStd_IN_Rd, &g_hChildStd_IN_Wr, &saAttr, 0)) 
      ErrorExit(TEXT("Stdin CreatePipe")); 

// Ensure the write handle to the pipe for STDIN is not inherited. 

   if ( ! SetHandleInformation(g_hChildStd_IN_Wr, HANDLE_FLAG_INHERIT, 0) )
      ErrorExit(TEXT("Stdin SetHandleInformation")); 

// Create the child process. 

   CreateChildProcess();

// Get a handle to an input file for the parent. 
// This example assumes a plain text file and uses string output to verify data flow. 

   if (argc == 1) 
      ErrorExit(TEXT("Please specify an input file.\n")); 

   g_hInputFile = CreateFile(
       argv[1], 
       GENERIC_READ, 
       0, 
       NULL, 
       OPEN_EXISTING, 
       FILE_ATTRIBUTE_READONLY, 
       NULL); 

   if ( g_hInputFile == INVALID_HANDLE_VALUE ) 
      ErrorExit(TEXT("CreateFile")); 

// Write to the pipe that is the standard input for a child process. 
// Data is written to the pipe's buffers, so it is not necessary to wait
// until the child process is running before writing data.

   WriteToPipe(); 
   printf( "\n->Contents of %s written to child STDIN pipe.\n", argv[1]);

// Read from pipe that is the standard output for child process. 

   printf( "\n->Contents of child process STDOUT:\n\n");
   ReadFromPipe(); 

   printf("\n->End of parent execution.\n");

// The remaining open handles are cleaned up when this process terminates. 
// To avoid resource leaks in a larger application, close handles explicitly. 

   return 0; 
} 

void CreateChildProcess()
// Create a child process that uses the previously created pipes for STDIN and STDOUT.
{ 
   TCHAR szCmdline[]=TEXT("C:\\Windows\\notepad.exe");//TEXT("C:\\Windows\\System32\\cmd.exe");
   PROCESS_INFORMATION piProcInfo; 
   STARTUPINFO siStartInfo;
   BOOL bSuccess = FALSE; 
    std::cout<<"\nCreateChildProcess.\n";
// Set up members of the PROCESS_INFORMATION structure. 

   ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );

// Set up members of the STARTUPINFO structure. 
// This structure specifies the STDIN and STDOUT handles for redirection.

   ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
   siStartInfo.cb = sizeof(STARTUPINFO); 
   siStartInfo.hStdError = g_hChildStd_OUT_Wr;
   siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
   siStartInfo.hStdInput = g_hChildStd_IN_Rd;
   siStartInfo.dwFlags |= STARTF_USESTDHANDLES;

// Create the child process. 

   bSuccess = CreateProcess(NULL, 
      szCmdline,     // command line 
      NULL,          // process security attributes 
      NULL,          // primary thread security attributes 
      TRUE,          // handles are inherited 
      0,             // creation flags 
      NULL,          // use parent's environment 
      NULL,          // use parent's current directory 
      &siStartInfo,  // STARTUPINFO pointer 
      &piProcInfo);  // receives PROCESS_INFORMATION 

   // If an error occurs, exit the application. 
   if ( ! bSuccess ) 
      ErrorExit(TEXT("CreateProcess"));
   else 
   {
      // Close handles to the child process and its primary thread.
      // Some applications might keep these handles to monitor the status
      // of the child process, for example. 

      CloseHandle(piProcInfo.hProcess);
      CloseHandle(piProcInfo.hThread);
   }
}

void WriteToPipe(void) 

// Read from a file and write its contents to the pipe for the child's STDIN.
// Stop when there is no more data. 
{ 
   DWORD dwRead, dwWritten; 
   CHAR chBuf[BUFSIZE];
   BOOL bSuccess = FALSE;

   for (;;) 
   { 
      bSuccess = ReadFile(g_hInputFile, chBuf, BUFSIZE, &dwRead, NULL);
      if ( ! bSuccess || dwRead == 0 ) break; 

      bSuccess = WriteFile(g_hChildStd_IN_Wr, chBuf, dwRead, &dwWritten, NULL);
      if ( ! bSuccess ) break; 
   } 
    std::cout<<"\nchBuf: "<<chBuf<<std::endl; 
// Close the pipe handle so the child process stops reading. 

   if ( ! CloseHandle(g_hChildStd_IN_Wr) ) 
      ErrorExit(TEXT("StdInWr CloseHandle")); 
} 

void ReadFromPipe(void) 

// Read output from the child process's pipe for STDOUT
// and write to the parent process's pipe for STDOUT. 
// Stop when there is no more data. 
{ 
   DWORD dwRead, dwWritten; 
   CHAR chBuf[BUFSIZE]; 
   BOOL bSuccess = FALSE;
   HANDLE hParentStdOut = GetStdHandle(STD_OUTPUT_HANDLE);

        bSuccess = ReadFile( g_hChildStd_OUT_Rd, chBuf, 4, &dwRead, NULL); 
        std::cout<<"\nchBuf: "<<chBuf<<std::endl;

   /*for (int i=0;i<2;i++) //read twice
   { 
      bSuccess = ReadFile( g_hChildStd_OUT_Rd, chBuf, BUFSIZE, &dwRead, NULL);
      if( ! bSuccess || dwRead == 0 ) break; 
      std::cout<<"\nchBuf: "<<chBuf<<std::endl;

      bSuccess = WriteFile(hParentStdOut, chBuf, 
                           dwRead, &dwWritten, NULL);
      if (! bSuccess ) break;
   } */

} 

void ErrorExit(PTSTR lpszFunction) 

// Format a readable error message, display a message box, 
// and exit from the application.
{ 
    LPVOID lpMsgBuf;
    LPVOID lpDisplayBuf;
    DWORD dw = GetLastError(); 

    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | 
        FORMAT_MESSAGE_FROM_SYSTEM |
        FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        dw,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR) &lpMsgBuf,
        0, NULL );

    lpDisplayBuf = (LPVOID)LocalAlloc(LMEM_ZEROINIT, 
        (lstrlen((LPCTSTR)lpMsgBuf)+lstrlen((LPCTSTR)lpszFunction)+40)*sizeof(TCHAR)); 
    StringCchPrintf((LPTSTR)lpDisplayBuf, 
        LocalSize(lpDisplayBuf) / sizeof(TCHAR),
        TEXT("%s failed with error %d: %s"), 
        lpszFunction, dw, lpMsgBuf); 
    MessageBox(NULL, (LPCTSTR)lpDisplayBuf, TEXT("Error"), MB_OK); 

    LocalFree(lpMsgBuf);
    LocalFree(lpDisplayBuf);
    ExitProcess(1);
}

Answer 1

The reason is that you are starting a GUI application, notepad.exe, which will never write to stdout. So stdout is sitting open and has nothing written to it so the call to ReadFile blocks waiting for something to show up. Switch back to cmd.exe which you have commented out and you’ll get output when from your ReadFromPipe call.

Also your ReadFromPipe function is just going to mostly output junk. To test it I changed it to:

void ReadFromPipe(void) 

// Read output from the child process's pipe for STDOUT
// and write to the parent process's pipe for STDOUT. 
// Stop when there is no more data. 
{ 
   DWORD dwRead;
   CHAR chBuf[BUFSIZE]; 
   BOOL bSuccess = TRUE;

   do
   {
        dwRead = 0;
        bSuccess = ReadFile( g_hChildStd_OUT_Rd, chBuf, BUFSIZE-1, &dwRead, NULL); 
        chBuf[dwRead] = '\0';
        std::cout<<"\nchBuf: "<<chBuf<<std::endl;
   } while (bSuccess && dwRead > 0);
}