Creating a Child Process with Redirected Input and Output [펌 msdn]

Creating a Child Process with Redirected Input and Output

The example in this topic demonstrates how to create a child process using the CreateProcess function from a console process. It also demonstrates a technique for using anonymous pipes to redirect the child process's standard input and output handles. Note that named pipes can also be used to redirect process I/O.

The CreatePipe function uses the SECURITY_ATTRIBUTES structure to create inheritable handles to the read and write ends of two pipes. The read end of one pipe serves as standard input for the child process, and the write end of the other pipe is the standard output for the child process. These pipe handles are specified in the STARTUPINFO structure, which makes them the standard handles inherited by the child process.

The parent process uses the opposite ends of these two pipes to write to the child process's input and read from the child process's output. As specified in the STARTUPINFO structure, these handles are also inheritable. However, these handles must not be inherited. Therefore, before creating the child process, the parent process uses the SetHandleInformation function to ensure that the write handle for the child process's standard input and the read handle for the child process's standard input cannot be inherited. For more information, see Pipes.

The following is the code for the parent process. It takes a single command-line argument: the name of a text file.

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#include <windows.h> 
#include <tchar.h>
#include <stdio.h> 
 
#define BUFSIZE 4096 
 
HANDLE hChildStdinRd, hChildStdinWr,  
   hChildStdoutRd, hChildStdoutWr, 
   hInputFile, hStdout;
 
BOOL CreateChildProcess(VOID); 
VOID WriteToPipe(VOID); 
VOID ReadFromPipe(VOID); 
VOID ErrorExit(LPSTR); 
 
int _tmain(int argc, TCHAR *argv[]) 
{ 
   SECURITY_ATTRIBUTES saAttr; 
   BOOL fSuccess; 
 
// Set the bInheritHandle flag so pipe handles are inherited. 
 
   saAttr.nLength = sizeof(SECURITY_ATTRIBUTES); 
   saAttr.bInheritHandle = TRUE; 
   saAttr.lpSecurityDescriptor = NULL; 

// Get the handle to the current STDOUT. 
 
   hStdout = GetStdHandle(STD_OUTPUT_HANDLE); 
 
// Create a pipe for the child process's STDOUT. 
 
   if (! CreatePipe(&hChildStdoutRd, &hChildStdoutWr, &saAttr, 0)) 
      ErrorExit("Stdout pipe creation failed\n"); 

// Ensure that the read handle to the child process's pipe for STDOUT is not inherited.

   SetHandleInformation( hChildStdoutRd, HANDLE_FLAG_INHERIT, 0);

// Create a pipe for the child process's STDIN. 
 
   if (! CreatePipe(&hChildStdinRd, &hChildStdinWr, &saAttr, 0)) 
      ErrorExit("Stdin pipe creation failed\n"); 

// Ensure that the write handle to the child process's pipe for STDIN is not inherited. 
 
   SetHandleInformation( hChildStdinWr, HANDLE_FLAG_INHERIT, 0);
 
// Now create the child process. 
   
   fSuccess = CreateChildProcess();
   if (! fSuccess) 
      ErrorExit("Create process failed with"); 

// Get a handle to the parent's input file. 
 
   if (argc == 1) 
      ErrorExit("Please specify an input file"); 

   printf( "\nContents of %s:\n\n", argv[1]);

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

   if (hInputFile == INVALID_HANDLE_VALUE) 
      ErrorExit("CreateFile failed"); 
 
// Write to pipe that is the standard input for a child process. 
 
   WriteToPipe(); 
 
// Read from pipe that is the standard output for child process. 
 
   ReadFromPipe(); 
 
   return 0; 
} 
 
BOOL CreateChildProcess() 
{ 
   TCHAR szCmdline[]=TEXT("child");
   PROCESS_INFORMATION piProcInfo; 
   STARTUPINFO siStartInfo;
   BOOL bFuncRetn = FALSE; 
 
// Set up members of the PROCESS_INFORMATION structure. 
 
   ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );
 
// Set up members of the STARTUPINFO structure. 
 
   ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
   siStartInfo.cb = sizeof(STARTUPINFO); 
   siStartInfo.hStdError = hChildStdoutWr;
   siStartInfo.hStdOutput = hChildStdoutWr;
   siStartInfo.hStdInput = hChildStdinRd;
   siStartInfo.dwFlags |= STARTF_USESTDHANDLES;
 
// Create the child process. 
    
   bFuncRetn = 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 (bFuncRetn == 0) 
      ErrorExit("CreateProcess failed\n");
   else 
   {
      CloseHandle(piProcInfo.hProcess);
      CloseHandle(piProcInfo.hThread);
      return bFuncRetn;
   }
}
 
VOID WriteToPipe(VOID) 
{ 
   DWORD dwRead, dwWritten; 
   CHAR chBuf[BUFSIZE]; 
 
// Read from a file and write its contents to a pipe. 
 
   for (;;) 
   { 
      if (! ReadFile(hInputFile, chBuf, BUFSIZE, &dwRead, NULL) || 
         dwRead == 0) break; 
      if (! WriteFile(hChildStdinWr, chBuf, dwRead, 
         &dwWritten, NULL)) break; 
   } 
 
// Close the pipe handle so the child process stops reading. 
 
   if (! CloseHandle(hChildStdinWr)) 
      ErrorExit("Close pipe failed\n"); 
} 
 
VOID ReadFromPipe(VOID) 
{ 
   DWORD dwRead, dwWritten; 
   CHAR chBuf[BUFSIZE]; 

// Close the write end of the pipe before reading from the 
// read end of the pipe. 
 
   if (!CloseHandle(hChildStdoutWr)) 
      ErrorExit("Closing handle failed"); 
 
// Read output from the child process, and write to parent's STDOUT. 
 
   for (;;) 
   { 
      if( !ReadFile( hChildStdoutRd, chBuf, BUFSIZE, &dwRead, 
         NULL) || dwRead == 0) break; 
      if (! WriteFile(hStdout, chBuf, dwRead, &dwWritten, NULL)) 
         break; 
   } 
} 
 
VOID ErrorExit (LPSTR lpszMessage) 
{ 
   fprintf(stderr, "%s\n", lpszMessage); 
   ExitProcess(0); 
}

The following is the code for the child process. It uses the inherited handles for STDIN and STDOUT to access the pipe created by the parent. The parent process reads from its input file and writes the information to a pipe. The child receives text through the pipe using STDIN and writes to the pipe using STDOUT. The parent reads from the read end of the pipe and displays the information to its STDOUT.

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#include <windows.h> 

#define BUFSIZE 4096 
 
VOID main(VOID) 
{ 
   CHAR chBuf[BUFSIZE]; 
   DWORD dwRead, dwWritten; 
   HANDLE hStdin, hStdout; 
   BOOL fSuccess; 
 
   hStdout = GetStdHandle(STD_OUTPUT_HANDLE); 
   hStdin = GetStdHandle(STD_INPUT_HANDLE); 
   if ((hStdout == INVALID_HANDLE_VALUE) || 
      (hStdin == INVALID_HANDLE_VALUE)) 
      ExitProcess(1); 
 
   for (;;) 
   { 
   // Read from standard input. 
      fSuccess = ReadFile(hStdin, chBuf, BUFSIZE, &dwRead, NULL); 
      if (! fSuccess || dwRead == 0) 
         break; 
 
   // Write to standard output. 
      fSuccess = WriteFile(hStdout, chBuf, dwRead, &dwWritten, NULL); 
      if (! fSuccess) 
         break; 
   } 
}