Membuat Proses Anak dengan Input dan Output yang Dialihkan

Contoh dalam topik ini menunjukkan cara membuat proses anak menggunakan fungsi CreateProcess dari proses konsol . Ini juga menunjukkan teknik untuk menggunakan pipa anonim untuk mengalihkan handel input dan output standar proses anak. Perhatikan bahwa pipa bernama juga dapat digunakan untuk mengalihkan proses I/O.

Fungsi CreatePipe menggunakan struktur SECURITY_ATTRIBUTES untuk membuat handel yang dapat diwariskan ke ujung baca dan tulis dari dua pipa. Ujung baca dari satu pipa berfungsi sebagai input standar untuk proses anak, dan ujung tulis pipa lain adalah output standar untuk proses anak. Handel pipa ini ditentukan dalam struktur STARTUPINFO , yang menjadikannya handel standar yang diwarisi oleh proses anak.

Proses induk menggunakan ujung yang berlawanan dari kedua pipa ini untuk menulis ke input proses anak dan membaca dari output proses anak. Seperti yang ditentukan dalam struktur SECURITY_ATTRIBUTES , handel ini juga dapat diwariskan. Namun, handel ini tidak boleh diwariskan. Oleh karena itu, sebelum membuat proses anak, proses induk menggunakan fungsi SetHandleInformation untuk memastikan bahwa handel tulis untuk input standar proses anak dan handel baca untuk output standar proses anak tidak dapat diwariskan. Untuk informasi selengkapnya, lihat Pipa.

Berikut ini adalah kode untuk proses induk. Dibutuhkan argumen baris perintah tunggal: nama file teks.

#include <windows.h> 
#include <tchar.h>
#include <stdio.h> 
#include <strsafe.h>

#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(PCTSTR); 
 
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("child");
   PROCESS_INFORMATION piProcInfo; 
   STARTUPINFO siStartInfo;
   BOOL bSuccess = FALSE; 
 
// 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);
      
      // Close handles to the stdin and stdout pipes no longer needed by the child process.
      // If they are not explicitly closed, there is no way to recognize that the child process has ended.
      
      CloseHandle(g_hChildStd_OUT_Wr);
      CloseHandle(g_hChildStd_IN_Rd);
   }
}
 
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; 
   } 
 
// 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);

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

      bSuccess = WriteFile(hParentStdOut, chBuf, 
                           dwRead, &dwWritten, NULL);
      if (! bSuccess ) break; 
   } 
} 
 
void ErrorExit(PCTSTR 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);
}

Berikut ini adalah kode untuk proses anak. Ini menggunakan handel yang diwariskan untuk STDIN dan STDOUT untuk mengakses pipa yang dibuat oleh induk. Proses induk membaca dari file inputnya dan menulis informasi ke pipa. Anak menerima teks melalui pipa menggunakan STDIN dan menulis ke pipa menggunakan STDOUT. Induk membaca dari ujung baca pipa dan menampilkan informasi ke STDOUT-nya.

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

#define BUFSIZE 4096 
 
int main(void) 
{ 
   CHAR chBuf[BUFSIZE]; 
   DWORD dwRead, dwWritten; 
   HANDLE hStdin, hStdout; 
   BOOL bSuccess; 
 
   hStdout = GetStdHandle(STD_OUTPUT_HANDLE); 
   hStdin = GetStdHandle(STD_INPUT_HANDLE); 
   if ( 
       (hStdout == INVALID_HANDLE_VALUE) || 
       (hStdin == INVALID_HANDLE_VALUE) 
      ) 
      ExitProcess(1); 
 
   // Send something to this process's stdout using printf.
   printf("\n ** This is a message from the child process. ** \n");

   // This simple algorithm uses the existence of the pipes to control execution.
   // It relies on the pipe buffers to ensure that no data is lost.
   // Larger applications would use more advanced process control.

   for (;;) 
   { 
   // Read from standard input and stop on error or no data.
      bSuccess = ReadFile(hStdin, chBuf, BUFSIZE, &dwRead, NULL); 
      
      if (! bSuccess || dwRead == 0) 
         break; 
 
   // Write to standard output and stop on error.
      bSuccess = WriteFile(hStdout, chBuf, dwRead, &dwWritten, NULL); 
      
      if (! bSuccess) 
         break; 
   } 
   return 0;
}