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Apertura di un file per la lettura o la scrittura

La funzione CreateFile può creare un nuovo file o aprire un file esistente. È necessario specificare il nome del file, le istruzioni di creazione e altri attributi. Quando un'applicazione crea un nuovo file, il sistema operativo lo aggiunge alla directory specificata.

Esempio: aprire un file per la scrittura

Nell'esempio seguente viene usato CreateFile per creare un nuovo file e aprirlo per la scrittura e writeFile per scrivere una stringa semplice in modo sincrono nel file.

Una chiamata successiva per aprire questo file con CreateFile avrà esito negativo fino alla chiusura dell'handle.

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

void DisplayError(LPTSTR lpszFunction);

int __cdecl _tmain(int argc, TCHAR *argv[])
{
    HANDLE hFile; 
    char DataBuffer[] = "This is some test data to write to the file.";
    DWORD dwBytesToWrite = (DWORD)strlen(DataBuffer);
    DWORD dwBytesWritten = 0;
    BOOL bErrorFlag = FALSE;

    printf("\n");
    if( argc != 2 )
    {
        printf("Usage Error:\tIncorrect number of arguments\n\n");
        _tprintf(TEXT("%s <file_name>\n"), argv[0]);
        return;
    }

    hFile = CreateFile(argv[1],                // name of the write
                       GENERIC_WRITE,          // open for writing
                       0,                      // do not share
                       NULL,                   // default security
                       CREATE_NEW,             // create new file only
                       FILE_ATTRIBUTE_NORMAL,  // normal file
                       NULL);                  // no attr. template

    if (hFile == INVALID_HANDLE_VALUE) 
    { 
        DisplayError(TEXT("CreateFile"));
        _tprintf(TEXT("Terminal failure: Unable to open file \"%s\" for write.\n"), argv[1]);
        return;
    }

    _tprintf(TEXT("Writing %d bytes to %s.\n"), dwBytesToWrite, argv[1]);

    bErrorFlag = WriteFile( 
                    hFile,           // open file handle
                    DataBuffer,      // start of data to write
                    dwBytesToWrite,  // number of bytes to write
                    &dwBytesWritten, // number of bytes that were written
                    NULL);            // no overlapped structure

    if (FALSE == bErrorFlag)
    {
        DisplayError(TEXT("WriteFile"));
        printf("Terminal failure: Unable to write to file.\n");
    }
    else
    {
        if (dwBytesWritten != dwBytesToWrite)
        {
            // This is an error because a synchronous write that results in
            // success (WriteFile returns TRUE) should write all data as
            // requested. This would not necessarily be the case for
            // asynchronous writes.
            printf("Error: dwBytesWritten != dwBytesToWrite\n");
        }
        else
        {
            _tprintf(TEXT("Wrote %d bytes to %s successfully.\n"), dwBytesWritten, argv[1]);
        }
    }

    CloseHandle(hFile);
}

void DisplayError(LPTSTR lpszFunction) 
// Routine Description:
// Retrieve and output the system error message for the last-error code
{ 
    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) // account for format string
                            * sizeof(TCHAR) );
    
    if (FAILED( StringCchPrintf((LPTSTR)lpDisplayBuf, 
                     LocalSize(lpDisplayBuf) / sizeof(TCHAR),
                     TEXT("%s failed with error code %d as follows:\n%s"), 
                     lpszFunction, 
                     dw, 
                     lpMsgBuf)))
    {
        printf("FATAL ERROR: Unable to output error code.\n");
    }
    
    _tprintf(TEXT("ERROR: %s\n"), (LPCTSTR)lpDisplayBuf);

    LocalFree(lpMsgBuf);
    LocalFree(lpDisplayBuf);
}

Esempio: Aprire un file per la lettura

L'esempio seguente usa CreateFile per aprire un file esistente per la lettura e readFile per leggere fino a 80 caratteri in modo sincrono dal file.

In questo caso , CreateFile ha esito positivo solo se il file specificato esiste già nella directory corrente. Una chiamata successiva per aprire questo file con CreateFile avrà esito positivo se la chiamata usa le stesse modalità di accesso e condivisione.

Suggerimento: è possibile usare il file creato con l'esempio WriteFile precedente per testare questo esempio.

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

#define BUFFERSIZE 5
DWORD g_BytesTransferred = 0;

void DisplayError(LPTSTR lpszFunction);

VOID CALLBACK FileIOCompletionRoutine(
  __in  DWORD dwErrorCode,
  __in  DWORD dwNumberOfBytesTransfered,
  __in  LPOVERLAPPED lpOverlapped
);

VOID CALLBACK FileIOCompletionRoutine(
  __in  DWORD dwErrorCode,
  __in  DWORD dwNumberOfBytesTransfered,
  __in  LPOVERLAPPED lpOverlapped )
 {
  _tprintf(TEXT("Error code:\t%x\n"), dwErrorCode);
  _tprintf(TEXT("Number of bytes:\t%x\n"), dwNumberOfBytesTransfered);
  g_BytesTransferred = dwNumberOfBytesTransfered;
 }

//
// Note: this simplified sample assumes the file to read is an ANSI text file
// only for the purposes of output to the screen. CreateFile and ReadFile
// do not use parameters to differentiate between text and binary file types.
//

int __cdecl _tmain(int argc, TCHAR *argv[])
{
    HANDLE hFile; 
    DWORD  dwBytesRead = 0;
    char   ReadBuffer[BUFFERSIZE] = {0};
    OVERLAPPED ol = {0};

    printf("\n");
    if( argc != 2 )
    {
        printf("Usage Error: Incorrect number of arguments\n\n");
        _tprintf(TEXT("Usage:\n\t%s <text_file_name>\n"), argv[0]);
        return;
    }

    hFile = CreateFile(argv[1],               // file to open
                       GENERIC_READ,          // open for reading
                       FILE_SHARE_READ,       // share for reading
                       NULL,                  // default security
                       OPEN_EXISTING,         // existing file only
                       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, // normal file
                       NULL);                 // no attr. template
 
    if (hFile == INVALID_HANDLE_VALUE) 
    { 
        DisplayError(TEXT("CreateFile"));
        _tprintf(TEXT("Terminal failure: unable to open file \"%s\" for read.\n"), argv[1]);
        return; 
    }

    // Read one character less than the buffer size to save room for
    // the terminating NULL character. 

    if( FALSE == ReadFileEx(hFile, ReadBuffer, BUFFERSIZE-1, &ol, FileIOCompletionRoutine) )
    {
        DisplayError(TEXT("ReadFile"));
        printf("Terminal failure: Unable to read from file.\n GetLastError=%08x\n", GetLastError());
        CloseHandle(hFile);
        return;
    }
    SleepEx(5000, TRUE);
    dwBytesRead = g_BytesTransferred;
    // This is the section of code that assumes the file is ANSI text. 
    // Modify this block for other data types if needed.

    if (dwBytesRead > 0 && dwBytesRead <= BUFFERSIZE-1)
    {
        ReadBuffer[dwBytesRead]='\0'; // NULL character

        _tprintf(TEXT("Data read from %s (%d bytes): \n"), argv[1], dwBytesRead);
        printf("%s\n", ReadBuffer);
    }
    else if (dwBytesRead == 0)
    {
        _tprintf(TEXT("No data read from file %s\n"), argv[1]);
    }
    else
    {
        printf("\n ** Unexpected value for dwBytesRead ** \n");
    }

    // It is always good practice to close the open file handles even though
    // the app will exit here and clean up open handles anyway.
    
    CloseHandle(hFile);
}

void DisplayError(LPTSTR lpszFunction) 
// Routine Description:
// Retrieve and output the system error message for the last-error code
{ 
    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) // account for format string
                            * sizeof(TCHAR) );
    
    if (FAILED( StringCchPrintf((LPTSTR)lpDisplayBuf, 
                     LocalSize(lpDisplayBuf) / sizeof(TCHAR),
                     TEXT("%s failed with error code %d as follows:\n%s"), 
                     lpszFunction, 
                     dw, 
                     lpMsgBuf)))
    {
        printf("FATAL ERROR: Unable to output error code.\n");
    }
    
    _tprintf(TEXT("ERROR: %s\n"), (LPCTSTR)lpDisplayBuf);

    LocalFree(lpMsgBuf);
    LocalFree(lpDisplayBuf);
}

Creazione e apertura di file