如何在win32中的文件上附加数据

Question

我已经搜索过这样做，但我找不到我做错了什么。我试图让这个函数在每次调用时都附加数据，但它总是执行一次。如果文件不存在，它会创建一个新文件并只在文件上写入一次，如果文件存在，它什么也不做（或者可能覆盖）

void WriteToFile (char data[],wchar_t filename[] )
{
    HANDLE hFile;
    DWORD dwBytesToWrite = (DWORD)strlen(data);
    DWORD dwBytesWritten ;
    BOOL bErrorFlag = FALSE;


    hFile = CreateFile((LPCWSTR)filename,            // 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"), filename);
        return;
    }


    bErrorFlag = WriteFile(
    hFile,              // open file handle
    data,               // 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, filename);
    }
}

CloseHandle(hFile);
}

这就是我在WM_COMMAND中调用函数的地方

//When a menu item selected execute this code
case IDM_FILE_SAVE:
        saveBool = true;
        char Str[] = "this is my own data";
        wchar_t filename[] = L"data.txt";
        WriteToFile(Str, filename);
        break;

Answer 1

如果文件存在，它什么也不做

应该如此。根据CreateFile() 文档：

CREATE_NEW
1
创建一个新文件，仅当它不存在时。

如果指定的文件存在，则函数失败，最后的错误代码设置为 ERROR_FILE_EXISTS (80)。

如果指定的文件不存在并且是可写位置的有效路径，则创建一个新文件。

对于您尝试做的事情，请改用OPEN_ALWAYS：

常开
4
总是打开一个文件。

如果指定的文件存在，则函数成功，最后的错误代码设置为 ERROR_ALREADY_EXISTS (183)。

如果指定的文件不存在并且是可写位置的有效路径，则该函数创建一个文件并将最后一个错误代码设置为零。

您可以使用FILE_APPEND_DATA 访问说明符让CreateFile() 在创建/打开文件后自动查找到文件末尾（否则，您必须使用SetFilePointer/Ex() 手动查找），然后再将新数据写入文件。

试试这个：

void WriteToFile (char *data, wchar_t *filename)
{
    HANDLE hFile;
    DWORD dwBytesToWrite = strlen(data);
    DWORD dwBytesWritten ;
    BOOL bErrorFlag = FALSE;

    hFile = CreateFileW(filename,  // name of the write
        FILE_APPEND_DATA,          // open for appending
        FILE_SHARE_READ,           // share for reading only
        NULL,                      // default security
        OPEN_ALWAYS,               // open existing file or create new file 
        FILE_ATTRIBUTE_NORMAL,     // normal file
        NULL);                     // no attr. template

    if (hFile == INVALID_HANDLE_VALUE)
    {
        DisplayError(TEXT("CreateFile"));
        wprintf(L"Terminal failure: Unable to create/open file \"%s\" for writing.\n", filename);
        return;
    }

    while (dwBytesToWrite > 0)
    {
        bErrorFlag = WriteFile(
            hFile,              // open file handle
            data,               // start of data to write
            dwBytesToWrite,     // number of bytes to write
            &dwBytesWritten,    // number of bytes that were written
            NULL);              // no overlapped structure

        if (!bErrorFlag)
        {
            DisplayError(TEXT("WriteFile"));
            printf("Terminal failure: Unable to write to file.\n");
            break;
        }

        wprintf(L"Wrote %u bytes to \"%s\" successfully.\n", dwBytesWritten, filename);

        data += dwBytesWritten;
        dwBytesToWrite -= dwBytesWritten;
    }

    CloseHandle(hFile);
}