c++ - 如何将文本附加到 Windows 中的文件？

Question

每次调用此函数时，旧文本数据都会丢失？？告诉我如何维护以前的数据和附加新数据。

这个函数被调用了 10 次:

void WriteEvent(LPWSTR pRenderedContent)
{
    HANDLE hFile; 
    DWORD dwBytesToWrite = ((DWORD)wcslen(pRenderedContent)*2);
    DWORD dwBytesWritten = 0;
    BOOL bErrorFlag = FALSE;

    printf("\n");

    hFile = CreateFile(L"D:\\EventsLog.txt", FILE_ALL_ACCESS, 0, NULL, OPEN_ALWAYS,   FILE_ATTRIBUTE_NORMAL, NULL);                 

    if (hFile == INVALID_HANDLE_VALUE) 
    { 
        printf("Terminal failure: Unable to open file \"EventsLog.txt\" for write.\n");
        return;
    }

    printf("Writing %d bytes to EventsLog.txt.\n", dwBytesToWrite);

    bErrorFlag = WriteFile( 
                    hFile,                // open file handle
                    pRenderedContent,      // 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)
    {
        printf("Terminal failure: Unable to write to file.\n");
    }
    else
    {
        if (dwBytesWritten != dwBytesToWrite)
        {
            printf("Error: dwBytesWritten != dwBytesToWrite\n");
        }
        else
        {
            printf("Wrote %d bytes to EventsLog.txt successfully.\n",dwBytesWritten);
        }
    }

    CloseHandle(hFile);
}

Answer 1

您应该将 FILE_APPEND_DATA 作为 dwDesiredAccess 传递给 CreateFile ，如 File Access Rights Constants 下所述(参见 Appending One File to Another File 处的示例代码)。虽然这会使用正确的访问权限打开文件，但您的代码仍负责设置 file pointer .这是必要的，因为:

Each time a file is opened, the system places the file pointer at the beginning of the file, which is offset zero.

可以使用 SetFilePointer 设置文件指针打开文件后的API:

hFile = CreateFile( L"D:\\EventsLog.txt", FILE_APPEND_DATA, 0x0, nullptr,
                    OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, nullptr );
if ( hFile == INVALID_HANDLE_VALUE ) {
    printf( "Terminal failure: Unable to open file \"EventsLog.txt\" for write.\n" );
    return;
}

// Set the file pointer to the end-of-file:
DWORD dwMoved = ::SetFilePointer( hFile, 0l, nullptr, FILE_END );
if ( dwMoved == INVALID_SET_FILE_POINTER ) {
    printf( "Terminal failure: Unable to set file pointer to end-of-file.\n" );
    return;
}

printf("Writing %d bytes to EventsLog.txt.\n", dwBytesToWrite);

bErrorFlag = WriteFile( // ...

---

与您的问题无关， dwBytesToWrite 的计算不应使用魔数(Magic Number)。您可能应该编写 * sizeof(*pRenderedContent) 而不是 * 2。 WriteEvent 的参数也应该是常量:

WriteEvent(LPCWSTR pRenderedContent)