Question

There are few question from the following code which i came across here:

// RawDataToPrinter - sends binary data directly to a printer 
//  
// szPrinterName: NULL-terminated string specifying printer name 
// lpData:        Pointer to raw data bytes 
// dwCount        Length of lpData in bytes 
//  
// Returns: TRUE for success, FALSE for failure. 
//  
BOOL RawDataToPrinter(LPTSTR szPrinterName, LPBYTE lpData, DWORD dwCount)
{
    BOOL     bStatus = FALSE;
    HANDLE     hPrinter = NULL;
    DOC_INFO_1 DocInfo;
    DWORD      dwJob = 0L;
    DWORD      dwBytesWritten = 0L;

    // Open a handle to the printer. 
    bStatus = OpenPrinter( szPrinterName, &hPrinter, NULL );  // question 1
    if (bStatus) {
        // Fill in the structure with info about this "document." 
        DocInfo.pDocName = (LPTSTR)_T("My Document");  // question 2
        DocInfo.pOutputFile = NULL;                 // question 3
        DocInfo.pDatatype = (LPTSTR)_T("RAW");   // question 4

        // Inform the spooler the document is beginning. 
        dwJob = StartDocPrinter( hPrinter, 1, (LPBYTE)&DocInfo );  // question 5
        if (dwJob > 0) {
            // Start a page. 
            bStatus = StartPagePrinter( hPrinter );
            if (bStatus) {
                // Send the data to the printer. 
                bStatus = WritePrinter( hPrinter, lpData, dwCount, &dwBytesWritten);
                EndPagePrinter (hPrinter);
            }
            // Inform the spooler that the document is ending. 
            EndDocPrinter( hPrinter );
        }
        // Close the printer handle. 
        ClosePrinter( hPrinter );
    }
    // Check to see if correct number of bytes were written. 
    if (!bStatus || (dwBytesWritten != dwCount)) {
        bStatus = FALSE;
    } else {
        bStatus = TRUE;
    }
    return bStatus;
}

Please look for the question number in comments

Question 1

• when i have set hPrinter = null then what is meant by &hPrinter ?

Question 2

• what does (LPTSTR)_T denote ? (see T )

Question 3

• What does null here denote ?

Question 4

• What does RAW mean ( what type of data it is ? ) ?

Question 5

• What does 1 denote here ?

Answer 1

1. &hPrinter is the address of the hPrinter variable. You need to pass it so the OpenPrinter function can write in it the actual handle to the printer.

2. _T takes your character string and converts it to the proper ANSI (8bits/character) or Unicode (16bits/character) representation based on your compilation settings. Basically it’s a macro that either expands to nothing (ANSI) or L (Unicode) plus the actual string.

3. Here where, in the code? NULL is a macro that expands to (basically) 0. It’s used to provide a decent default value.

4. Raw data is exactly what it sounds, raw data. It’s not structured in any way, it’s just a soup of bytes you pass to a (usually) device.

5. From http://msdn.microsoft.com/en-us/library/dd145115(v=vs.85).aspx:

   The version of the structure to which pDocInfo points. This value must be 1.