Getting the status of the selected printer from Visual Basic
Shows how you can use the Windows API to return additional information about the printer above and beyond that which is available through the Visual Basic Printer object.
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Codice sorgente
<p align=left><font face="Arial" style="BACKGROUND-COLOR: yellow"> 1. What the <i>Printer</i> object missed</font></p>
<p align=justify> <font face="Arial" >Printing has long been a very problematic part of developing complete
and professional applications in Visual Basic. This was redressed to a large degree with the new <i>Printer</i>
object introduced in Visual Basic 4.<br>
However, there are shortcomings with this object. The biggest shortcoming is that you cannot find out whether
the printer is ready, busy, out of paper etc. from your application.<br>
However, there is an API call, <i>GetPrinter</i> which returns a great deal more information about a printer.
</font></p>
<!-- API Declaration -->
<p align=left class="sourcecode">
<font class="keyword">
Private Declare Function</font> GetPrinterApi <font class="keyword">Lib</font> "winspool.drv" <font class="keyword">Alias</font> _ <br>
"GetPrinterA" <font class="keyword">(ByVal</font> hPrinter <font class="keyword">As Long,</font> _ <br>
<font class="keyword">ByVal</font> Level <font class="keyword">As Long,</font> _ <br>
buffer <font class="keyword">As Long,</font> _ <br>
<font class="keyword">ByVal</font> pbSize <font class="keyword">As Long,</font> _ <br>
pbSizeNeeded <font class="keyword">As Long) As Long
</font>
</p>
<p align=justify> <font face="Arial" >This takes the handle to a printer in <i>hPrinter</i> and fills
the buffer provided to it with information from the printer driver. To get the handle from the Printer object,
you need to use the <i>OpenPrinter</i> API call. <br>
This handle must be released using the <i>ClosePrinter</i> API call as soon as you are finished with it.
</font></p>
<!-- API Declaration -->
<p align=left class="sourcecode">
<font class="keyword">
Private Type</font> PRINTER_DEFAULTS <br>
pDatatype <font class="keyword">As String</font> <br>
pDevMode <font class="keyword">As DEVMODE</font> <br>
DesiredAccess <font class="keyword">As Long</font> <br>
<font class="keyword">End Type</font> <br>
<br>
<font class="keyword">Private Declare Function</font> OpenPrinter <font class="keyword">Lib</font> "winspool.drv" _ <br>
<font class="keyword">Alias</font> "OpenPrinterA" <font class="keyword">(ByVal</font> pPrinterName <font class="keyword">As String,</font> _ <br>
phPrinter <font class="keyword">As Long</font>, pDefault <font class="keyword">As</font> PRINTER_DEFAULTS) As Long <br>
<br>
<font class="keyword">Private Declare Function</font> ClosePrinter <font class="keyword">Lib</font> "winspool.drv" _ <br>
<font class="keyword">(ByVal</font> hPrinter <font class="keyword">As Long) As Long</font> <br>
</p>
<p align=justify> <font face="Arial" >You pass the Printer.DeviceName to this to get the handle.
</font></p>
<!-- Use -->
<p align=left class="sourcecode">
<font class="keyword">
Dim</font> lret <font class="keyword">As Long</font> <br>
<font class="keyword">Dim</font> pDef <font class="keyword">As</font> PRINTER_DEFAULTS <br>
<br>
lret = OpenPrinter(<font class="keyword">Printer.DeviceName</font>, mhPrinter, pDef)
</font>
</p>
<p align=left><font face="Arial" style="BACKGROUND-COLOR: yellow"> 2. The different statuses</font></p>
<p align=justify> <font face="Arial" >There are a number of standard statuses that can be returned by the
printer driver.
</font></p>
<!-- Enumerated type -->
<p align=left class="sourcecode">
<font class="keyword">
Public Enum</font> <a href="http://www.merrioncomputing.com/EventVB/Printer_Status.html">Printer_Status</a> <br>
PRINTER_STATUS_READY = &H0 <br>
PRINTER_STATUS_PAUSED = &H1 <br>
PRINTER_STATUS_ERROR = &H2 <br> PRINTER_STATUS_PENDING_DELETION =
&H4 <br>
PRINTER_STATUS_PAPER_JAM = &H8 <br> PRINTER_STATUS_PAPER_OUT =
&H10 <br> PRINTER_STATUS_MANUAL_FEED =
&H20 <br> PRINTER_STATUS_PAPER_PROBLEM =
&H40 <br>
PRINTER_STATUS_OFFLINE = &H80 <br> PRINTER_STATUS_IO_ACTIVE =
&H100 <br> PRINTER_STATUS_BUSY =
&H200 <br>
PRINTER_STATUS_PRINTING = &H400 <br>
PRINTER_STATUS_OUTPUT_BIN_FULL = &H800 <br> PRINTER_STATUS_NOT_AVAILABLE =
&H1000 <br>
PRINTER_STATUS_WAITING = &H2000 <br> PRINTER_STATUS_PROCESSING =
&H4000 <br> PRINTER_STATUS_INITIALIZING =
&H8000 <br> PRINTER_STATUS_WARMING_UP =
&H10000 <br> PRINTER_STATUS_TONER_LOW =
&H20000 <br> PRINTER_STATUS_NO_TONER =
&H40000 <br> PRINTER_STATUS_PAGE_PUNT =
&H80000 <br> PRINTER_STATUS_USER_INTERVENTION =
&H100000 <br> PRINTER_STATUS_OUT_OF_MEMORY =
&H200000 <br> PRINTER_STATUS_DOOR_OPEN =
&H400000 <br> PRINTER_STATUS_SERVER_UNKNOWN =
&H800000 <br> PRINTER_STATUS_POWER_SAVE =
&H1000000 <br>
<font class="keyword">End Enum
</font>
</p>
<p align=left><font face="Arial" style="BACKGROUND-COLOR: yellow"> 3. The data structures</font></p>
<p align=justify>
<font face=Arial >
As each printer driver is responsible for returning
this data there has to be a standard to which this returned data conforms
in order for one application to be able to query a number of different
types of printers. As it happens, there are nine different standard data
types that can be returned by the <EM>GetPrinter</EM>
API call in Windows 2000 (only the first two are universal to all current versions of Windows). <br>
Of these, the second is the most interesting - named PRINTER_INFO_2
</font>
<!-- Data structure -->
<p align=left class="sourcecode">
<font class="keyword">
Private Type</font> PRINTER_INFO_2 <br>
pServerName <font class="keyword">As String</font> <br>
pPrinterName <font class="keyword">As String</font> <br>
pShareName <font class="keyword">As String</font> <br>
pPortName <font class="keyword">As String</font> <br>
pDriverName <font class="keyword">As String</font> <br>
pComment <font class="keyword">As String</font> <br>
pLocation <font class="keyword">As String</font> <br>
pDevMode <font class="keyword">As Long</font> <br>
pSepFile <font class="keyword">As String</font> <br>
pPrintProcessor <font class="keyword">As String</font> <br>
pDatatype <font class="keyword">As String</font> <br>
pParameters <font class="keyword">As String</font> <br>
pSecurityDescriptor <font class="keyword">As Long</font> <br>
Attributes <font class="keyword">As Long</font> <br>
Priority <font class="keyword">As Long</font> <br>
DefaultPriority <font class="keyword">As Long</font> <br>
StartTime <font class="keyword">As Long</font> <br>
UntilTime <font class="keyword">As Long</font> <br>
Status <font class="keyword">As Long</font> <br>
JobsCount <font class="keyword">As Long</font> <br>
AveragePPM <font class="keyword">As Long</font> <br>
<font class="keyword">End Type
</font>
</p>
<p align=justify>
<font face=Arial >
However, it is not as simple as just passing this structure to the <i>GetPrinter</i> API call as a printer can
return more information than is in that structure and if you do not allocate sufficent buffer space for it to
do so your application will crash. <br>
Fortunately the API call caters for this - if you pass zero in the <i>pbSize</i> parameter then the API call will
tell you how big a buffer you will require in the <i>pbSizeNeeded</i>. <br>
This means that filling the information from the printer driver becomes a two step process:
</font>
</p>
<!-- Using the GetPrinter API call -->
<p align=left class="sourcecode"> <font class="keyword">
Dim</font> lret <font class="keyword">As Long</font> <br>
<font class="keyword">Dim</font> SizeNeeded <font class="keyword">As Long</font><br>
<br>
<font class="keyword">Dim</font> buffer() <font class="keyword">As Long</font><br>
<br>
<font class="keyword">ReDim Preserve</font> buffer(0 To 1) <font class="keyword">As Long</font> <br>
lret = GetPrinterApi(mhPrinter, Index, buffer(0), UBound(buffer), SizeNeeded) <br>
<font class="keyword">ReDim Preserve</font> buffer(0 To (SizeNeeded / 4) + 3) <font class="keyword">As Long</font> <br>
lret = GetPrinterApi(mhPrinter, Index, buffer(0), UBound(buffer) * 4, SizeNeeded) <br>
</p>
<!-- Retrieving the string part -->
<p align=justify>
<font face=Arial >
However the buffer is just an array of <i>Long</i> data types. Some of the data within the PRINTER_INFO_2
data structure is String data. This must be collected from the addresses which are stored in the appropriate
buffer position.
</font>
</p><p align=justify>
<font face=Arial >
To get a string from a pointer the <i>CopyMemory</i> API call is used and there is also an API call,
<i>IsBadStringPtr</i>, which can be used to verify that the address pointed to does actually contain a valid
string.
</font>
</p>
<!-- Declarations -->
<p align=left class="sourcecode">
<font class="comment">
'\\ Memory manipulation routines </font><br>
<font class="keyword">Private Declare Sub</font> CopyMemory <font class="keyword">Lib</font> "kernel32" <font class="keyword">Alias</font> "RtlMoveMemory" (Destination <font class="keyword">As Any,</font> Source <font class="keyword">As Any, ByVal</font> Length <font class="keyword">As Long)</font> <br>
<font class="comment">'\\ Pointer validation in StringFromPointer </font><br>
<font class="keyword">Private Declare Function</font> IsBadStringPtrByLong <font class="keyword">Lib</font> "kernel32" <font class="keyword">Alias</font> "IsBadStringPtrA" <font class="keyword">(ByVal</font> lpsz <font class="keyword">As Long, ByVal</font> ucchMax <font class="keyword">As Long) As Long</font> <br>
</p>
<p align=justify >
<font face=Arial >
Retrieving the string from a pointer is a common thing to have to do so it is worth having this utility function
in your arsenal.
</font>
</p>
<p align=left class="sourcecode">
<font class="keyword">
Public Function</font> StringFromPointer(lpString <font class="keyword">As Long</font>, lMaxLength <font class="keyword">As Long) As String</font><br>
<br>
<font class="keyword">Dim</font> sRet <font class="keyword">As String</font><br>
<font class="keyword">Dim</font> lret <font class="keyword">As Long</font><br>
<br>
<font class="keyword">If</font> lpString = 0 <font class="keyword">Then</font><br>
StringFromPointer = ""<br>
<font class="keyword">Exit Function</font><br>
<font class="keyword">End If</font><br>
<br>
<font class="keyword">If</font> IsBadStringPtrByLong(lpString, lMaxLength) <font class="keyword">Then</font><br>
<font class="comment">'\\ An error has occured - do not attempt to use this pointer</font><br>
StringFromPointer = ""<br>
<font class="keyword">Exit Function</font><br>
<font class="keyword">End If</font><br>
<br>
<font class="comment">'\\ Pre-initialise the return string...</font><br>
sRet = <font class="keyword">Space$</font>(lMaxLength)<br>
CopyMemory <font class="keyword">ByVal</font> sRet, <font class="keyword">ByVal</font> lpString, <font class="keyword">ByVal Len(</font>sRet)<br>
<font class="keyword">If Err.LastDllError</font> = 0 <font class="keyword">Then</font><br>
<font class="keyword">If InStr(</font>sRet, Chr$(0)) > 0 <font class="keyword">Then</font> <br>
sRet = <font class="keyword">Left$(</font>sRet, InStr(sRet, Chr$(0)) - 1)<br>
<font class="keyword">End If</font><br>
<font class="keyword">End If</font><br>
<br>
StringFromPointer = sRet<br>
<br>
<font class="keyword">End Function</font><br>
</p>
<p align=justify> So to use this to populate your PRINTER_INFO_2 variable:
</p>
<p class="sourcecode">
<font class="keyword">With</font> mPRINTER_INFO_2 <font class="comment">'\\ This variable is of type PRINTER_INFO_2</font><br>
.pServerName = StringFromPointer(buffer(0), 1024)<br>
.pPrinterName = StringFromPointer(buffer(1), 1024)<br>
.pShareName = StringFromPointer(buffer(2), 1024)<br>
.pPortName = StringFromPointer(buffer(3), 1024)<br>
.pDriverName = StringFromPointer(buffer(4), 1024)<br>
.pComment = StringFromPointer(buffer(5), 1024)<br>
.pLocation = StringFromPointer(buffer(6), 1024)<br>
.pDevMode = buffer(7)<br>
.pSepFile = StringFromPointer(buffer(8), 1024)<br>
.pPrintProcessor = StringFromPointer(buffer(9), 1024)<br>
.pDatatype = StringFromPointer(buffer(10), 1024)<br>
.pParameters = StringFromPointer(buffer(11), 1024)<br>
.pSecurityDescriptor = buffer(12)<br>
.Attributes = buffer(13)<br>
.Priority = buffer(14)<br>
.DefaultPriority = buffer(15)<br>
.StartTime = buffer(16)<br>
.UntilTime = buffer(17)<br>
.Status = buffer(18)<br>
.JobsCount = buffer(19)<br>
.AveragePPM = buffer(20)<br>
<font class="keyword">End With</font>
</p>
<p align=center><IMG alt="Source code for this article to download" src="../Images/source.gif" align=middle ></p>
<p align=justify>
<font face=Arial>The complete source code for these examples is available for download <a href="http://groups.yahoo.com/group/MerrionComputing/files/PrintWatchClient.zip
">here</a><br>
You will be asked to register with <b>Yahoo!Groups</b> in order to access it.
</font>
</p>
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