Visual Basic/VB, Accessing char ** C-programming Construct via DLL in VB5

Accessing char ** C-programming Construct via DLL in VB5

Author	Message
sCoT #1 / 3	Accessing char C-programming Construct via DLL in VB5 Gang: Please bare with me, I am new to VB5... I am trying to access the Ghostscript 5.03 for Windows DLL "gsdll32.dll" via the Private Declare Sub construct in VB5. There are several functions present in this DLL, but I started with gsdll_revision which returns the product name, copyright information, the revision number, and the revision date. In the documentation for gsdll_revision, we are told that the function's arguments are: char product; char *copyright; long revision; long revisiondate; I created my VB declare as follows: Private Declare Sub gsdll_revision Lib "gsdll32" (ByVal product As String, ByVal copyright As String, rev As Long, revdate As Long) I then ran the program to test the results: Private Sub Command1_Click() Dim product As String Dim copyright As String Dim revision As Long Dim revisiondate As Long Dim rc As Integer gsdll_revision product, copyright, revision, revisiondate rc = MsgBox("Product=" + product, vbOKOnly, "Product Information") rc = MsgBox("Copyright=" + copyright, vbOKOnly, "Copyright Information") rc = MsgBox("Revision=" + Str(revision), vbOKOnly, "Revision Information") rc = MsgBox("Product=" + Str(revisiondate), vbOKOnly, "Revision Date Information") End Sub The messagebox for product and copyright both returned blank strings, while the revision returned the number 503 and revisiondate returned 19970808, both of which are correct. I have checked the documentation provided with VB5 on accessing DLLs, but I am not able to find anything about character pointers to pointers. I have tried to remove the "ByVal"s in the declaration and running it that way, but I get a crash. Curious, though, when I force the product and copyright string Dim's to a fixed length of 10... Dim product As String 10 Dim copyright As String * 10 ...both the product and copyright return jibberish characters. (I am assuming that what I am getting is the address to a memory location, but I do not know that for sure.) Any help would be greatly appreciated! Thanks, Scott
Fri, 24 Nov 2000 03:00:00 GMT

Jim Mac #2 / 3	Accessing char C-programming Construct via DLL in VB5 Scott - Char is a fair representation of "ByRef xx As String", but the catch is that you have to provide a pre-allocated string. Fixed-length strings are not very efficient for this. Also, VB/32 expects Unicode strings, so you'll probably have to use StrConv to get them into the proper form. Dim sProduct As String sProduct = String$(100, 0) ... your call ... sProduct = StrConv(sProduct, vbUnicode) -- Jim Mack MicroDexterity, Inc PO Box 5372 Plymouth, MI 48170-5372 http://www.microdexterity.com Fax: +1 734-453-8942 Quote: > Gang: > Please bare with me, I am new to VB5... > I am trying to access the Ghostscript 5.03 for Windows > DLL "gsdll32.dll" via the Private Declare Sub construct > in VB5. There are several functions present in this DLL, but > I started with gsdll_revision which returns the product name, > copyright information, the revision number, and the revision > date. > In the documentation for gsdll_revision, we are told that the > function's arguments are: > char product; > char copyright; > long revision; > long revisiondate; > I created my VB declare as follows: > Private Declare Sub gsdll_revision Lib "gsdll32" (ByVal product As > String, ByVal copyright As String, rev As Long, revdate As Long) > I then ran the program to test the results: > Private Sub Command1_Click() > Dim product As String > Dim copyright As String > Dim revision As Long > Dim revisiondate As Long > Dim rc As Integer > gsdll_revision product, copyright, revision, revisiondate > rc = MsgBox("Product=" + product, vbOKOnly, "Product Information") > rc = MsgBox("Copyright=" + copyright, vbOKOnly, "Copyright > Information") > rc = MsgBox("Revision=" + Str(revision), vbOKOnly, "Revision > Information") > rc = MsgBox("Product=" + Str(revisiondate), vbOKOnly, "Revision Date > Information") > End Sub > The messagebox for product and copyright both returned blank strings, > while the revision returned the number 503 and revisiondate returned > 19970808, > both of which are correct. > I have checked the documentation provided with VB5 on accessing DLLs, > but I am not able to find anything about character pointers to > pointers. I have > tried to remove the "ByVal"s in the declaration and running it that way, > but I > get a crash. > Curious, though, when I force the product and copyright string Dim's to > a fixed length of 10... > Dim product As String * 10 > Dim copyright As String * 10 > ...both the product and copyright return jibberish characters. (I am > assuming that what I am getting is the address to a memory location, but > I do not know that > for sure.) > Any help would be greatly appreciated! > Thanks, > Scott
Fri, 24 Nov 2000 03:00:00 GMT

Shane Kruege #3 / 3	Accessing char C-programming Construct via DLL in VB5 Whenever you pass a string to a DLL ByVal, the string is automatically converted to ANSI during the call, and back again upon the return of the dll call. As Jim Mack said though, strings do have to be preallocated, but never use the StrConv function when passing a string to a DLL byval. If used, a ANSI-->Unicode conversion would occur twice for the same string and/or vice versa. See a simplified example for the correct use of the GetComputerName API call (tested & works perfectly) below. ---------------------- Shane Krueger '-------Start VB GetComputerName demonstration code------ Private Declare Function APIGetComputerName Lib "kernel32" Alias _ "GetComputerNameA" (ByVal lpBuffer As String, nSize As Long) As Long Public Function GetComputerName() As String Dim Dr$ Dr$ = Space$(1000) If APIGetComputerName(Dr$, Len(Dr$)) = 0 Then Err.Raise 75 GetComputerName = Left$(Dr$, InStr(Dr$, Chr$(0)) - 1) End Function '-------End VB GetComputerName demonstration code------ Quote: >Scott - > Char is a fair representation of "ByRef xx As String", but the catch >is that you have to provide a pre-allocated string. Fixed-length strings are >not very efficient for this. > Also, VB/32 expects Unicode strings, so you'll probably have to use >StrConv to get them into the proper form. > Dim sProduct As String > sProduct = String$(100, 0) > ... your call ... > sProduct = StrConv(sProduct, vbUnicode) >-- > Jim Mack > MicroDexterity, Inc > PO Box 5372 > Plymouth, MI 48170-5372 > http://www.microdexterity.com > Fax: +1 734-453-8942 >> Gang: >> Please bare with me, I am new to VB5... >> I am trying to access the Ghostscript 5.03 for Windows >> DLL "gsdll32.dll" via the Private Declare Sub construct >> in VB5. There are several functions present in this DLL, but >> I started with gsdll_revision which returns the product name, >> copyright information, the revision number, and the revision >> date. >> In the documentation for gsdll_revision, we are told that the >> function's arguments are: >> char product; >> char copyright; >> long revision; >> long revisiondate; >> I created my VB declare as follows: >> Private Declare Sub gsdll_revision Lib "gsdll32" (ByVal product As >> String, ByVal copyright As String, rev As Long, revdate As Long) >> I then ran the program to test the results: >> Private Sub Command1_Click() >> Dim product As String >> Dim copyright As String >> Dim revision As Long >> Dim revisiondate As Long >> Dim rc As Integer >> gsdll_revision product, copyright, revision, revisiondate >> rc = MsgBox("Product=" + product, vbOKOnly, "Product Information") >> rc = MsgBox("Copyright=" + copyright, vbOKOnly, "Copyright >> Information") >> rc = MsgBox("Revision=" + Str(revision), vbOKOnly, "Revision >> Information") >> rc = MsgBox("Product=" + Str(revisiondate), vbOKOnly, "Revision Date >> Information") >> End Sub >> The messagebox for product and copyright both returned blank strings, >> while the revision returned the number 503 and revisiondate returned >> 19970808, >> both of which are correct. >> I have checked the documentation provided with VB5 on accessing DLLs, >> but I am not able to find anything about character pointers to >> pointers. I have >> tried to remove the "ByVal"s in the declaration and running it that way, >> but I >> get a crash. >> Curious, though, when I force the product and copyright string Dim's to >> a fixed length of 10... >> Dim product As String * 10 >> Dim copyright As String * 10 >> ...both the product and copyright return jibberish characters. (I am >> assuming that what I am getting is the address to a memory location, but >> I do not know that >> for sure.) >> Any help would be greatly appreciated! >> Thanks, >> Scott
Sat, 25 Nov 2000 03:00:00 GMT

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