ASCIIZ and VB String and DLL = GPF

Posted on 2002-03-23
Last Modified: 2010-05-18
Please help, this is driving me crazy.

I have written a DLL (in PowerBasic) which has the following declaration:


ASCIIZ is a null terminated string.

1. The following does NOT work and causes a GPF in MS VB 6.0 within about 20 calls:

Dim MyMessage as String
MyMessage =""
MyMessage = Chr$(65) & Chr$(66) & Chr$(67) & Chr$(68) & Chr$(69)
MyStr = TEST_DLL(MyMessage)

2. HOWEVER the following always works fine in MS VB 6.0:

Dim MyMessage as String
MyMessage = "ABCDE"
MyStr = TEST_DLL(MyMessage)

I need to build the string like in #1 above but it crashes. I look forward to your solution.

Question by:dougdeming
  • 2
LVL 16

Accepted Solution

Richie_Simonetti earned 100 total points
ID: 6891067
Dim MyMessage as String
MyMessage =""
MyMessage = Chr$(65) & Chr$(66) & Chr$(67) & Chr$(68) & Chr$(69) & chr$(0)
MyStr = TEST_DLL(MyMessage)


Expert Comment

ID: 6891435
Have you tried null-terminating your string. VB strings are BSTR (OLE) strings and not null terminated.

ie try
MyMessage = Chr$(65) & Chr$(66) & Chr$(67) & Chr$(68) & Chr$(69) & Chr$(0)

Author Comment

ID: 6891448
Okay here's what I found out. Adding the CHR$(0) did NOT fix the problem. After pouring thru lots of documentation and code, I discovered the problem was in my DLL.

The DLL defined the parameter as BYREF which is correct and VB defined it as BYVAL which is also correct.

However, I inadvertantly was changing the value of the parameter inside the DLL which messed up data in VB because I was increasing the length of the string inside the DLL, hence causing a GPF in VB.

Whew!  Thank you for everyone's help and insight. I am awarding the points to the first person who suggested the CHR$(0) because I appreciate his super fast response.

Thank you!

Author Comment

ID: 6891449
See my comment.

Featured Post

Courses: Start Training Online With Pros, Today

Brush up on the basics or master the advanced techniques required to earn essential industry certifications, with Courses. Enroll in a course and start learning today. Training topics range from Android App Dev to the Xen Virtualization Platform.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
String manipulation in Visual Basic 7 62
using web browser with BING 40 119
VB6 - Convert HH:MM into Decimal 8 54
VB6 - Scroll Mouse wheel on Picturebox 13 43
If you have ever used Microsoft Word then you know that it has a good spell checker and it may have occurred to you that the ability to check spelling might be a nice piece of functionality to add to certain applications of yours. Well the code that…
Most everyone who has done any programming in VB6 knows that you can do something in code like Debug.Print MyVar and that when the program runs from the IDE, the value of MyVar will be displayed in the Immediate Window. Less well known is Debug.Asse…
Get people started with the process of using Access VBA to control Excel using automation, Microsoft Access can control other applications. An example is the ability to programmatically talk to Excel. Using automation, an Access application can laun…
Show developers how to use a criteria form to limit the data that appears on an Access report. It is a common requirement that users can specify the criteria for a report at runtime. The easiest way to accomplish this is using a criteria form that a…

776 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question