Solved

Annoying:  Access Violation / Null Reference

Posted on 2004-03-31
7
740 Views
Last Modified: 2007-12-19
Hi Experts,
I have a C# application that calls a DLL written by me in C++.
GPGUI.exe calls gpmain() in simplegpdll.dll with the following line:

if (simplegpdll.gpmain(...,...,...) == 1) {... return true;}


Most of the time it runs with no problems - however, sometimes (and I can't spot a pattern), it crashes with the following error:

   First-chance exception at 0x77f5b2ab in GPGUI.exe: 0xC0000005: Access violation reading location 0x00000034.
   An unhandled exception of type 'System.NullReferenceException' occurred in simplegpdll.dll

   Additional information: Object reference not set to an instance of an object.

The DLL reads a text file, and writes three (once each per operation).  All reads and writes look like this:
FILE *solstream;
solstream=fopen("solution.csv","w+");
fprintf(solstream,"Gen,Best Program\n%d,%d\n", generation, best_prog);
fclose(solstream);
fcloseall();

Any suggestions on what's at 0x00...34 (or how to find out), and also what is throwing the Null Ref?

Thanks in advance, J
0
Comment
Question by:Jazzle
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 4
  • 3
7 Comments
 
LVL 12

Accepted Solution

by:
vascov earned 100 total points
ID: 10728298
Hi Jazzle,

Compile your C++ DLL with debuging info. That may help you showing more info when an AV occurs.

Regarding your code, check to see that solstream is NOT null after opening it. You might be experiencing issues with file sharing, or something like that.
The NullRef might be a consequence of the other error... (the access violation)

hth

Vasco
0
 
LVL 1

Author Comment

by:Jazzle
ID: 10728376
I have compiled it with the debuging info (it was anyway) but unfortunately it doesn't give me any extra info. :(
also, I know that solstream can't be null because of the way it's created.

I did wonder about file sharing, but i have found found that it gives a different error if the file is in use.

thanks for looking.

J


PS - 'hth'?
0
 
LVL 12

Assisted Solution

by:vascov
vascov earned 100 total points
ID: 10730184
Launch your application using the debugger.

Please, do check if solstream is NOT null anyway. It wont hurt, and it might surprise you the situations that can occur. (i'm not saying with 100% sure that the problem is in this area, but since this is the only code you're showing i definetely recommend for you to test it...)

That exception can also be happening in some marshalling between managed / unmanaged.

hope this helps (hth),

Vasco
0
Instantly Create Instructional Tutorials

Contextual Guidance at the moment of need helps your employees adopt to new software or processes instantly. Boost knowledge retention and employee engagement step-by-step with one easy solution.

 
LVL 1

Author Comment

by:Jazzle
ID: 10731085
Okay, so I have now found that the exception is caused by the fprintf() command - so i suppose it's refering to the file i want to write to.  all the files written to are closed afterwards - so could it be a matter of threads confusing each other? (I haven't specifed any thread usage)

************* Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at fprintf(_iobuf* , SByte* )
   at gpmain(Int32 POPULATION_SIZ.....
0
 
LVL 1

Author Comment

by:Jazzle
ID: 10731683
You were right it seems the fopen() command is sometimes returning NULL - but I can't see why - any ideas anyone?

J
0
 
LVL 12

Assisted Solution

by:vascov
vascov earned 100 total points
ID: 10731756
Use GetLastError()

VV

0
 
LVL 1

Author Comment

by:Jazzle
ID: 10731905
thanks - will look into GetLastError().
0

Featured Post

Major Incident Management Communications

Major incidents and IT service outages cost companies millions. Often the solution to minimizing damage is automated communication. Find out more in our Major Incident Management Communications infographic.

Question has a verified solution.

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

In my previous two articles we discussed Binary Serialization (http://www.experts-exchange.com/A_4362.html) and XML Serialization (http://www.experts-exchange.com/A_4425.html). In this article we will try to know more about SOAP (Simple Object Acces…
For those of you who don't follow the news, or just happen to live under rocks, Microsoft Research released a beta SDK (http://www.microsoft.com/en-us/download/details.aspx?id=27876) for the Xbox 360 Kinect. If you don't know what a Kinect is (http:…
NetCrunch network monitor is a highly extensive platform for network monitoring and alert generation. In this video you'll see a live demo of NetCrunch with most notable features explained in a walk-through manner. You'll also get to know the philos…
Michael from AdRem Software explains how to view the most utilized and worst performing nodes in your network, by accessing the Top Charts view in NetCrunch network monitor (https://www.adremsoft.com/). Top Charts is a view in which you can set seve…

705 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