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Annoying: Access Violation / Null Reference

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

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vascov

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Jazzle

ASKER

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'?

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vascov

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Jazzle

ASKER

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.....

Jazzle

ASKER

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

J

SOLUTION

vascov

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Jazzle

ASKER

thanks - will look into GetLastError().