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Dynamically allocate memory

Posted on 2016-09-02
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Last Modified: 2016-09-03
Hi Guys.

I'm not a C++ developer and with this project I'm doing I was forced to use C++. The problem is that when I run my app and try to open a service its says Access violation reading location 0x00000000. The pointer is
lphService

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I've looked around on how to allocate memory but I don't seem to understand as to how could I assign this as the size will be returned by a function and I don't know the size by head. Any help?
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Question by:MCSI Developers
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Expert Comment

by:Zoppo
ID: 41781174
Hi MCSI Developers,

in general the error occurs in case a NULL-pointer is dereferenced, in your case the program tries to access (read) the value where the pointer lphService points to.

We need more info about this to help you, could you post some code (especially all parts where the lphService is instanced and used)?

Best regards,

ZOPPO
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Author Comment

by:MCSI Developers
ID: 41781205
@ZOPPO, here is the code, and I get error -26.

    LPHSERVICE lphService;
    lphService = 0; // I put 0 because the EXIT_SUCCESS is also 0
    HRESULT hResult;
    WFSVERSION SvcVersion, SpiVersion;
    char szLogicalName[]="Epson1200";
    hResult = (WFSOpen(szLogicalName,WFS_DEFAULT_HAPP,"Epson Print",WFS_TRACE_NONE,TWO_MINUTES,RECOGNISED_VERSIONS,&SvcVersion,&SpiVersion,lphService));
    if(hResult == WFS_ERR_CANCELED)
    {
    cout << WFS_ERR_CANCELED << endl; // This returns -4
    }else if(hResult == WFS_ERR_SERVICE_NOT_FOUND)
    {
    cout << WFS_ERR_SERVICE_NOT_FOUND << endl; // This is -43
    }else if(hResult == WFS_ERR_CONNECTION_LOST)
    {
    cout << WFS_ERR_CONNECTION_LOST << endl; // -54
    }else if(hResult == WFS_ERR_INTERNAL_ERROR)
    {
    cout << WFS_ERR_INTERNAL_ERROR << endl; // -15
    }else if(hResult == WFS_ERR_INVALID_APP_HANDLE)
    {
    cout << WFS_ERR_INVALID_APP_HANDLE << endl; // -17
    }else if(hResult == WFS_ERR_INVALID_POINTER)
    {
    cout << WFS_ERR_INVALID_POINTER << endl; // This is the error I get which is -26
    }

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Accepted Solution

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Zoppo earned 500 total points
ID: 41781716
ok, I guess the LPHSERVICE is a typedef to a HSERVICE pointer, so you should be able to solve it using one of these methods:

1. Use a pointer: change line 2 to
lphService = new HSERVICE;

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and add this line after the last use of lphservice:
delete lphservice;

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2. Use an instance: change line 1 to
HSERVICE hService;

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delete line 2 and replace lphService by &hService in line 6.

In both cases the pointer passed to WSFOpen points to a valid instance, with 1. this instance is created in heap (and therefor has to be released after last use), with 2. the instance is put onto the stack, thus released automatically when the function terminates.

Hope this helps,

ZOPPO
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Expert Comment

by:Zoppo
ID: 41781726
BTW, in addition two hints:

1. you don't need the ( and ) surrounding the WSFOpen call
2. using a chain of if/else to check a variable against constant values is not optimal for performance, in your case a switch/case statement would perform better.

ZOPPO
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Author Closing Comment

by:MCSI Developers
ID: 41782249
Thanks thats really helped, now I get this error: WFS_ERR_SERVICE_NOT_FOUND. Is this because of missing device or the name is not found on the registry?
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Expert Comment

by:Zoppo
ID: 41782922
Hm - sorry, that's hard to say for me since I don't know the API you're using - you could try if the program acts different when you dis- and re-connect the device, if so I would guess it's a problem with an unregistered (in registry) service.
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Author Comment

by:MCSI Developers
ID: 41782924
I would guess it's a problem with an unregistered (in registry) service
do you mean my App is not registered or the device/service?
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Expert Comment

by:Zoppo
ID: 41783110
I would guess the service is unregistered or it is registered but not running.
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Author Comment

by:MCSI Developers
ID: 41783219
Thanks,
registered but not running
that does make seance.
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