Problem with CreateFileMapping() function

This is a discussion on Problem with CreateFileMapping() function within the Windows Programming forums, part of the Platform Specific Boards category; Dear Experts, i have a problem with CreateFileMapping() function , i am trying in this way, Code: hnd = (Mbyte*)CreateFile(fname, ...

  1. #1
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    Problem with CreateFileMapping() function

    Dear Experts,

    i have a problem with CreateFileMapping() function ,
    i am trying in this way,

    Code:
    hnd = (Mbyte*)CreateFile(fname, GENERIC_WRITE, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
    
    printf("\n in if _hMap==============%d",(int)_hnd);//which gives me some 2060 
    _hMap = (char *)CreateFileMapping(_hnd,NULL,MAPPROTWRMODE,_fileSize,NULL);
    
    printf("\n in if mapHandle==============%d",(int)_hmap);//which gives me NULL
    why my map is getting failed when i can succesfully create a file,
    i want the map for the file the "fname" specified in createfile please help me what can be the reason please provide me some solution.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Did you just type that out of your head?

    1. Remove all the casts, and declare the variables properly.
    2. http://msdn.microsoft.com/en-us/libr...37(VS.85).aspx - 6 params, you have 5.
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  3. #3
    and the hat of sweating
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    BTW, since this question is clearly Windows specific (because you're talking about Windows API functions), you should be posting it in the Windows Programming forum instead of the general C++ forum. Just to keep things more organized and get the right kind of help.
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    Oh yes, now that you mention it... moved.
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  5. #5
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    Problem with creartedFilemapping

    dear Experts,
    sorry that i might have posted in the wrong forum,
    but please tell me the solution to my problem,
    if i remove the casts also the createFileMapping() is getting failed,
    please tell me what is wrong because i used many examples to test ,
    but there was not output please tell me what to do,

    Code:
    hnd = CreateFile(fname, GENERIC_WRITE, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
    
    _hMap =CreateFileMapping(_hnd,NULL,MAPPROTWRMODE,_fileSize,NULL);
    this _hmap is failing,what is wrong plz help me.

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    You couldn't have found any examples like that, since as Salem points out it takes six arguments not five. (The last being "optional" only means you can pass in 0/NULL, not that you can pass in nothing at all.) Also, MAPPROTWRMODE is not listed as a valid value for the third value. (Actually, it looks like you're trying to make "_fileSize" play the part of the fourth and fifth arguments together.)

  7. #7
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    Dear experts,

    sorry for sending that code actually it was

    Code:
    #define MAPPROTWRMODE	PAGE_READWRITE
    
    hnd = CreateFile(fname, GENERIC_WRITE, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
    
    _hMap =CreateFileMapping(_hnd,NULL,MAPPROTWRMODE,0,_fileSize,NULL);
    this is the actual one which is syntactically correct but returning NULL,
    so know any corrections you suggest.

  8. #8
    and the Hat of Guessing tabstop's Avatar
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    You should check GetLastError.

    But I would guess that since you opened the file with write privileges, trying to map it with read/write privileges may not work.

  9. #9
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    createFileMapping()

    Dear experts,
    thanks for reply,
    but that is not the solution as iam opening the file in GENERIC_WRITE,so it will write instead of read,
    any more solutions.

  10. #10
    and the Hat of Guessing tabstop's Avatar
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    I doubt that's what GetLastError said.

    (Edit: by that I mean "but that is not the solution as iam opening the file in GENERIC_WRITE,so it will write instead of read" is not a Windows system message.)

    Edit: And I've been assuming that the fact that one line says "hnd" and the next line says "_hnd" is just a copy-n-paste error. If not, well.
    Last edited by tabstop; 10-16-2008 at 11:06 PM.

  11. #11
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    problem with CreateFileMapping()

    Thank you for reply ,

    i am using the getLastError() in this way

    Code:
    #define MAPPROTWRMODE	PAGE_READWRITE
    
    _hnd = CreateFile(fname, GENERIC_WRITE, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL);
    
    _hMap =CreateFileMapping(_hnd,NULL,MAPPROTWRMODE,0,_fileSize,NULL);
    
    register int errstat=0;
     if(_hMap == NULL)
    {
       errstat=GetLastError();
        if(errstat)
       printf("\n ERROR* CreateFileMapping: error=%d\n",errstat);//which  returns me 5 as number 	
    }
    i am using like this which gives me error=5 what does it mean ,
    is it the right way i am using or any other way but does 5 indicates.
    Last edited by vin_pll; 10-17-2008 at 10:25 PM.

  12. #12
    and the hat of wrongness Salem's Avatar
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    Well if you clicked around where tabstop pointed, you would get to
    http://msdn.microsoft.com/en-us/libr...82(VS.85).aspx

    ERROR_ACCESS_DENIED 5 0x5 Access is denied.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  13. #13
    and the Hat of Guessing tabstop's Avatar
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    I still say GENERIC_WRITE should be GENERIC_READ|GENERIC_WRITE. Especially since the page that has been linked two or three times now says this:
    Quote Originally Posted by msdn
    PAGE_READWRITE Allows views to be mapped for read-only, copy-on-write, or read/write access.

    The file handle that the hFile parameter specifies must be created with the GENERIC_READ and GENERIC_WRITE access rights.

  14. #14
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    problem with CreateFileMapping()

    Thank you experts thanks a lot,

    thanks especially to tabstop who guessed the problem exactly, thanks to you specially,
    the problem got solved , really very pleasurefull morning to me.

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