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Linking Linked List To Hard Disk

Posted on 2006-07-19
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Last Modified: 2008-03-06
A simple linked list--

struct node{
int data;
node *next;
}


Generally when we create LL it gets created in RAM , what if I want to store it hard disk so that I can add / delete more nodes to it.



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Question by:sinha_anshul26
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by:sin_
sin_ earned 30 total points
ID: 17140890
Anshu,

Since it doesn't make any sense to store the pointers in the hard disk, you can store the data of the linked list into the hard disk by writing method like:

void makePersistent(NODE* root)
{
   // get each element and store the values in some file.
}

NODE* getPersistentList()
{
   //Read from the file.
   //create the link list and associated next ptrs.
   //return it.
}

The bottom line is, you store the data of the linked list, not the ptrs.
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by:sinha_anshul26
ID: 17143413
Hi Sin_
Just to confirm I got it right...
you  mean to say,(In other words) that there is no way that we can store data at a specific location of the Hard disk (using pointers)?

I read somewhere that OS does this to manage files using FAT file system
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manish_regmi earned 20 total points
ID: 17143899
In Fat it is not pointers of address but the pointers of block.

for eg. if a file occupies four blocks in block 3 5 6 9,

fat 3 points to 5 points to 6 points to 9.
it means a 32 bit fat value has block number 5.

No there is no way you can store pointers in disk. In virtual memory system you get different address each time you run (esp in dynamically allocated memory).

regards
Manish Regmi
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Expert Comment

by:sin_
ID: 17144178
That's right Anshu! Compiler generates the virtual memory addresses, which  are mapped to a physical page address by MMU while running the application. Therefore, everytime you run the application, you will find that your pointer is pointing to a different address.

Just imagine if you store this address; it may be a garbage next time. Persistence mechanism works by storing the content; you can reconstruct the data structure while reading it back.

Hope it helps.

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Author Comment

by:sinha_anshul26
ID: 17145400
>>Hope it helps
It Helped alot ...thank you both of you
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Author Comment

by:sinha_anshul26
ID: 17145443
Oh !! I choose the other way round sin_ 's  answer should be  Accepted and manish's assisted.
But any way thank you both for  helping


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Expert Comment

by:sin_
ID: 17147130
Not a problem! I too didn't know anything about FAT ptrs :)

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