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Memory allocation usin malloc

Posted on 2012-03-19
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Last Modified: 2012-08-13
Hi Experts,

I am experimenting with how memory is allocated on the heap.  It appears, when I do malloc back to back, the first one returns an address in one page, and the second one returns an address in another page(4096 bytes page size).  I was expecting( most likely incorrectly) that it will be in the same page, but would be separated by the allocation table data.  Does every malloc goes and fetch addresses from different pages?
Thank you.


        int i, j;

	int * firstArray = malloc(sizeof(int)*20);
	int *secondArray = malloc(sizeof(int)*10);
	for( j = 0; j < 5; j++)
	{
		secondArray[j] = 11;
	}
	for( i = 0; i < 25; i++)
	{
		firstArray[i] = 10;
	}

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Question by:ambuli
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Expert Comment

by:jkr
ID: 37740673
Yup, the 'usual' algorithms for allocating on the heap will either a) rely on undelying OS code or b) offer a scheme where it is most worthwile to fragment the heap as least as possible. So, in your case, the allocator code might have found regions that are better suitable to place your request in other than two consecutive pieces of memory (yet that might happen as well). As a bottom line, assumptions about what heap allocators return are futile at best, better not worry about that in the first place.
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Author Comment

by:ambuli
ID: 37740712
sure:-) i was curious as to why it is allocated on a new page. thank you. one more question. I believe the allocation info is usually placed in the block before or after the allocated block. how much of memory is used for this data and what information is stored there. is there any reference doc for this.
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jkr earned 2000 total points
ID: 37740720
This is completely implementation dependant. I was about to mention that such a 'context record' could have been in the way between you and your contiguous allocations, but if it was me to create a (sub-)allocator, I'd keep that separated from the actual memory pool I was to supervise about. So, I guess my best advice is to make no guesses about what your underlying heap does nor how it is implemented.
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Author Comment

by:ambuli
ID: 37740742
thanks.  i was studying about heap corruptions and that is why i was interested in.
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Expert Comment

by:jkr
ID: 37740768
As a 'real life' example - I had to design an allocator for a UDF2 file system for real time video recording, and *speed* therefore was the primary  factor - so, find the first slot that satisfies your requirements and move on... (in hindsight, I see much room for optimization ;o)
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