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Reading integers from disk

Posted on 2006-11-09
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Last Modified: 2010-04-15
When I read from disk I usually read into a void* type memory block, and then sort out the types using pointers.

But, I find strange discrepencies sometimes when dealing with integers.  For example, if I have a 64-bit integer stored in a void memory block, I access it by dereferencing the region in memory, and casting it to the appropriate type.

uint64_t number = (uint64_t) *( (byte*) membuffer + offset);

I also have to cast the void* pointer itself to a char*, or my byte* typedef, in order to add the offset distance.

I can also access the integer like this:

memcpy(&number, (byte*) membuffer + offset, sizeof(number));

The first method seems more direct, but it sometimes results in garbage numbers, whereas the second method always works properly.  Is there some reason that accessing the data via dereferencing the pointer and casting might not work properly?
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Question by:chsalvia
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ozo earned 250 total points
ID: 17909726
does
uint64_t number = *( (uint64_t*) ( (byte*) membuffer + offset));
work any beter?
If not, there may be alignment issues for pointers to different types
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by:chsalvia
ID: 17910014
That worked correctly, thanks.  But I'm confused.  Why would casting to a pointer-to-integer type, and then dereferencing that, result in something different than dereferncing a pointer and casting to an integer type?  It's the same memory region.
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Expert Comment

by:ozo
ID: 17910088
when you dereference a byte pointer, you get a byte.  when you dereference a uint64_t pointer, you get a uint64_t
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