Writing A Pointer To Memory

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2 total messages Started by Nathan T. Wild Mon, 18 Mar 1996 13:46

#7943

Author: Nathan T. Wild
Date: Mon, 18 Mar 1996 13:46

26 lines
1194 bytes


I know this is s fairly stupid question, but I am having a remarkable
amount of confusion over it...


I have a far * to a structure.  I need to share this with a device driver, by
writing the pointer to a location in memory.

the location in RAM that I have to write to is 2xsizeof(far *) or 8 bytes back
from a specific location in memory.

I know where I need to write the pointer, and what I need to write, but for
some strange reason I cannot get it to do what I want...

take:

void far *ptr = far pointer to a structure;

desiredDestinationPtr = (locationPtr - sizeof(void far *));

All I want to do is copy the 4 byte seg/offset pointer that ptr points to
into physical memory...  Aargh...  It shouldn't be this difficult...  
Too much coffee...  Please respond via email asap...

Thanks

Re: Writing A Pointer To Memory

#7944

Author: Phlash
Date: Mon, 18 Mar 1996 16:59

41 lines
2099 bytes

Nathan T. Wild wrote:
> 
> I know this is s fairly stupid question, but I am having a remarkable
> amount of confusion over it...

Such is life :-)

> I have a far * to a structure.  I need to share this with a device driver, by
> writing the pointer to a location in memory.
> 
> the location in RAM that I have to write to is 2xsizeof(far *) or 8 bytes back
> from a specific location in memory.

assume declarations of this form:

char far *locationPtr = <specific location in memory>
struct far *structPtr = &myStructure;

then it should be possible to do this:

  *((struct far * far *)(locationPtr - sizeof(struct far *) * 2)) = structPtr;

ie: Do the math on a char far *, then cast to a pointer to a pointer to struct,
then copy the pointer to struct into this memory location by de-referencing the
new location...

or for the less pointer-oriented:

  _fmemcpy(locationPtr - sizeof(struct far *) * 2, (char far *)&structPtr,
     sizeof(struct far *));

ie: calculate the memory location, give to _fmemcpy as destination address, give
address of pointer to copy, give size of pointer to copy...

PS: What wierd device driver needs this sort of thing?

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