Thread: int memCopy(void* , void*, int size)

Hi,

I am having some fun these days trying to implement c functions using pointers and c prototypes. So far i got strlength and stringCopy done. However i am moving along to memcopy and have to admit that i lack some knowlegde or pointers here. The following is what i started out with. I have been going back and forth a while now. Can someone explain how the memcpy function is implemented in c. Or do they use bit manipulation etc.
I know the below is wrong, but i backed up to the start of me mucking around with it.

Code:

int memCopy(void *destination, void *source, int size){

   char *dest = (char*)&destination;
   char *sour = (char*)&source;
   int c;

   for(c=0;c<size;c++)
      *dest++ = *sour++;

    return c;



}

Cheers,
G'n'R

You're working with raw memory, so use 'unsigned' instead. For speed, don't work with a char unless you have to-- use a long instead. Why copy 1 byte at a time, when you could copy 4? or more (depending on the native word-size of your processor)?

Never modify your caller's pointers. Always work with a copy. Check for valid pointers, and do some range-checking if necessary. All these things make your routine fast, reliable, robust, and friendly.

"BADPTR" (below) is a user-defined code-- you could enum it to match whatever this real memcpy() function would return-- I just couldn't remember it at this time.

This example, just uses, byte-sized copying (but normally I would use unsigned longs)--

---

int memCopy(void *dPtr,void *sPtr)
{
unsigned byte *d;
unsigned byte *s;
unsigned long blockSize;

if(!dPtr || !sPtr) /* basic validity check */
return(BADPTR); /* bad pointer, bail */

d = dPtr; /* copy original pointers */
s = sPtr;

blockSize = labs((long)dPtr - (long)sPtr); /* get size being copied */

while(blockSize--) /* do the copy */
d++ = s++;

return(NOERR); /* success! */
}


This is not tested, but it's either right or close.

cool, thanks for that info Sayah. I will get into your example and write some code soon. I will get back to you on how it goes.


Cheers
G'n'R

Just one minor change:

Code:

int memCopy(void *destination, void *source, int size)
{

   char *dest = (char*)destination;
   char *sour = (char*)source;
   int c;

   for(c=0;c<size;c++)
      *dest++ = *sour++;

    return c;
}

Sayah,

I have been trying out you code and modified it some and more questions have popped into my head. Bear with me. I want to have a clear understanding of what is going on. Here goes.

>Never modify your caller's pointers. Always work with a copy.
Question 1
Arent we here modifying the callers pointers anyway, since the changes made to d and s affect dPtr and sPtr which are the orignal pointers

Code:

d = dPtr; /* copy original pointers */ 
s = sPtr;

i could only get this to work

Code:

while(blockSize--)
   d++ = s++;

like this. Am i moving the address of the pointer one bit up here or actually dereferencing the pointer an asigning data or doing both?

Code:

while(blockSize--)
  *d++ = *s++;

Question 3
> You're working with raw memory, so use 'unsigned' instead.
Why are unsigned variables better.

Question 4
>Use unsigned long (in the cast) instead
Is this because a long is the maximum c variable?

However, the function does work, except for i really need to insert a null terminator at the end if a pass char arrays. Is it safe to do so on any type passed to this function.

Thanks again
G'n'R