c - Meaning of overlapping when using memcpy

Question

I am trying to understand the function memcpy() which is defined in the C library <string.h>

Syntax: void *memcpy(void*dst,const void*src,size_t n);

I know that this function is used to copy the contents of the memory pointed by pointer src to the location pointed by the dst pointer and return a address pointed by dst pointer.

I am not able to understand the following important statement regarding memcpy():

• When using memcpy(), memory address should not overlap, if it overlaps then the memcpy() is undefined.

Another query is: Is the value passed to third argument of the function i.e size_t n is always an integer value?

Answer 1

From the comments your problem is that you don't understand what "overlapping" means:

Overlapping means this:

Here the two memory regions src and dst do overlap:

But here they don't:

So if you have overlapping memory regions, then you cannot use memcpy but you have to use memmove.

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Second question:

Yes, size_t is an unsigned integer type. The third argument is the number of bytes to copy, so it can hardly be anything else than an unsigned integer type.