Can I call memcpy() and memmove() with "number of bytes" set to zero?
C++CPointersMemcpyMemmoveC++ Problem Overview
Do I need to treat cases when I actully have nothing to move/copy with memmove()
/memcpy()
as edge cases
int numberOfBytes = ...
if( numberOfBytes != 0 ) {
memmove( dest, source, numberOfBytes );
}
or should I just call the function without checking
int numberOfBytes = ...
memmove( dest, source, numberOfBytes );
Is the check in the former snippet necessary?
C++ Solutions
Solution 1 - C++
From the C99 standard (7.21.1/2):
> Where an argument declared as size_t n
specifies the length of the array for a
function, n
can have the value zero on a call to that function. Unless explicitly stated
otherwise in the description of a particular function in this subclause, pointer arguments
on such a call shall still have valid values, as described in 7.1.4. On such a call, a
function that locates a character finds no occurrence, a function that compares two
character sequences returns zero, and a function that copies characters copies zero
characters.
So the answer is no; the check is not necessary (or yes; you can pass zero).
Solution 2 - C++
As said by @You, the standard specifies that the memcpy and memmove should handle this case without problem; since they are usually implemented somehow like
void *memcpy(void *_dst, const void *_src, size_t len)
{
unsigned char *dst = _dst;
const unsigned char *src = _src;
while(len-- > 0)
*dst++ = *src++;
return _dst;
}
you should not even have any performance penality other than the function call; if the compiler supports intrinsics/inlining for such functions, the additional check may even make the code a micro-little-bit slower, since the check is already done at the while.