The function memmove is defined like this:
void *memmove(void *dest, const void *src, size_t n);
In the Linux manual page, it says:
RETURN VALUE
The memmove() function returns a pointer to dest.
Why isn't the function just defined as void memmove(…) when it always returns one of the input parameters? Can the return value be different from dest?
Or is the return value really always dest and it is just done to be able to compose the function in some creative ways?
memmove will never return anything other than dest.
Returning dest, as opposed to making memmove void, is useful when the first argument is a computed expression, because it lets you avoid computing the same value upfront, and storing it in a variable. This lets you do in a single line
void *dest = memmove(&buf[offset] + copiedSoFar, src + offset, sizeof(buf)-offset-copiedSoFar);
what you would otherwise need to do on two lines:
void *dest = &buf[offset] + copiedSoFar;
memmove(dest, src + offset, sizeof(buf)-offset-copiedSoFar);