Re: source(builtin) and read(2)

[Paul Eggert]

"Schwarz, Konrad" <address@hidden> writes:

> The argument that since ptrdiff_t is intended to represent the
> difference of pointers makes it somehow unsuitable to represent the
> result of read()/write() is missing the point: on any useful ABI,
> ptrdiff_t can represent the size of any object, can represent 0, and can
> represent -1, which is what read() and cousins require.

That's true nowadays, but in the old days there may have been a point
to distinguishing between ptrdiff_t and ssize_t.  I wasn't there, but
I imagine that the original point of ptrdiff_t was that it could be
wider than size_t.  Consider the case of a machine with 16-bit
addresses, 32-bit registers, and where objects can be larger than
2**15 bytes.  (The Motorola 68000 in 16-bit address mode comes to
mind.)  In such a machine size_t (and therefore ssize_t) is 16 bits,
and but it's useful to make ptrdiff_t 32 bits.

These days the distinction isn't all that relevant, because it's not
so useful to have objects that are between 2**31 and 2**32 bytes in
size.  So it would make sense now to require that ssize_t and
ptrdiff_t must be the same width.  I don't know of any modern
counterexamples.