Re: source(builtin) and read(2)

[Clive D.W. Feather]

Schwarz, Konrad said:
> While we're at it, I would be interested in the rational for "inventing"
> ssize_t and not using ptrdiff_t instead, if anyone would care to
> comment.

size_t has to be able to hold the size of the largest object. It is an
unsigned type.

ptrdiff_t has to be able to hold the largest difference between two
pointers; it is a signed type.

The two need not be a signed/unsigned pair. For example, if a system has a
maximum object size of 64,000 bytes, then size_t needs to be at least 16
bits but ptrdiff_t needs to be at least 17 bits. Thus it would be
legitimate for size_t to be (16 bit) unsigned short but ptrdiff_t to be
(32 bit) signed int.

ssize_t is the signed type corresponding to size_t. In the above example,
it is required to be signed short. It's possible for a legitimate
subtraction of two pointers to generate a value that can't fit in ssize_t.
However, because size_t and ssize_t are a signed/unsigned pair, they have
useful properties (for example, when dealing with varargs lists).

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