On Fri, Apr 29, 2016, at 14:11, Marcos Dione wrote:
> These are not output parameters, even if they're pointers. they'r
> using the NULL pointer to signal that the current offsets should not be
> touched, to differentiate from a offset of 0. Something that in Python we
> would use None.
That's not actually true according to the documentation. (And if it
were, they could simply use -1 rather than a null pointer)
If you pass a null pointer in, the file's offset is used and *is*
updated, same as if you used an ordinary read/write call. If you pass a
value in, that value is used *and updated* (which makes it an output
parameter) and the file's offset is left alone.
Documentation below, I've >>>highlighted<<< the part that shows they are
used as output parameters:
The following semantics apply for off_in, and similar statements
apply to off_out:
* If off_in is NULL, then bytes are read from fd_in starting
from
the file offset, and the file offset is adjusted by the number
of
bytes copied.
* If off_in is not NULL, then off_in must point to a buffer that
specifies the starting offset where bytes from fd_in will be
read.
The file offset of fd_in is not changed, >>>but off_in is
adjusted
appropriately.<<<
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