view release on metacpan or  search on metacpan

lib/AnyEvent/IO.pm  view on Meta::CPAN

difference. Here is a short guideline that should leave you less
confused. It only talks about read operations, but the reasoning works
with other I/O operations as well.

Non-blocking I/O means that data is delivered by some external means,
automatically - that is, something I<pushes> data towards your file
handle, without you having to do anything. Non-blocking means that if
your operating system currently has no data (or EOF, or some error)
available for you, it will not wait ("block") as it would normally do,
but immediately return with an error (e.g. C<EWOULDBLOCK> - "I would have
blocked, but you forbid it").

Your program can then wait for data to arrive by other means, for example,
an I/O watcher which tells you when to re-attempt the read, after which it
can try to read again, and so on.

Often, you would expect this to work for disk files as well - if the data
isn't already in memory, one might want to wait for it and then re-attempt
the read for example. While this is sound reasoning, the POSIX API does
not support this, because disk drives and file systems do not send data
"on their own", and more so, the OS already knows that data is there, it