6.12 Summary
There are five different models for I/O provided
by Unix:
-
Blocking
-
Nonblocking
-
I/O multiplexing
-
Signal-driven I/O
-
Asynchronous I/O
The default is blocking I/O, which is also the
most commonly used. We will cover nonblocking I/O and signal-driven
I/O in later chapters and have covered I/O multiplexing in this
chapter. True asynchronous I/O is defined by the POSIX
specification, but few implementations exist.
The most commonly used function for I/O
multiplexing is select. We tell the select
function what descriptors we are interested in (for reading,
writing, and exceptions), the maximum amount of time to wait, and
the maximum descriptor number (plus one). Most calls to
select specify readability, and we noted that the only
exception condition when dealing with sockets is the arrival of
out-of-band data (Chapter 24). Since select
provides a time limit on how long a function blocks, we will use
this feature in Figure 14.3 to place a
time limit on an input operation.
We used our echo client in a batch mode using
select and discovered that even though the end of the user
input is encountered, data can still be in the pipe to or from the
server. To handle this scenario requires the shutdown
function, and it lets us take advantage of TCP's half-close
feature.
The dangers of mixing stdio buffering (as well
as our own readline buffering) with select caused
us to produce versions of the echo client and server that operated
on buffers instead of lines.
POSIX defines the function pselect,
which increases the time precision from microseconds to nanoseconds
and takes a new argument that is a pointer to a signal set. This
lets us avoid race conditions when signals are being caught and we
talk more about this in Section 20.5.
The poll function from System V
provides functionality similar to select and provides
additional information on STREAMS devices. POSIX requires both
select and poll, but the former is used more
often.
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