i/o models

The five I/O models

• blocking I/O
• nonblocking I/O
• I/O multiplexing (select，poll，epoll)
• signal driven I/O (SIGIO)
• asynchronous I/O (the POSIX aio_functions)

The two distinct phases for an input operation:

As we show in all the examples in this section, there are normally two distinct phases for an input operation:

1. Waiting for the data to be ready
2. Copying the data from the kernel to the process

For an input operation on a socket, the first step normally involves waiting for data to arrive on the network. When the packet arrives, it is copied into a buffer within the kernel. The second step is copying this data from the kernel’s buffer into our application buffer.

Comparison of the I/O Models

It shows that the main
difference between the first four models is the first phase, as the second phase in the
first four models is the same: the process is blocked in a call to recvfrom while the
data is copied from the kernel to the caller’s buffer. Asynchronous I/O, however,
handles both phases and is different from the first four.

Synchronous I/O versus Asynchronous I/O

POSIX defines these two terms as follows:

• A synchronous I/O operation causes the requesting process to be blocked until that I/O operation completes.
• An asynchronous I/O operation does not cause the requesting process to be blocked.

Using these definitions, the first four I/O models—blocking, nonblocking, I/O multiplexing, and
signal-driven I/O—are all synchronous because the actual I/O operation (recvfrom) blocks the process. Only the asynchronous I/O model matches the asynchronous I/O definition.