A BIO is an I/O abstraction, it hides many of the underlying
I/O details from an application. If an application uses a
BIO for its I/O it can transparently handle SSL connections,
unencrypted network connections and file I/O.
There are two type of BIO, a source/sink BIO and a filter
BIO.
As its name implies a source/sink BIO is a source and/or
sink of data, examples include a socket BIO and a file BIO.
A filter BIO takes data from one BIO and passes it through
to another, or the application. The data may be left
unmodified (for example a message digest BIO) or translated
(for example an encryption BIO). The effect of a filter BIO
may change according to the I/O operation it is performing:
for example an encryption BIO will encrypt data if it is
being written to and decrypt data if it is being read from.
BIOs can be joined together to form a chain (a single BIO is
a chain with one component). A chain normally consist of one
source/sink BIO and one or more filter BIOs. Data read from
or written to the first BIO then traverses the chain to the
end (normally a source/sink BIO).