bio - I/O abstraction
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).