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).
Some BIOs (such as memory BIOs) can be used immediately after calling BIO_new(). Others (such as file BIOs) need some additional initialization, and frequently a utility function exists to create and initialize such BIOs.
If BIO_free() is called on a BIO chain it will only free one BIO resulting in a memory leak.
Calling BIO_free_all() on a single BIO has the same effect as calling BIO_free() on it other than the discarded return value.
Normally the type argument is supplied by a function which returns a pointer to a BIO_METHOD. There is a naming convention for such functions: a source/sink BIO is normally called BIO_s_*() and a filter BIO BIO_f_*();
BIO *mem = BIO_new(BIO_s_mem());
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