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OpenSSL/Using BIOs
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A BIO is an I/O stream abstraction; essentially OpenSSL's answer to the C library's FILE *. OpenSSL comes with a number of useful BIO types predefined, or you can create your own.
BIOs come in two flavors: source/sink, or filter. BIOs can be chained together. Each chain always has exactly one source/sink, but can have any number (zero or more) of filters.
Reading from a BIO can be done with BIO_read and BIO_gets.
Writing to a BIO can be done with BIO_write, BIO_puts, BIO_printf, and BIO_vprintf.