BIO

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 Manual:BIO_read(3) and BIO_gets.

Writing to a BIO can be done with BIO_write, BIO_puts, BIO_printf, and BIO_vprintf.

Filter BIOs

• Manual:BIO_f_base64(3)
• Manual:BIO_f_buffer(3)
• Manual:BIO_f_cipher(3)
• Manual:BIO_f_md(3)
• Manual:BIO_f_ssl(3)

Source/sink BIOs

• Manual:BIO_s_accept(3)
• Manual:BIO_s_bio(3)
• Manual:BIO_s_connect(3)
• Manual:BIO_s_fd(3)
• Manual:BIO_s_file(3)
• Manual:BIO_s_mem(3)
• Manual:BIO_s_null(3)
• Manual:BIO_s_socket(3)

implementation of those bio are within bio/bss_xxx.c bss stading for Bio Source Sink

Combining Filters and Source Sink BIOs

• Manual:BIO_push(3)

new_head = BIO_push(BIO * local_head, BIO * tail)

will connect tail at end of local_head chain.

WARNING BIO_push will never fail, but can create invalid chains.

In standard usage end of a chain is a source sink, and all other elements are filters.

• Manual:BIO_s_bio(3)

two separated BIOs can then be connected with BIO_make_bio_pair() into a connected pair.