BIO_f_readbuffer - read only buffering BIO that supports BIO_tell() and
  BIO_seek()
 #include <openssl/bio.h>
 const BIO_METHOD *BIO_f_readbuffer(void);
BIO_f_readbuffer() returns the read buffering BIO method.
This BIO filter can be inserted on top of BIO's that do not
    support BIO_tell() or BIO_seek() (e.g. A file BIO that uses
    stdin).
Data read from a read buffering BIO comes from an internal buffer
    which is filled from the next BIO in the chain.
BIO_gets() is supported for read buffering BIOs. Writing
    data to a read buffering BIO is not supported.
Calling BIO_reset() on a read buffering BIO does not clear
    any buffered data.
Read buffering BIOs implement BIO_read_ex() by using BIO_read_ex()
  operations on the next BIO (e.g. a file BIO) in the chain and storing the
  result in an internal buffer, from which bytes are given back to the caller as
  appropriate for the call. BIO_read_ex() is guaranteed to give the
  caller the number of bytes it asks for, unless there's an error or end of
  communication is reached in the next BIO. The internal buffer can grow to
  cache the entire contents of the next BIO in the chain. BIO_seek() uses
  the internal buffer, so that it can only seek into data that is already read.
BIO_f_readbuffer() returns the read buffering BIO method.
bio(7), BIO_read(3), BIO_gets(3), BIO_reset(3),
  BIO_ctrl(3).
Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License").
    You may not use this file except in compliance with the License. You can
    obtain a copy in the file LICENSE in the source distribution or at
    <https://www.openssl.org/source/license.html>.