Clarification.

buffer is full request a retry. BIO_ctrl_get_write_guarantee() is a function

whereas BIO_get_write_guarantee() is a macro.

BIO_get_read_request() and BIO_ctrl_get_read_request() return the amount of data

requested (or the buffer size if it is less) if the last read failed due to an

empty buffer. This can be used to determine how much data should be written to the

other half of the pair so the next read will succeed: this is most useful in TLS/SSL

applications where the amount of data read is usually meaningful rather than just

a buffer size. After a successful read this call will return zero.

BIO_get_read_request() and BIO_ctrl_get_read_request() return the

amount of data requested, or the buffer size if it is less, if the

last read attempt at the other half of the BIO pair failed due to an

empty buffer.  This can be used to determine how much data should be

written to the BIO so the next read will succeed: this is most useful

in TLS/SSL applications where the amount of data read is usually

meaningful rather than just a buffer size. After a successful read

this call will return zero.  It also will return zero once new data

has been written satisfying the read request or part of it.

Note that BIO_get_read_request() never returns an amount larger

than that returned by BIO_get_write_guarantee().

BIO_ctrl_reset_read_request() can also be used to reset the value returned by

BIO_get_read_request() to zero.