BIO_number_written
str = calloc(BIO_number_written(b) + 1, 1);
BIO_read(b, str, BIO_number_written(b));
BIO_printf(bio_err, "bytes written: %8ju\n", BIO_number_written(out));
BIO_number_written(SSL_get_wbio(s)));
uint64_t BIO_number_written(BIO *bio);