BIO_FLAGS_IO_SPECIAL
#define BIO_FLAGS_RWS (BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
BIO_set_flags(b, (BIO_FLAGS_IO_SPECIAL|BIO_FLAGS_SHOULD_RETRY))
#define BIO_should_io_special(a) BIO_test_flags(a, BIO_FLAGS_IO_SPECIAL)
BIO_FLAGS_IO_SPECIAL|BIO_FLAGS_SHOULD_RETRY);