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