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_set_flags(b, BIO_FLAGS_WRITE | BIO_FLAGS_SHOULD_RETRY);
err = BIO_FLAGS_WRITE;
while (!clienterr && !retc && err == BIO_FLAGS_WRITE) {