non-blocking BIO_do_connect()

nb_bio_conn.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h> /* strerror */
#include <errno.h>
#include <poll.h>

#include <openssl/bio.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

static void print_ssl_err(unsigned long ssl_err, const char * const func)
{
	const char * const err_str = ERR_reason_error_string(ssl_err);
	if (err_str)
		fprintf(stderr, "%s\n", err_str);
	else
		fprintf(stderr, "%s failed: %lu (0x%lx)\n", func, ssl_err, ssl_err);
}

/*
 * Compile:
 *   macOS:
 *     gcc -o a.out nb_bio_conn.c -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib -lssl -lcrypto
 */

int main(int argc, char **argv)
{
	if (argc != 3) {
		fprintf(stderr, "%s host port\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	const char *host = argv[1];
	int port = atoi(argv[2]);

	if (errno != 0) {
		fprintf(stderr, "atoi(): %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	(void)SSL_library_init();
	SSL_load_error_strings();

	SSL_CTX *ctx = NULL;
	SSL *ssl = NULL;
	BIO *bio = NULL;
	long ret;
	unsigned long ssl_err;
	char hostname[4096];

	const SSL_METHOD *method = TLS_client_method();
	if (!method) {
		ssl_err = ERR_get_error();
		print_ssl_err(ssl_err, "TLS_client_method");
		goto err;
	}

	ctx = SSL_CTX_new(TLS_client_method());
	if (!ctx) {
		ssl_err = ERR_get_error();
		print_ssl_err(ssl_err, "SSL_CTX_new");
		goto err;
	}

	bio = BIO_new_ssl_connect(ctx);
	if (!bio) {
		ssl_err = ERR_get_error();
		print_ssl_err(ssl_err, "BIO_new_ssl_connect");
		goto err;
	}

	snprintf(hostname, sizeof(hostname), "%s:%d", host, port);
	ret = BIO_set_conn_hostname(bio, hostname);
	if (ret != 1) {
		ssl_err = ERR_get_error();
		print_ssl_err(ssl_err, "BIO_set_conn_hostname");
		goto err;
	}

	BIO_get_ssl(bio, &ssl);
	SSL_set_tlsext_host_name(ssl, host);
	BIO_set_nbio(bio, 1);

	while (1) {
		ret = BIO_do_connect(bio);
		if (ret == 1) {
			break;
		}

		int n;
		struct pollfd pollfd; 
		int ms = 1000;  /* timeout */

		int err = SSL_get_error(ssl, (int)ret);
		switch (err) {
		case SSL_ERROR_WANT_WRITE:
		case SSL_ERROR_WANT_READ:
			pollfd.fd = SSL_get_fd(ssl);
			pollfd.events = POLLIN | POLLOUT;
			n = poll(&pollfd, 1, ms);
			if (n == 1) {
				continue;
			} else {
				fprintf(stdout, "connect to %s:%d timeouted: %d ms\n",
					host, port, ms);
				goto err;
			}

		case SSL_ERROR_WANT_CONNECT:
			continue;

		default:
			ssl_err = ERR_get_error();
			print_ssl_err(ssl_err, "BIO_do_connect");
			break;
		}
	}

	if (ret != 1) {
		fprintf(stderr, "connect to %s:%d failed\n", host, port);
		goto err;
	}

	fprintf(stdout, "connect to %s:%d done\n", host, port);
	ret = BIO_do_handshake(bio);
	if (ret != 1) {
		ssl_err = ERR_get_error();
		print_ssl_err(ssl_err, "BIO_do_handshake");
		goto err;
	}

	fprintf(stdout, "tls handshake done\n");

	/* do the read and write things */

err:
	if (ctx) SSL_CTX_free(ctx);
	if (bio) BIO_free(bio);

	exit(EXIT_SUCCESS);
}