Example OpenSSL Server + Client code to exchange data

client.c

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

#define HOST_ADDR "localhost:4433"
#define MSG_SIZE 512

int main(void) {
    BIO *out = NULL;
    char buf[MSG_SIZE];
    char *msg = "GET / HTTP/1.0\r\n\r\n";
    size_t msg_len = strlen(msg);

    fprintf(stdout, "Len of string -> %d\n", msg_len);

    int left_off = 0; 
    SSL_CTX *ctx = NULL;
    SSL *ssl = NULL; 

    ctx = SSL_CTX_new(TLS_client_method());
    if (ctx == NULL) {
        fprintf(stderr, "Failed creating SSL context\n");
        return EXIT_FAILURE;
    }

    ssl = SSL_new(ctx);
    if (ssl == NULL) {
        fprintf(stderr, "Failed creating SSL object\n");
        return EXIT_FAILURE;
    }

    out = BIO_new(BIO_s_connect());
    BIO_set_conn_hostname(out, HOST_ADDR);

    BIO_set_nbio(out, 1);

    for (;;) {
        int bytes = BIO_write(out, &(msg[left_off]), msg_len);
        if (bytes <= 0) {
            if (BIO_should_retry(out)) {
                fprintf(stderr, "Retrying..\n");
                sleep (1); 
                continue;
            }

            fprintf(stderr, "Failed writing a message through the socket\n");
            return EXIT_FAILURE;
        }

        left_off += bytes; 
        msg_len -= bytes; 

        if (msg_len <= 0) 
            break;
    }

    fprintf(stdout, "Finished writing the whole message to the server\n");
    fprintf(stdout, "Exiting..\n");

    BIO_free(out);
    SSL_CTX_free(ctx);

    return EXIT_SUCCESS;
}

server.c

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

#define HOST_PORT "4433"
#define MSG_SIZE 512

int main(void) {

    SSL_library_init();

    SSL_CTX *ctx = NULL; 
    BIO *in = NULL;
    BIO *tmp = NULL;
    char *buf[MSG_SIZE]; 

    ctx = SSL_CTX_new(TLS_server_method());
    if (ctx == NULL) {
        fprintf(stderr, "Failed creating ctx object\n");
        return EXIT_FAILURE; 
    }

    in = BIO_new_accept(HOST_PORT);
    if (in == NULL) {
        fprintf(stderr, "Failed starting up service on port %s\n", HOST_PORT);
        return EXIT_FAILURE;
    }

    fprintf(stdout, "Listening on port %s...\n", HOST_PORT);

again: 

    if (BIO_do_accept(in) <= 0) {
        fprintf(stderr, "Failed accepting bio\n");
        return EXIT_FAILURE;
    }

    for (;;) {
        int bytes = BIO_read(in, buf, MSG_SIZE);
        if (bytes == 0) {
            printf("We don't got anything to read bro we're chilling\n");
            tmp = BIO_pop(in);            
            BIO_free_all(tmp);
            goto again;
        }
        if (bytes < 0) {
            fprintf(stderr, "Failed reading from stream\n");
            ERR_print_errors_fp(stderr);
            return EXIT_FAILURE;
        }

        fflush(stdout);
        fprintf(stdout, "Reading %d bytes from stream: '%s'\n", bytes, buf);
    }

    BIO_free(in);
    return EXIT_SUCCESS;
}