How to set SSL_connect on non blocking socket with select on linux platform - select

Question

I am trying to use the select function to have non-blocking connection from a client to server.I found a tutorial with some code and tried to adapt to this:
...
sockfd = socket(AF_INET, SOCK_STREAM, 0);
err = connect(sockfd,(struct sockaddr*)&sa,sizeof(sa));
...
SSL_set_fd(pssl,sockfd);
err = SSL_connect_nonb(pssl,sockfd,60);
if(err <=0 ){
printf("SSL_connect:%s\n",ERR_error_string(SSL_get_error(pssl,err),NULL));
return -1;
}
...
The SSL_connect_nonb function is defined as bellow:
int SSL_connect_nonb(SSL*pssl,int sockfd, int nsec)
{
int flags, error;
socklen_t len;
fd_set rset, wset;
struct timeval tval;
flags = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);
int err = SSL_connect(pssl);
int err2 = SSL_get_error(pssl,err);
switch(err2) {
default:
printf("SSL_connect err=%s\n",ERR_error_string(err2,0));
return -1;
break;
case SSL_ERROR_WANT_READ:
case SSL_ERROR_WANT_WRITE:
break;
}
FD_ZERO(&rset);
FD_ZERO(&wset);
FD_SET(sockfd, &rset);
FD_SET(sockfd, &wset);
tval.tv_sec = nsec;
tval.tv_usec = 0;
if (select(sockfd+1, &rset, &wset, NULL,nsec ? &tval:NULL) == 0) {
return -1;
}
if(FD_ISSET(sockfd,&rset) || FD_ISSET(sockfd, &wset )) {
len = sizeof(error);
if(getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &error, &len) < 0){
return -1;
}
}else{
printf("sockfd not set\n");
return -1;
}
fcntl(sockfd, F_SETFL, flags);
if (error) {
return -1;
}
return 1;
}
The sockfd is correct with connect，the problem is that in SSL_connect_nonb the select function return avalue=1 (actually the condition FD_ISSET(sockfd, &wset) is successful all time),but if I use blocking method as this :
....
SSL_set_fd(pssl,sockfd);
err = SSL_connect(pssl);
if(err <=0 ){
printf("SSL_connect:%s\n",ERR_error_string(SSL_get_error(pssl,err),NULL));
return -1;
}
...
the value of err is 0 because the SSL_connect is not successful, so, how to do with SSL_connect on non blocking socket by calling select function?

Answer 1

SSL_connect(), aka SSL client handshake, is a complicated process, which requires several roundtrip with servers. So, when you run SSL_connect() on a non-blocking socket, it's not enough to just run it only once. When you get SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, you must retry SSL_connect() again, until it succeeded or failed with other errors.