Select() problem in FD_SET

[Damon123]

I am writing a multiserver program.
I use I/O mutiplexing (SELECT function).

I am going to use one read set (rset_1) of file descriptor for clients, and another set (rset_2) for server, so as to distinguish connection between client and server.

I want to ask how sould I combine these 2 sets in select function?

select(maxfd + 1, &allset, 0,0,0);

or should I call select function twice?

select(maxfd + 1, &rset_1, 0,0,0);
if ( FD_ISSET(listenfd, &rset_1 ) )
{.......}
select(maxfd + 1, &rset_2, 0,0,0);
if ( FD_ISSET(listenfd, &rset_2 ) )
{.......}

However, as I know, select function is blocking and cannot be called twice, am I right??

[Naumaan]

I want to ask how sould I combine these 2 sets in select function?

select(maxfd + 1, &allset, 0,0,0);

or should I call select function twice?

select(maxfd + 1, &rset_1, 0,0,0);
if ( FD_ISSET(listenfd, &rset_1 ) )
{.......}
select(maxfd + 1, &rset_2, 0,0,0);
if ( FD_ISSET(listenfd, &rset_2 ) )
{.......}

You have to call

FD_SET(s, *set) 
Adds descriptor s to set.

Twice to insert your socket descriptor into your fd_set .
if u have a client socket then call FD_SET(s,&rset_1) and if have a server descriptor then call FD_SET(s,&rset_2).

here i assume that u have want to handle two groups of socket some are called client sockets and some are called server sockets.

[Damon123]

thank you for your reply and I know that 2 different FD SET should be used for client and server.

however, if I am in the case that I am waiting for either connection of client and server, how should I implement the SELECT function?

[Richard.J]

What exactly do you want to achieve? Is it a server listening on multiple ports and serving multiple clients at a time?
If yes, simply add all sockets (server and client) to one read-set. Thjis way you would get notified if the clients send data to the server as well as when a new connection request might be accepted.

HTH
Richard