question about asynchronous development

Hi,
(sorry for the bad English)

I'am developping a C program with NET-SNMP.
This program collects SNMP data from a large number of targets.

There are two designs:
1) synchronous and multi-threads
one thread do a sysnchronous collection from one target
demo: http://net-snmp.sourceforge.net/wiki...nc_Application
2) asynchronous
use asynchronous send function like the demo on the website
Which one is better? (stable, faster ...)
Or any other better design?

Then, some problem about asynchronous send function:
1) I know there is a call-back function.
Does the call-back function run multi-instances. (like multi-threads)
2) When will the call-back function run?
After snmp_read() ?
Or whenever a response is received? (even before the second snmp_send())
3) I don't quite understand the code below. (these code comes from the demo)
>/* loop while any active hosts */
> while (active_hosts) {
> int fds = 0, block = 1;
> fd_set fdset;
> struct timeval timeout;
>
> FD_ZERO(&fdset);
> snmp_select_info(&fds, &fdset, &timeout, &block);
> fds = select(fds, &fdset, NULL, NULL, block ? NULL :
&timeout);
> if (fds < 0) {
> perror("select failed");
> exit(1);
> }
> if (fds)
> snmp_read(&fdset);
> else
> snmp_timeout();
> }
Why we need this part of code? Whitout these code, will the
call-back function work?
Will these code block themselves when no response is received?
Or cost all CPU resource?

Thanks,
pgf
email: pgf00a@gmail.com

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