FD_ZERO and FD_SET.. i still don't get it

yahn · 02-04-2006, 12:35 PM

ok i know that select picks the sockets that are set, but do the sockets set themselves when they have information to be received?

i was told that in the begining of the loop to set all the sockets, but that doesn't really make much sense to me. i want to have a loop and it pick out sockets that have information to be received in it. can anyone tell me if i need to set the sockets before hand or if they become set when they have information. i also need to know if when i process the socket if i should FD_CLEAR it.

**Salem** · 02-04-2006, 01:21 PM

Something like

Code:

fd_set socketSet, selectSet;
// socketSet is the permanent list of sockets you want to monitor.
// having a separate variable saves a lot of FD_SET calls.

// just before calling select, do
selectSet = socketSet;
int rval = select ( maxFd+1, &selectSet, NULL, NULL, NULL );

// if rval > 0, then use FD_ISSET on the selectSet to see which particular sockets have something
// available for reading.

yahn · 02-04-2006, 06:35 PM

ok well that helps some but i still don't get the whole picture. when i accept a socket and i want to add it to the list what should i do? just FD_SET(sock, list_to_add_to)? whenever a message is sent to that socket is it FD_SET? how does select help any if if doesn't get set when it has something in it?

nkhambal · 02-04-2006, 07:45 PM

Check the link below

select()--Synchronous I/O Multiplexing

**Salem** · 02-05-2006, 01:21 AM

The bits you FD_SET in a set on input to select() will be those things you want to watch.
When select returns, the set is modified to indicate which fd's have changed, and these you test with FD_ISSET.

02-04-2006, 12:35 PM	#1
yahn Registered User Join Date: Nov 2005 Posts: 137	FD_ZERO and FD_SET.. i still don't get it ok i know that select picks the sockets that are set, but do the sockets set themselves when they have information to be received? i was told that in the begining of the loop to set all the sockets, but that doesn't really make much sense to me. i want to have a loop and it pick out sockets that have information to be received in it. can anyone tell me if i need to set the sockets before hand or if they become set when they have information. i also need to know if when i process the socket if i should FD_CLEAR it.
yahn is offline

02-04-2006, 06:35 PM	#3
yahn Registered User Join Date: Nov 2005 Posts: 137	ok well that helps some but i still don't get the whole picture. when i accept a socket and i want to add it to the list what should i do? just FD_SET(sock, list_to_add_to)? whenever a message is sent to that socket is it FD_SET? how does select help any if if doesn't get set when it has something in it?
yahn is offline

02-04-2006, 07:45 PM	#4
nkhambal Registered User Join Date: Apr 2005 Posts: 134	Check the link below select()--Synchronous I/O Multiplexing
nkhambal is offline

02-05-2006, 01:21 AM	#5
Salem and the hat of Jobseeking Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,710	The bits you FD_SET in a set on input to select() will be those things you want to watch. When select returns, the set is modified to indicate which fd's have changed, and these you test with FD_ISSET.
Salem is offline

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