ns_sockselect - Determine processable channels
ns_sockselect uses a select() call to determine which channels are readable, writable, or have exceptional conditions. ns_sockselect returns a list of three lists channels, readable, writable, exception channels. The -timeout option specifies the length of time to wait in seconds for ns_sockselect to return before timing out. The arguments rfds, wfds, efds are lists of channels for readable, writable, and exception channels.
# Start nonblocking connect()'s to www01 through
# www09.foo.com and remember the read channelId which
# corresponds to each write channelId.
for {set n 1} {$n < 10} {incr n} {
set host [format "www%2d.foo.com" $n]
set fds [ns_sockopen -nonblock $host 80]
set r [lindex $fds ]
set w [lindex $fds 1]
set w2r($w) $r
lappend wfds $w
}
# All connect()'s are in progress, use select to wait for one or
# more to become writable in the next two seconds which means
# they may have connected. We're not interested in readable or
# exception sockets so the corresponding lists are empty
# (i.e., {}).
set sel [ns_sockselect -timeout 2 {} $wfds {}]
# Select returned - get the sockets ready to write to.
set wfds [lindex $sel 1]
# Use ns_sockcheck to see if the sockets actually connected and
# didn't become writable because the connect() failed (e.g., no
# Web server was running on port 80 on the corresponding machine).
# Note that the wfds list may be empty, meaning all sockets timed
# out on connect.
set ok ""
foreach w $wfds {
if [ns_sockcheck $w] {
# Socket is connected - send a GET HTTP request.
lappend ok $w
puts $w "GET /index.htm HTTP/1.0\r\n\r"
# The flush is important, otherwise the remote
# server may never see the data.
flush $w
}
}
# Get the read ids for each socket which we sent the GET request to.
foreach w $ok {
lappend rfds $w2r($w)
}
# Use select again to wait for the read sockets to have data
# available in response to the GET request.
set sel [ns_sockselect -timeout 2 $rfds {} {}]
set rfds [lindex $sel 0]
# Read the pages which came back.
foreach r $rfds {
if [ns_sockcheck $r] {
set page($r) [read $r]
}
}
# Close all the sockets
foreach w [array names w2r] {
close $w
close $w2r($w)
}