ns_sockaccept accepts a new connection pending on sockid.
Example
#listen for connections on port 9000
set sock [ns_socklisten * 9000]
#wait for new connection
set fds [ns_sockaccept $sock]
set rfd [lindex $fds 0]
set wfd [lindex $fds 1]
puts $wfd "Hello!"
close $rfd
close $wfd
close $sock