Regarding named pipes behaviour in tcl - tcl

Question

I have a question regarding named pipes in tcl.
First I created the pipe with mkfifo:
mkfifo foo
Then execute the following tcl script:
set fifo [open "foo" r]
fconfigure $fifo -blocking 1
proc read_fifo {} {
global fifo
puts "calling read_fifo"
gets $fifo x
puts "x is $x"
}
puts "before file event"
fileevent $fifo readable read_fifo
puts "after file event"
When i run the tcl script it waits for an event without outputting anything.
Then, when I write to the fifo:
echo "hello" > foo
Now, the tcl scripts prints out :
before file event
after file event
Why is 'read_fifo' function call not getting triggered here ?
Could anyone help me in understanding this behaviour.

Answer 1

fileevent relies on the the eventloop, which you don't enter.
fileevent just tells Tcl to call read_fifo when it is readable.
If you want blocking IO, then just call gets. This blocks until an entire line has been read.
set fifo [open "foo" r]
fconfigure $fifo -blocking 1
gets $fifo x
puts "x is $x"
If you do it event-driven, you need fileevent, use non-blocking IO and you have to enter the event-loop (e.g. with vwait forever).
set fifo [open "foo" r]
fconfigure $fifo -blocking 0
proc read_fifo {fifo} {
puts "calling read_fifo"
if {[gets $fifo x] < 0} {
if {[eof $fifo]} {
# Do some cleanup here.
close $fifo
}
}
puts "x is $x"
}
fileevent $fifo readable [list read_fifo $fifo]
vwait forever; #enter the eventloop
Don't mix event-driven with blocking IO. This does not really work.
Note that you don't have to call vwait in Tk, doing so would reenter the event-loop, which is considered bad practice.