how to clean up the previous expect_buf in an expect script - tcl

Question

I've written a expect function to get the output of a command and my code is like below
proc do_cmd {cmd id} {
set spawn_id $id
send "$cmd\r"
expect "$cmd\r"
expect {
-re "\n(.*)\r\n" {return $expect_out(1,string)}
default {exit 1}
}
}
If I call the function just once it would works fine and return something I want, but if I call it continually without a break, it would return something unwanted.
# test case 1
set ret [do_cmd $mycmd $spawn_id]
puts "$mycmd returns $ret" # the return value is ok
# test case 2
set ret [do_cmd $mycmd $spawn_id]
set ret [do_cmd $mycmd $spawn_id]
puts "$mycmd returns $ret" # the return value is not something I want
I use the 'exp_internal 1' to debug it and found that the expect_out in the second called command still holds the previous output info and caused the matched problem, so how can I clean up the expect_out buffer(I tried to set it an empty string but it doesn't work,) or is there anything else I can do to avoid this problem? Thanks in advance.

Answer 1

Don Libes's suggestion for your scenario is as follows,
Sometimes it is even useful to say:
expect *
Here the * matches anything. This is like saying, "I don't care what's
in the input buffer. Throw it away." This pattern always matches, even
if nothing is there. Remember that * matches anything, and the empty
string is anything! As a corollary of this behavior, this command
always returns immediately. It never waits for new data to arrive. It
does not have to since it matches everything.
Reference : Exploring Expect
In this case, after your required match, better try to save the match to some variable then simply add the code expect * at the last. This will empty the buffer. Your code can altered as below.
proc do_cmd {cmd id} {
set spawn_id $id
send "$cmd\r"
#Looks like you are looking for a particular command to arrive
expect "$cmd\r"
#Then you have one more expect here which is you want to get it
expect {
#Saving the value sub match to the variable 'result'
-re "\n(.*)\r\n" {set result $expect_out(1,string)}}
}
#Causing the buffer to clear and it will return quickly
expect *
return $result
}
Apart from this, there is one more way can be unsetting the expect_out(buffer) content itself which will remove the 'buffer' index from expect_out array which can be depicted as
unset expect_out(buffer)
When the next match happens, expect_out array will be updated the index 'buffer' and we can have the fresh expect_out(buffer) value. Replace the expect * with the above code if you prefer to use this way.
This is quite a workaround kind of stuff to get what we want actually. You can go ahead with any approach. Choice is yours. :)