Exploring Expect

Example—Logging By Tracing

The log_file command does not provide any special support for writing the output of different processes to different log files. This can be simulated by tracing the expect_out array. Remember that the spawn_id element identifies the particular spawn id that produced the output.

The following trace procedure writes the value the expect_out(buffer) to a file specific to the spawn id.

proc log_by_tracing {array element op} {
    uplevel {
        global logfile
        set file $logfile($expect_out(spawn_id))
        puts -nonewline $file $expect_out(buffer)
    }
}

The association between the spawn id and each log file is made in the array logfile which contains a pointer to the log file based on the spawn id. Such an association could be made with the following code when each process is spawned.

spawn ...
set logfile($spawn_id) [open ... w]

The trace is armed in the usual way:

trace variable expect_out(buffer) w log_by_tracing

Internally, the expect command saves the spawn_id element of expect_out after the X,string elements but before the buffer element. For this reason, the trace must be triggered by the buffer element rather than the spawn_id element. A trace triggered on expect_out(spawn_id) will see an old value of expect_out(buffer).

In Chapter 6 (p. 147), I described how process output could be discarded if more arrived than was permitted by match_max. In fact, a copy of the output is saved in expect_out(buffer) before it is removed from the internal buffer. So even if the expect command ...