Exploring Expect

Stepping Into Procedure Calls

The n command executes a procedure atomically. In contrast, it is possible to step into a procedure with the s command. (If the command that is about to be executed is not a procedure, then s and n behave identically.)

Imagine that p4 is just about to be executed. After the s command, the debugger stops before the first command in the procedure and waits for more interactive commands.

4: p4 $b
dbg5.5> s
7: expr 1+$x
dbg8.6>

"expr 1+$x" is the first command to be executed inside of p4. It is nested inside of two brackets, plus the procedure call of p4, so the stack level is increased by three.

s, n, and N take an optional argument in the form of a number describing how many commands to execute. For example:

s 2
s 100
s $b
s [expr 2+[p4 $b]]

The arguments are evaluated according to the usual Tcl rules because s, n, and N are commands known to Tcl.

The debugger will not interrupt procedures invoked from the command line. This is usually the desired behavior although it is possible to change this.

s, n, and N are action commands. This means that the debugger remembers the last one and uses it if you press return without a command. The arguments are remembered as well. So if you want to execute 10 steps at a time, you need only enter "s 10" once and then press returns thereafter.