ModelSim freezes when it executes [gets stdin]

Question

I have a TCL stript steering my ModelSim simulation. I would like to be able to break the simulation, ask user for some input and continue. vsim_break is not exactly what I want. I've tried set user_input [gets stdin], but the simulator freezes immediately. Are there other ways?

Answer 1

When you do gets stdin, Tcl (normally) waits until it receives a line from standard input. If you want to keep a GUI (or other background-processing system) going but still wait for a line, you need a slightly different approach.

proc getStdinLine {} {
    set ::user_input [gets stdin]
}
fileevent stdin readable getStdinLine
vwait ::user_input
# Unregister the callback
fileevent stdin readable {}

BEWARE! This code can have problems with reentrancy; if your script manages to get back to the same point while it is waiting, it can very easily get itself in a real mess. This is because vwait keeps a record of what is being waited for on the C stack; this remains true even in Tcl 8.6 (though that has coroutines which you can use to create a workaround, if ModelSim is cooperative enough).