Race conditions / buffer issues in console output

[Markus]

I had troubles concerning the order of console output, esp. in combination with ModelSim/Questasim.

Problem Description:
Assume the following proc in VendorScript_Mentor.tcl (pseudocode):

proc analyze {FileName} {
  puts Starting VCOM
  vcom ... $FileName
  puts VCOM Done
}

I got the following output:
Starting VCOM
VCOM Done
…
<actual output from vcom>

So there seems to be a problem concerning output buffering or something similar. The actual output from the vcom command is displayed AFTER the command has long finished and the tcl script has already proceeded.

I narrowed it down to the usage of puts (built-in tcl command to write to console) vs. echo (ModelSim/QuestaSim specific command that writes to transcript and console). It seems those two have “race conditions”.

[Markus]

It seems the problem was on my end after all
When I call
vsim -batch -do "build project.pro"
the output is fine. However, when I call it with a pipeline operator to postprocess the output
vsim -batch -do "build project.pro" | cat
the output is somehow mixed up.

This is a buffering problem which can be solved with unbuffer:
unbuffer -p vsim -batch -do "build project.pro" | cat

See also the respective github isse:
Issue on Github

[Author]

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