The Question is:
This is a followup to 9378.
I stumbled upon a partial solution viz. don't allow the "input-file" argument
(the second argument) to default. For many calls to LIB$SPAWN within an
application there is no intention for the spawned command to read input and so
a value of "NL:" for this
argument suffices. This then avoids the TRMMBX error.
I note that the LIB$SPAWN doco does not mention TRMMBX as a possible error and
hence or otherwise does not mention the situations in which it is unavoidable
and those in which it is avoidable.
The Answer is :
TRMMBX, terminal has associated mailbox - terminate image and then
Facility: CLI, Command Language Interpreter (DCL)
Explanation: A SPAWN command failed because the current image has
associated a mailbox with the terminal.
User Action: Terminate the current image, then reenter the command.