Library /sys$common/syshlp/dbg$help.hlb DEBUG, Built in Symbols, %CALLER TASK  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    (Applies only to Ada tasking programs.) Gives you the task which
    is the entry caller of the active task during a task rendezvous.

    If the active task (%ACTIVE_TASK) is not currently executing
    an accept statement (that is, a rendezvous is not in progress),
    %CALLER_TASK returns %TASK 0.

    Example:

    The following command sets a breakpoint within an accept
    statement. The breakpoint is triggered only when %TASK 3 is the
    task making the entry call of the rendezvous.

    DBG> TYPE 51:53
    module SAMPLE
        51:         accept RENDEZVOUS do
        52:            PUT_LINE("Beginning the rendezvous");
        53:         end RENDEZVOUS;
    DBG> SET BREAK %LINE 52 WHEN (%CALLER_TASK = %TASK 3)