Each pvmd
maintains a list of all tasks under its management
(Figure 
).
Every task, regardless of state, is a member of a
threaded list,
sorted by task id.
Most tasks are also in a second list,
sorted by process id.
The head of both lists is
locltasks.
PVM provides a simple debugging system
described in Section .
More complex debuggers can be built by using
a special type of task called a tasker, introduced in version 3.3.
A tasker starts (execs, and is the parent of)
other tasks.
In general,
a debugger is a process that
controls the execution of other processes - can read and write
their memories and start and stop instruction counters.
On many species of Unix,
a debugger must be the direct parent of any processes it controls.
This is becoming less common with growing availability of the
attachable ptrace interface.
The function of the tasker interface overlaps with the simple debugger starter, but is fundamentally different for two reasons: First, all tasks running under a pvmd (during the life of the tasker) may be children of a single tasker process. With PvmTaskDebug, a new debugger is necessarily started for each task. Second, the tasker cannot be enabled or disabled by spawn flags, so it is always in control, though this is not an important difference.
If a tasker is registered (using pvm_reg_tasker()) with a pvmd when a DM_EXEC message is received to start new tasks, the pvmd sends a SM_STTASK message to the tasker instead of calling execv(). No SM_STTASKACK message is required; closure comes from the task reconnecting to the pvmd as usual. The pvmd doesn't get SIGCHLD signals when a tasker is in use, because it's not the parent process of tasks, so the tasker must send notification of exited tasks to the pvmd in a SM_TASKX message.
