Suspending Thread from other Thread under *nix properly
lazarus / 32-Linux

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Skotty · 2020-03-30 22:54

Today I ran into a problem with an application I originally programmed
under Windows:

A thread should be suspended by another thread. The Unix
implementation (tthread.inc) of FPC throws me an error here:
QuoteSuspending one thread from inside another one is unsupported
(because it is unsafe and deadlock prone) by *nix and posix operating
systems
The comment of the person who wrote this implementation for FPC says:
QuotePosix doesn't define anything for suspending threads as this is
inherintly unsafe. Just don't suspend threads at points they cannot
control. Therefore, I didn't implement .Suspend() if its called from
outside the threads execution flow (except on Linux _without_ NPTL).
My question is now: How do I properly tell one thread to wait, until
another thread tells it to resume?

Usually the answer I found, and would use myself, are RTLEvents.

Edit:
I found the actual solution myself after some more testing
I created a "Pause" method on my thread to set a boolean flag via
InterlockedExchange to switch it by any outside thread. Then in my
thread's main loop I check if this flag is set. If yes, I use
RTLWaitFor. Then I created a "Continue" method which sets the RTLEvent
for this, to resume (and of course sets the flag to false).

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eljo · 2020-03-30 23:32

As far as I understand threading
1) threadFunc is called from inside the threads execution context to
set up things so calling suspend from it is calling it from inside the
thread.
2) since suspend calls RtlEventWaitFor(FSuspendEvent) if it is called
from an external thread the external thread will get suspended not the
thread you wish to suspend.

to accomplish what you are after you need to create your own threads
and add a pause method or something along those lines.

This will set/reset an internal rtlevent and set a FPause variable to
true. Inside your threads execute procedure you need to check the
fpause value regualarly and if it is set then call
RtlEventWaitFor(FPause).

You can extend the logic with an extra event that will inform the
external thread you are paused and waiting to be unpaused if the
external thread needs to make sure that its paused before continuing.