Calling TIdTCPClient.IOHandler.Write from main thread
lazarus / 38-Networking and Web Programming

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PawelO · 2025-07-30 15:17

Hello,

Recently, I found in this thread:
https://en.delphipraxis.net/topic/1010-using-indy-for-cross-platform-
tcpip/ that calling TIdTCPClient.IOHandler.Write... (which is
something that I've been doing since forever - thanks to almost every
tutorial - and that seems to work fine) from main thread can be
"unsafe" and it's not recommended.

However, I cannot call it from the thread I have already created and
used for listening (TIdTCPClient.IOHandler.Read...), since this thread
is usually blocked waiting for incoming messages. Do I need a third
thread then? Or can I use same thread, sequentially reading with some
small timeout (but how small can it be), and writing from queue?

I basically want to create a TCPClient wrapper that has Connect,
Disconnect, WriteLn methods, some OnReadLn event and Connected
property all working with main (UI) thread, in an asynchronous, non-
blocking way. Are somewhere examples how to do it in the most simple
and "right" way?

Best regards

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Remy Lebeau · 2025-07-30 17:13

Quote from: PawelO on July 30, 2025, 05:17:49 pmDo I need a third
thread then? Or can I use same thread, sequentially reading with some
small timeout (but how small can it be), and writing from queue?

Basically, yes.  Either approach will work. Use whichever one makes
more sense for your app's design.

Quote from: PawelO on July 30, 2025, 05:17:49 pmI basically want to
create a TCPClient wrapper that has Connect, Disconnect, WriteLn
methods, some OnReadLn event and Connected property all working with
main (UI) thread, in an asynchronous, non-blocking way.

Then perhaps Indy is not the best choice for your app. Since Indy is
primarily a synchronous library, threads and timeouts are basically
your only options for asynchronous behaviors. Otherwise, look into
other options if you need truly asynchronous sockets.

Quote from: PawelO on July 30, 2025, 05:17:49 pmAre somewhere examples
how to do it in the most simple and "right" way?

Nothing official, no.  Just posts floating around the various forum
sites over the years.