Catching Indy IdUDPClient.SendBuffer error
lazarus / 38-Networking and Web Programming

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Dzandaa · 2023-03-09 15:49

Hi,

Using Lazarus 2.2.4 and Indy on Windows and Linux.

I have an ESP8266 server that respond to an UDP command and send back
Data.

I have an Indy IdUDPClient that send command and receive the data.

All works good except if I close the server or if I connect when  the
IP and port are wrong, IdUDPClient.SendBuffer crash.

I want to catch the error and close correctly.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
 MyUDPClient.Host := IP;   MyUDPClient.Port := Port; 
 MyUDPClient.Connect;  // No error even if the IP or Port doesn't
exist    ...   MyUDPClient.SendBuffer(DataSend); // I want to catch
the error  
Any ide?

Thank you.

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Warfley · 2023-03-09 16:00

It's UDP, so you can't know if the message reached it's destination.

Example:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i var  sock:
TSocket;  Res: SizeInt;begin  sock := UDPSocket(stIPv4);  Res :=
SendStrTo(sock, '153.2.111.10', 5917, 'Test'); // Made up IP and port 
WriteLn(Res); // Result: 4 = Length('Test')  CloseSocket(sock);end.As
you can see, when sending data to this made up ip and port, where
there is no server behind, it still works as expected and tells me
that 4 bytes have been sent, even though they never reached any
destination.

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Dzandaa · 2023-03-09 16:16

Hi,

Yes, I know that you can't rely on UDP and send data to an unknown IP,

All I want to know if it is possible to catch the
IdUDPClient.SendBuffer error with try, except finally.

Thank you.

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Warfley · 2023-03-09 16:18

If an exception is thrown, you you probably can just use try-except.
But you should note that lazarus breaks on exceptions anyway, if they
are enclosed in try-except or not. This is just a debugging
functionality and will not occur in production code running without a
debugger. You can disable handling of certain exception types to avoid
this. Otherwise, just click the continue button when the exception
message pops up.

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Remy Lebeau · 2023-03-10 02:08

Quote from: Warfley on March 09, 2023, 05:00:24 pmIt's UDP, so you
can't know if the message reached it's destination.

Actually, you can, but not immediately.  When you send a packet to a
non-existent IP/port, the network is likely to send back an ICMP
packet explaining that the host/port was unreachable.  If you
Connect() a UDP socket to a peer, and the socket internally receives
such an ICMP packet after a send to that peer, then subsequent sends
on that same peer will fail with an error that you can catch, as long
as the socket remains "connected" to that peer.

Quote from: Dzandaa on March 09, 2023, 05:16:56 pmAll I want to know
if it is possible to catch the IdUDPClient.SendBuffer error with try,
except finally.

Of course it is possible.  It is just a normal 'raise'd exception,
like any other.  Why do you think it is not possible to catch it?

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Dzandaa · 2023-03-10 09:50

Hi,

Thank you Warfley, I forgot that the debugger showed an alert message
when using try, except, etc...
Thank you.

B->