TObject, InstanceSize and thread synchronization

[chkaufmann 17]

A TObject has an InstanceSize of 8 bytes. I searched the internet to understand why. 4 bytes would make sence (pointer to the classtype or whatever).

 

All I find are old postings about this change back in 2009, but is this still valid for the current version? And if the second pointer is available for object locking, can somebody point me to an explanation for this. To be honest, I did not really understand how this works.

 

Currently I use a "Spring.Lock" in all my objects for thread synchronization, but maybe this is not necessary and I could use the built in lock feature of TObject?

 

Christian

[Remy Lebeau 1260]

31 minutes ago, chkaufmann said:

A TObject has an InstanceSize of 8 bytes. I searched the internet to understand why. 4 bytes would make sence (pointer to the classtype or whatever).

The extra bytes are for a hidden pointer to a System.TMonitor instance (not to be confused with Vcl.Forms.TMonitor).  Every TObject instance has an optional TMonitor instance associated with it.  The actual TMonitor instance is created the 1st time it is accessed, but the parent TObject's InstanceSize includes space for a pointer to the TMonitor instance.

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All I find are old postings about this change back in 2009, but is this still valid for the current version?

AFAIK, yes.

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And if the second pointer is available for object locking, can somebody point me to an explanation for this.

Read the TMonitor documentation, and also see What is TMonitor in Delphi System unit good for?

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Currently I use a "Spring.Lock" in all my objects for thread synchronization, but maybe this is not necessary and I could use the built in lock feature of TObject?

You could, yes.  You would simply call TMonitor.Enter(obj) and TMonitor.Exit(obj) where needed.

Edited August 29, 2022 by Remy Lebeau

[chkaufmann 17]

On 8/29/2022 at 7:34 PM, Remy Lebeau said:

The extra bytes are for a hidden pointer to a System.TMonitor instance (not to be confused with Vcl.Forms.TMonitor).  Every TObject instance has an optional TMonitor instance associated with it.  The actual TMonitor instance is created the 1st time it is accessed, but the parent TObject's InstanceSize includes space for a pointer to the TMonitor instance.

AFAIK, yes.

Read the TMonitor documentation, and also see What is TMonitor in Delphi System unit good for?

You could, yes.  You would simply call TMonitor.Enter(obj) and TMonitor.Exit(obj) where needed.

When I read https://www.delphitools.info/2013/06/06/tmonitor-vs-trtlcriticalsection/, then I should not use TMonitor. Is this still the case? Is there another simpler solution to lock an object in order to avoid that multiple threads change an internal value. Locktime will be very short and conflicts probably happen very rarely.

[chkaufmann 17]

And what about using a TLightweightMREW for each of my objects compared to TMonitor and Spring.Lock?

I probably need this to have a reentrant version of the lock:
https://www.thedelphigeek.com/2021/02/readers-writ-47358-48721-45511-46172.html

 

Christian

[Fr0sT.Brutal 897]

On 9/6/2022 at 4:02 PM, chkaufmann said:

 https://www.delphitools.info/2013/06/06/tmonitor-vs-trtlcriticalsection/

Haven't you noticed that:

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Edit 2013-08-24: looks like the issue discussed here should be resolved in XE5