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    <p>Here's a sample piece of code that raises an exception into an other thread. It uses <code>SuspendThread</code> to stop the thread, <code>GetThreadContext</code> to read the thread's registers, alters <code>EIP</code> (the instruction pointer), uses <code>SetThreadContext</code> and then <code>ResumeThread</code> to restart the thread. It works!</p> <h2>UKilThread unit</h2> <p>Nicely packaged for reuse unit that provides the <code>AbortThread()</code> routine:</p> <pre><code>unit UKillThread; interface uses Classes, Windows, SysUtils; procedure AbortThread(const Th: TThread); implementation // Exception to be raized on thread abort. type EThreadAbort = class(EAbort); // Procedure to raize the exception. Needs to be a simple, parameterless procedure // to simplify pointing the thread to this routine. procedure RaizeThreadAbort; begin raise EThreadAbort.Create('Thread was aborted using AbortThread()'); end; procedure AbortThread(const Th: TThread); const AlignAt = SizeOf(DWORD); // Undocumented; Apparently the memory used for _CONTEXT needs to be aligned on DWORD boundary var Block:array[0..SizeOf(_CONTEXT)+512] of Byte; // The _CONTEXT structure is probably larger then what Delphi thinks it should be. Unless I provide enough padding space, GetThreadContext fails ThContext: PContext; begin SuspendThread(Th.Handle); ZeroMemory(@Block, SizeOf(Block)); ThContext := PContext(((Integer(@Block) + AlignAt - 1) div AlignAt) * AlignAt); ThContext.ContextFlags := CONTEXT_FULL; if not GetThreadContext(Th.Handle, ThContext^) then RaiseLastOSError; ThContext.Eip := Cardinal(@RaizeThreadAbort); // Change EIP so we can redirect the thread to our error-raizing routine SetThreadContext(Th.Handle, ThContext^); ResumeThread(Th.Handle); end; end. </code></pre> <h2>Demo project</h2> <p>Here's how to use <code>AbortThread</code>:</p> <pre><code>program Project23; {$APPTYPE CONSOLE} uses SysUtils, Classes, Windows, UKillThread; var Th: TThread; type TTestThread = class(TThread) public procedure Execute;override; end; { TTestTrehad } procedure TTestThread.Execute; var N: Integer; begin try N := 1; while not Terminated do begin WriteLn(N); Inc(N); Sleep(1000); end; except on E:Exception do WriteLn(E.ClassName + ' / ' + E.Message); end; end; begin Th := TTestThread.Create(False); WriteLn('Press ENTER to raize exception in Thread'); ReadLn; AbortThread(Th); WriteLn('Press ENTER to exit'); ReadLn; end. </code></pre> <h2>Disclaimer</h2> <p>Please make sure you understand what this code does before you actually use it. This is by no means a replacement for proper <code>Terminate - Terminated</code> logic (that is, cooperative thread shut-down), but it's a better alternative to <code>TerminateThread()</code>. This has been modeled after the <a href="http://msdn.microsoft.com/en-us/library/ty8d3wta.aspx" rel="nofollow">.NET Thread.Abort()</a> method. I have no idea how the actual .NET method was implemented but none the less read up on that because the potential problems of using this code are similar:</p> <ul> <li>The method doesn't actually terminate the thread, it raises an <code>EAbort</code> -derived exception in the context of the thread. The thread's code might catch the exception. That's very unlikely because <code>EAbort</code> exceptions are not supposed to be handled.</li> <li>The method might stop the thread <em>at any time</em>. It might stop the thread while it's handling a <code>finally</code> section or while setting up a new exception frame. Even if your thread uses proper <code>try-finally</code> blocks, it might cause memory or resource leaks if the exception is raised after a resource has been allocated but before the resource has been assigned to a variable.</li> <li>The code might cause deadlocks if the thread is interrupted immediately after <code>EnterCriticalSection</code> and just before the <code>try-finally</code> that normally follows. The <a href="http://msdn.microsoft.com/en-us/library/ms682608%28v=vs.85%29.aspx" rel="nofollow">MSDN page for EnterCriticalSection</a> mentions: <code>"If a thread terminates while it has ownership of a critical section, the state of the critical section is undefined."</code>. This came as a surprise to me, I'd intuitively expect the critical section to be "released" when the owning thread terminates, but apparently that's not so.</li> </ul>
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    1. USCosmin Prund
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    1. CO+1; nice answer
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    2. COVery clever indeed but rather impractical. This is deadlock bait. You should say so in your answer and recommend that nobody actually uses the code for real.
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    3. CO@David, I implemented something very similar to what [.NET Thread.Abort()](http://msdn.microsoft.com/en-us/library/ty8d3wta.aspx) does, complete with the long list of potential problems listed for that method! Any thread should be somewhat prepared for an exception to pop up anytime, for example because of `out-of-memory` conditions. Since this pops the exception into the thread's stack, all code in `finally` sections should be executed, giving the thread a chance to clean up nicely. This is clearly much nicer then simply terminating the background thread. I don't think this is deadlock bait.
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