Thread Debugging with the Thread Information Block

• Download demo project - 8 Kb

Introduction

I have found one of most difficult things about thread programming is debugging, because you must constantly keep track of which thread is active, and what thread is supposed to be doing. While I was looking in MSDN for something else, I came across an article by John Robbins. Toward very end of article, Robbins shows how to store descriptive text in Thread Information Block (TIB) (which Matt Pietrek has described), and then how to display this information in VC debugger using a watch expression.

IMPORTANT NOTE:   I am not claiming authorship for idea of using TIB to store thread description. Please see citations listed below for original articles by Pietrek and Robbins. I have merely repackaged TIB routines into handy header file, and would like to bring this to attention of others here at CodeProject, because I believe this technique is so obscure that few may be aware of it.

Thread Information Block

Matt Pietrek documented fields of TIB in his May 1996 article in MSJ. There is TIB for each thread running in Win32 system, and in all Intel-based Win32 implementations, FS register points to TIB. This leads to following function to obtain TIB address of active thread:

static tagXTIB * GetTIB()
{
	tagXTIB * pTib;

	__asm
	{
		MOV  EAX , FS:[18h]
		MOV  pTib , EAX
	}
	return pTib;
}

Among TIB structure fields that are common to all Win32 implementations, there is one at 14H called pvArbitrary, which is available for applications to use however they want. This is field that XTib will use to store pointer to descriptive string.

How To Use

First thing to do is to include header file XTib.h. Next, call SetThreadName() from thread function, with string that will uniquely identify thread. This string can be generated from thread function's lpParameter (thread function argument) - for example, lpParameter could be some integer value, or pointer to some string. Final step is to set watch expression in debugger. To do this, set breakpoint (for example, somewhere in thread function). When debugger breaks, enter this expression in watch window:

(char*)(DW(@TIB+0x14))

Now debugger will automatically display string pointed to by pvArbitrary field in TIB. This saves you from having to go to Debug | Threads to find out what thread you are in.

For applications running under NT, another useful watch expression is

DW(@tib+0x24)

This will display the thread ID, which is stored at 24H in the TIB on NT 4, Win2000, and XP.

XTib Demo

Demo project provides sample app that creates five threads and lets you see what is displayed in watch window. Here is what watch window will display for thread 1:

      

Acknowledgments

XTib code is based on:

• "Under The Hood", May 1996 Microsoft Systems Journal, by Matt Pietrek.
• "Bugslayer", January 2000 Microsoft Systems Journal, by John Robbins.

Usage

This software is released into the public domain. You are free to use it in any way you like. If you modify it or extend it, please to consider posting new code here for everyone to share. This software is provided "as is" with no expressed or implied warranty. I accept no liability for any damage or loss of business that this software may cause.