The usage of SetFunction() is shown by the example debugging program at the end of this article, ISpy. ISpy uses a semaphore to gain access to its jump table. This jump table contains pointers to the wedge routines. When executed, each wedge routine puts a shared lock on the semaphore to indicate that the code is being executed. To get an idea of who is calling the debugger entries, ISpy uses a little assembler stub to load a4 with the address of the stack of the caller and calls a C routine where the actual (simple) argument checking is done. When ISpy is signalled to exit, it tries to get an exclusive lock on the semaphore in a Forbid()en state. If this succeeds, it can safely assume its code is not being executed at the moment and can therefore place the original function vectors in the jump table and exit, leaving the semaphore behind. This semaphore is also used to check whether ISpy is already installed. If so, the new instance will exit immediately. Because of the use of shared semaphore locks, this program will only run with V37. By using a global counter (which is incremented each time a function is entered and decremented when it is exited) ISpy can be adapted to V33. Because of the way ISpy is set up, it is very easy to add argument checking front ends for functions, and have multiple versions of ISpy for different libraries. Memoration and Scratcher by Bill Hawes. IO_Torture by Bryce Nesbitt. Wedge and DevMon by Carolyn Scheppner. MungWall by Ewout Walraven (inspired by Memwall by Randell Jesup and MemMung by Bryce Nesbitt).
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