Texas Instruments TI89 Developer Guide - Page 140

98 Chapter 10: Error Handling 10.6. 10.6.1. Handle variables h1, h2, and h3 must be initialized to H_NULL so the error handling block can distinguish between allocated and unallocated handles. HeapAllocThrow returns a memory handle (a value other than H_NULL) if it succeeds. If HeapAllocThrow fails, it throws ER_MEMORY, thereby transferring execution to the ONERR block. HeapFreeIndir frees the handle if it is non-null and sets the variable back to H_NULL. The ONERR block is skipped if the entire TRY block is executed without throwing an error. Sometimes your app needs to clean up after a section of code whether an error occurs or not. TRY . . . FINALLY . . . ENDFINAL blocks are helpful in this case. volatile HANDLE h1 = H_NULL, h2 = H_NULL; TRY h1 = HeapAllocThrow(BUF1_SIZE); h2 = HeapAllocThrow(BUF2_SIZE); . . /* do something with handles h1 and h2 */ . FINALLY /* free handles h1 and h2 */ HeapFreeIndir(&h1); HeapFreeIndir(&h2); ENDFINAL If any error is thrown in the TRY block, execution transfers to the FINALLY block where handles h1 and h2 are released, if necessary, and the error is passed on up. If no error is thrown, the remainder of the TRY block is executed, memory handles h1 and h2 are released in the FINALLY block, and execution continues after the ENDFINAL macro. The FINALLY block is always executed, thus guaranteeing that handles h1 and h2 will be freed. TRY . . . ENDFINAL blocks and TRY . . . ENDTRY blocks can be nested within each other. Caveats You should be aware of some coding and design issues dealing with raising and catching exceptions. Jumping Out of TRY Blocks Do not do it. Jumping out of TRY blocks causes big trouble. TRY blocks maintain a stack of saved execution contexts in order to determine where execution should resume when an error is thrown. The TRY stack is automatically popped when an error is thrown or when an ONERR or FINALLY macro is reached. It is very important that the code in a TRY block does nothing to corrupt the TRY stack. TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001