1 JMH Associates © 2004, All rights reserved Chapter 4 Structured Exception Handling

2 JMH Associates © 2004, All rights reserved OBJECTIVESOBJECTIVES Upon completion of this chapter, you will be able to:  Describe Windows and Visual C++ Structured Exception Handling (SEH)  Use SEH to detect and analyze exceptions  Both user- and system-generated  Simplify your code’s response to exceptions and errors  Using exception and termination handlers  Protect your code from unexpected exceptions  And properly clean up and terminate your program

3 JMH Associates © 2004, All rights reserved OVERVIEWOVERVIEW Structured Exception Handling (SEH) provides a robust mechanism for applications to respond to unexpected events  Hardware faults  Addressing exceptions  System errors  User generated exceptions SEH assures the ability to free resources  Perform clean-up SEH simplifies program logic

4 JMH Associates © 2004, All rights reserved PARTSPARTS Part IException Handlers Part IITermination Handlers Lab 4–A

5 JMH Associates © 2004, All rights reserved PART I Exception Handlers

6 JMH Associates © 2004, All rights reserved TRY AND EXCEPT BLOCKS __try and __except  Keywords recognized by Visual C++ compiler  Actual keywords are specific to the compiler __try { /* Block of monitored code */ } __except (filter_expression) { /* Exception handling block */ }

7 JMH Associates © 2004, All rights reserved SEH, BLOCKS, AND FUNCTIONS { DWORD x1; /* Block 1 */... _try { /* Block 2 */ DWORD x2;.... x2 = f (x1);.... } _except () { /* SEH 2 */ } DWORD f (DWORD y) { /* Block f */ DWORD z; z = y / (y - 1); return z / y; } STACK Windows Exception Handler Block 1 x1 Block 2 x2 SEH 2 Block f y z Exception Occurs Execute this SEH

8 JMH Associates © 2004, All rights reserved FILTER EXPRESSIONS The filter_expression is evaluated after an exception It is a literal constant, expression, or a function call Returns one of three values  EXCEPTION_EXECUTE_HANDLER  Normal case  EXCEPTION_CONTINUE_SEARCH  This handler does not process this exception; unwind the stack for the next exception handler  EXCEPTION_CONTINUE_EXECUTION  Some exceptions cannot be continued, and another exception would occur immediately

9 JMH Associates © 2004, All rights reserved EXCEPTION CODES (1 of 2) How do you know what exception occurred?  And how do you know what filter expression value to generate? DWORD GetExceptionCode (VOID)  The filter function itself cannot call GetExceptionCode, so a common usage is: __except (MyFilter (GetExceptionCode ())) {... }

10 JMH Associates © 2004, All rights reserved EXCEPTION CODES (2 of 2) An alternative function that returns additional information: LPEXCEPTION_POINTERS GetExceptionInformation (VOID) including information on the virtual address causing an access violation

11 JMH Associates © 2004, All rights reserved EXCEPTION CODE VALUES (1 of 2) 1.Program violations, including:  EXCEPTION_ACCESS_VIOLATION  EXCEPTION_DATATYPE_MISALIGNMENT  EXCEPTION_NONCONTINUABLE_EXECUTION 2..Memory allocation exceptions (See Chapter 4) ( HeapAlloc and HeapCreate )  STATUS_NO_MEMORY

12 JMH Associates © 2004, All rights reserved EXCEPTION CODE VALUES (2 of 2) 3.User-defined exception codes 4.Arithmetic codes, such as:  EXCEPTION_INT_DIVIDE_BY_ZERO  EXCEPTION_FLT_DIVIDE_BY_ZERO 5.Debugger exceptions — EXCEPTION_BREAKPOINT

13 JMH Associates © 2004, All rights reserved EXCEPTION CONTROL FLOW _try {... i = j / 0;... } _except (Filter (GetExceptionCode ())) {... }... DWORD Filter (DWORD ExCode) { switch (ExCode) {... case EXCEPTION_INT_DIVIDE_BY_ZERO:... return EXCEPTION_EXECUTE_HANDLER; case... }

14 JMH Associates © 2004, All rights reserved USER-GENERATED EXCEPTIONS (1 of 2) VOID RaiseException (DWORD dwExceptionCode, DWORD dwExceptionFlags, DWORD cArguments, LPDWORD lpArguments) dwExceptionCode bits 31, 30: 0 — Success1 — Informational 2 — Warning3 — Error Bit 29: Set Bit 28: 0Bits 27–0: User Specified Typical value: 0XE

15 JMH Associates © 2004, All rights reserved USER-GENERATED EXCEPTIONS (2 of 2) dwExceptionFlags — EXCEPTION_NONCONTINUABLE indicates filter expression should not generate EXCEPTION_CONTINUE_EXECUTION lpArguments — If not NULL, points to an array of cArguments EXCEPTION_MAXIMUM_PARAMETERS == 15

16 JMH Associates © 2004, All rights reserved ReportException Function (1 of 2) VOID ReportException (LPCTSTR UserMessage, DWORD ExceptionCode) /* ReportException */ /* Extension of ReportError to generate a user exception code rather than terminating. */ /* UserMessage: Message to be displayed ExceptionCode: 0 - Return > 0 - ExitProcess with this code */ { /* Report as a non-fatal error */ if (lstrlen (UserMessage) > 0) ReportError (UserMessage, 0, TRUE);

17 JMH Associates © 2004, All rights reserved ReportException Function (2 of 2) /* If fatal, raise an exception */ /* Mask out any high order bits in the */ /* user-supplied exception code */ if (ExceptionCode != 0) RaiseException ( (0x0FFFFFFF & ExceptionCode) | 0xE , 0, 0, NULL); return; }

18 JMH Associates © 2004, All rights reserved ENABLING FLOATING POINT EXCEPTIONS (1 of 2) Skip if you are not interested in floating point arithmetic To enable floating point exceptions, you need to set the fp mask (set the bit off to enable the exception) int iFPMask; /* Save old control mask */ iFPMask = _controlfp (0, 0); iFPMask &= ~(EM_OVERFLOW | EM_UNDERFLOW | EM_INEXACT | EM_ZERODIVIDE | EM_DENORMAL); _controlfp (iFPMask, MCW_EM); /* Set new control mask*/ /* Restore the old mask to disable exceptions */ _controlfp (iFPMask, 0xFFFFFFFF);

19 JMH Associates © 2004, All rights reserved ENABLING FLOATING POINT EXCEPTIONS (2 of 2) Formula: (current_mask & ~mask) | (new & mask) After a FP exception, clear it with _clearfp()

20 JMH Associates © 2004, All rights reserved PART II Termination Handlers

21 JMH Associates © 2004, All rights reserved TRY-FINALLY BLOCKS Example: The finally block (almost) always exectutes while (...) __try { hTemp = CreateFile (TempFileName,...); /* Block of guarded code */... if (...) break; /* finally block will run */... } __finally { /* Executes on every loop iteration */ /* termination handler (finally block) */ CloseHandle (hTemp); DeleteFile (TempFileName); }

22 JMH Associates © 2004, All rights reserved TERMINATION HANDLERS Executed whenever control flow leaves the try block because of:  Reaching the end of the __try block  Leaving the block because of execution of: returnbreaklongjump continuegoto__leave  An exception Executed in the context of the block or function it guards Control can pass from end of termination handler to next statement

23 JMH Associates © 2004, All rights reserved ABNORMAL TERMINATION To detect termination for any reason other than reaching the end of the try block—use BOOL AbnormalTermination (VOID) to determine how the try block terminated TRUE — For abnormal termination FALSE — For normal termination

24 JMH Associates © 2004, All rights reserved COMBINING FINALLY AND EXCEPT BLOCKS (1 of 2) __try { /* Block of guarded code */ } __except (filter_expression) { /* Except block */ } __finally { /* You cannot do this ! */ }

25 JMH Associates © 2004, All rights reserved COMBINING FINALLY AND EXCEPT BLOCKS (2 of 2) __try { /* Outer try-except block */ while (...) __try { /* Inner try-fin block */ hFile = CreateFile (TempFile,...); if (...) __try { /* Inner try-ex block */... } __except (EXCEPTION_EXECUTE_HANDLER) {... /* Process FP exception. */ _clearfp(); } __finally { /* End of while loop */ CloseHandle (hFile); DeleteFile (TempFile); } __except (filter-expression) { }

26 JMH Associates © 2004, All rights reserved LAB 4–A Write a program, toupper, which processes one or more files on the command line  The processing consists of converting lower case characters to upper case and writing the results to a new file  If an input file is missing, ignore it and process the next  Process the files in memory (allocate enough memory to read the entire file)  Use exception handlers to detect and respond to all errors that occur during the processing