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

1JMH Associates © 2004, All rights reserved

Chapter 4Chapter 4Chapter 4Chapter 4

Structured Exception Handling


OBJECTIVESOBJECTIVESOBJECTIVESOBJECTIVES

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


OVERVIEWOVERVIEWOVERVIEWOVERVIEW

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


PARTSPARTSPARTSPARTS

Part I Exception Handlers

Part II Termination Handlers

Lab 4–A


PART IPART IPART IPART I

Exception Handlers


TRY AND EXCEPT BLOCKSTRY AND EXCEPT BLOCKSTRY AND EXCEPT BLOCKSTRY 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 */}


SEH, BLOCKS, AND FUNCTIONSSEH, BLOCKS, AND FUNCTIONSSEH, BLOCKS, AND FUNCTIONSSEH, 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 2x2

SEH 2

Block fyz

Exception OccursExecute this SEH


FILTER EXPRESSIONSFILTER EXPRESSIONSFILTER EXPRESSIONSFILTER 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


EXCEPTION CODES (1 of 2)EXCEPTION CODES (1 of 2)EXCEPTION CODES (1 of 2)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 ())) { . . .

}


EXCEPTION CODES (2 of 2)EXCEPTION CODES (2 of 2)EXCEPTION CODES (2 of 2)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


EXCEPTION CODE VALUES (1 of 2)EXCEPTION CODE VALUES (1 of 2)EXCEPTION CODE VALUES (1 of 2)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


EXCEPTION CODE VALUES (2 of 2)EXCEPTION CODE VALUES (2 of 2)EXCEPTION CODE VALUES (2 of 2)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


EXCEPTION CONTROL FLOWEXCEPTION CONTROL FLOWEXCEPTION CONTROL FLOWEXCEPTION 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 . . .}

}

2

6

7

3

5

4

1


USER-GENERATED EXCEPTIONSUSER-GENERATED EXCEPTIONS(1 of 2)(1 of 2)


VOID RaiseException (DWORD dwExceptionCode,DWORD dwExceptionFlags, DWORD cArguments,LPDWORD lpArguments)

dwExceptionCode bits 31, 30:

0 — Success 1 — Informational

2 — Warning 3 — Error

Bit 29: Set

Bit 28: 0 Bits 27–0: User Specified

Typical value: 0XE0000006




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


ReportExceptionReportException Function (1 of 2) Function (1 of 2)ReportExceptionReportException Function (1 of 2) 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);


ReportExceptionReportException Function (2 of 2) Function (2 of 2)ReportExceptionReportException Function (2 of 2) 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) | 0xE0000000, 0, 0, NULL); return;}


ENABLING FLOATING POINT ENABLING FLOATING POINT EXCEPTIONS (1 of 2)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);




Formula: (current_mask & ~mask) | (new & mask)

After a FP exception, clear it with _clearfp()


PART IIPART IIPART IIPART II

Termination Handlers


TRY-FINALLY BLOCKSTRY-FINALLY BLOCKSTRY-FINALLY BLOCKSTRY-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);}


TERMINATION HANDLERSTERMINATION HANDLERSTERMINATION HANDLERSTERMINATION 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:

return breaklongjump

continue goto __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


ABNORMAL TERMINATIONABNORMAL TERMINATIONABNORMAL TERMINATIONABNORMAL 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


COMBINING FINALLY ANDCOMBINING FINALLY ANDEXCEPT BLOCKS (1 of 2)EXCEPT BLOCKS (1 of 2)


__try {

/* Block of guarded code */

}

__except (filter_expression) {

/* Except block */

}

__finally {

/* You cannot do this ! */

}




__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) { }


LAB LAB 44–A–ALAB LAB 44–A–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

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

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