1 Exceptions (Section 8.5) CSCI 431 Programming Languages Fall 2003 A compilation of material developed by Felix Hernandez-Campos and Michael Scott

2 Exception Handling ExceptionException –unusual condition detected at run time –may require to "back-out" from several levels of subroutine calls Examples:Examples: "Traditional" ways to handle such situations:"Traditional" ways to handle such situations: –arithmetic overflow –end-of-file on input –wrong type for input data –user-defined conditions (not necessarily errors), raised explicitly –return some default value when the real one cannot be obtained –return (or have an extra parameter) an explicit "status" value, to be inspected after each call –pass a closure for error handling, to be called in case of trouble

3 Exception Handling Example (C++):Example (C++): void f () {...try{ g(); g();} catch (exc) { // handle exception of type exc // handle exception of type exc}...} void g () {...h();...} void h() {... if (...) throw exc; throw exc;...}

4 Exception Handling Exception handlerException handler –code executed when exception occurs –different handler for each type of exception Introduced by PL/I:Introduced by PL/I: ON condition statement –the statement is "remembered" for later use, if the condition (e.g. OVERFLOW) arises –if the condition arises: »execute statement »for "fatal" errors - terminate program »for "recoverable" errors – jump back to where exception occurred and continue execution

5 Exception Handling C++, Ada, Java, ML – more structured approach:C++, Ada, Java, ML – more structured approach: –handlers (catch in C++) are lexically bound to blocks of protected code (the code inside a try block in C++) Exception propagation:Exception propagation: –if an exception is raised (throw in C++): »if the exception is not handled in the current subroutine, return abruptly from subroutine »return abruptly from each subroutine in the dynamic chain of calls, until a handler is found »if found, execute the handler, then continue with code after handler »if no handler is found until outermost level (main program), terminate program

6 Exception Handling Usage for exception handling mechanisms:Usage for exception handling mechanisms: –perform operations to recover, and then continue execution »allocate more memory »recover from errors in a compiler –handle errors exactly where we want, without checking for them explicitly everywhere they might occur –subroutine documentation – specify what exceptions might be raised by a subroutine → subroutine user may want to catch them –cannot recover locally, but: »may want a local handler just to clean up some local resources »then re-raise the exception to be handled by a "higher authority" –terminate, but first print a helpful error message Advantages:Advantages: –uniform manner of handling errors

7 Hierarchy of Exceptions In PL/1, exception do not have a typeIn PL/1, exception do not have a type In Ada, all exceptions are of type exceptionIn Ada, all exceptions are of type exception –Exception handler can handle one specific exception or all of them Since exceptions are classes in Java, exception handlers can capture an entire class of exceptions (parent classes and all its derived classes)Since exceptions are classes in Java, exception handlers can capture an entire class of exceptions (parent classes and all its derived classes) –Hierarchy of exceptions

8 Definition of Exceptions Exception types:Exception types: –Predefined – dynamic semantic errors: »arithmetic overflow »division by zero »end-of-file on input »subscript and subrange out of bounds »null pointer dereference –User-defined: declare empty_queue : exception;-- Ada class empty_queue { };// C ++

9 Definition of Exceptions Parameterized exceptions – the code which raises the exception can pass additional information with itParameterized exceptions – the code which raises the exception can pass additional information with it –C++: class duplicate_in_set { item dup;// element that was inserted twice }... throw duplicate_in_set (d); –ML: exception duplicate_in_set of item;... raise duplicate_in_set (d); –Ada, Common Lisp: »exceptions are just tags – no other information than the exception name

10 Definition of Exceptions Documenting exceptions:Documenting exceptions: –Modula-3 – mandatory to declare (in the subroutine header) all exceptions that may propagate out of the subroutine »if another exception propagates (not caught internally) → run-time error –C++ – optional list of exceptions »if the list is specified → same behavior as in Modula-3 »if the list is omitted → any exception can propagate –Java »“checked” exceptions – must be declared in subroutine headers »“unchecked” exceptions – may be omitted (e.g. fatal errors – array subscript out of bounds)

11 Exception Propagation Exception handler – attached to a block of code (protected code)Exception handler – attached to a block of code (protected code) –Ada: with text_io;-- import I/O routines (and exceptions) procedure read_record (...) is begin...begin protected code - potentially complex sequence of operations -- involving many calls to text_io.get...exception when end_error => handler to catch any attempt to read past end_of_file -- in any of the I/O calls end;... end read_record;

12 Exception Propagation –C++ (predefined exceptions): try {... // protected block of code...} catch (end_of_file) {...} catch (io_error e) {// any io_error other than end_of_file...} catch (...) {// all other exceptions;... is part of syntax...} »handler matches exception if it names a class from which exception is derived »can declare an exception object (e) – access additional information passed with the exception

13 Exception Handlers Java Java uses lexically scoped exception handlersJava uses lexically scoped exception handlers try { int a[] = new int[2]; int a[] = new int[2]; a[4]; a[4]; } catch (ArrayIndexOutOfBoundsException e) { System.out.println("exception: " + e.getMessage()); System.out.println("exception: " + e.getMessage()); e.printStackTrace(); e.printStackTrace();} The dynamic chain is available using printStackTrace()The dynamic chain is available using printStackTrace() – ang/Throwable.html ang/Throwable.htmlhttp://java.sun.com/products/jdk/1.2/docs/api/java/l ang/Throwable.html

14 Implementation Linked-list of dynamically-bound handlers maintained at run-timeLinked-list of dynamically-bound handlers maintained at run-time –Each subroutine has a default handler that takes care of the epilogue before propagating an exception –This is slow, since the list must be updated for each block of code Compile-time table of blocks and handlersCompile-time table of blocks and handlers –Two fields: starting address of the block and address of the corresponding handler –Exception handling using a binary search indexed by the program counter –Logarithmic cost on the number of handlers

15 Java Each subroutine has a separate exception handling tableEach subroutine has a separate exception handling table –Thanks to independent compilation of code fragments Each stack frame contains a pointer to the appropriate tableEach stack frame contains a pointer to the appropriate table

16 C Exception can be simulatedException can be simulated setjmp() can store a representation of the current program state in a buffersetjmp() can store a representation of the current program state in a buffer –Returns 0 if normal return, 1 if return from long jump longjmp() can restore this statelongjmp() can restore this state Example:Example: if (!setjmp(buffer)) { /* protected code */ } else { /* handler */ } For more information: LongjmpFor more information: LongjmpLongjmp

17 C The state is usually the set of registersThe state is usually the set of registers longjmp() restores this set of registerslongjmp() restores this set of registers – 0.02/library_20.html /library_20.htmlhttp:// 0.02/library_20.html Is this good enough?Is this good enough? Changes to variables before the long jump are committed, but changes to registers are ignoredChanges to variables before the long jump are committed, but changes to registers are ignored If the handler needs to see changes to a variable that may be modified in the protected code, the programmer must include the volatile keyword in the variable’s declarationIf the handler needs to see changes to a variable that may be modified in the protected code, the programmer must include the volatile keyword in the variable’s declaration