Error Detection _ Recovery

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ERROR DETECTION AND RECOVERY

ERROR

• Program submitted to a compiler often have errors of various kinds So, good compiler should be able to detect as many errors as possible in various ways and also recover from them

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SOURCES OF ERROR

 • * ALGORITHMIC ERRORS: The algorithm used to meet the design may be inadequate or incorrect *CODING ERRORS: The programmer may introduce errors in implementing the algorithms, either by introducing logical errors or using the programming language constructs improperly

• *The program may exceed a compiler or machine limit not implied by the definition of the programming language.• Eg: • An array may be declared with too many dimensions to fit in the symbol table ,• an array may be declared with too large to be allocated at runtime. • *COMPILER ERRORS:• compiler can insert errors as it translates source program into an object program.

ERROR CLASSIFICATION

• CRETERIA FOR THE CLASSIFICATION OF ERRORS:• *Compile time,• *Link / Load time • * Run time errors.

• The lexical and syntactic errors are found during the execution of the program.• Most of the run time errors are semantic in nature.• In compile-link-go systems, the compile and link errors will be trapped seperately by the compiler and the linkage editor / loader.• In compile-and-go systems, the compile and link errors will be trapped by the compiler itself. 

• Execution time errors are detected by the run time environment, which includes runtime control routine, the machine hardware and the standard OS interfaces through which status of the hardware can be accessed or monitored as when required.

PLAN OF ERROR DETECTOR / CORRECTOR

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Lexical analyzer

Lexical corrector

Diagnostic Message PrinterSyntactic corrector

Parser

Symbol Table

Semantic checkerSource code Tokens Intermediate

code

LEXICAL AND SYNTAX ERRORS

• Two frequent sources of these errors are:1.Spelling errors,2.Missing operators and keywords

• These errors can happen due to genuine oversight or due to typing mistake.• They are common mistakes for even for professional programmers.

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SPELLING ERRORS- WHEN DO THEY OCCUR???

If a program uses variables names which differs in only one or two characters ,then there exits great scope for spelling errors. There is less chance for using automatic procedures for detecting and correcting these errors. Only the programmer can able to tackle the problem because he only know about significance of variable names.

MAJORITY SPELLING ERRORS

1. One character is wrong,2. One character is missing,3. One character is extra,4. Two adjacent characters are transposed.• Testing for these four types of error will not enable us to catch all the spelling mistakes but practical consideration limit searches to these four only• The implementation these four checks is quite expensive because an associative search has to be performed over all names in symbol table to locate resembling name

DUPLICATE MASSAGE

• It is to find that many message appear owing to the same error.• Ex: If a is used as a simple variable and later goes to declare and use it as array a[10,10] then all references to the array will be flagged as erroneous use of variable name.• This can achieved by setting a flag in the symbol table entry of a.• This will enable to detect and indicate all possible illegal use of identifier.

RECOVERING FROM SYNTAX ERROR

• The chief concern while recovering from the syntax error is to attain a parser state from where the parser can safely resume parsing the input string.• Many parsers detects errors when it doesnot have legal move from it’s correct configuration , which is determined by it’s state , stack content and current input symbol.• To recover from an error a parser should ideally locate it , correct and resume parsing

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TIME OF DETECTION – VALID PREFIX PROPERTY

• LL1 AND LR1 parsers will announce errors as soon as the prefix of the input has been seen for which there is no valid continuation• This is the earliest time at which a parser that reads it’s input from left to right can announce an error• Adv – reports errors as soon as possible• Limits amount of erraneous output

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Panic mode recoveryon discovering an errorthe parser discards input symbols one at a timeuntil is found one of a designated set of synchronizing tokens

◦ delimiters ; or }◦ have a clear and unambiguous role◦ must be selected by the compiler designer

skips considerable amount of inputno checking for additional errorssimpleguaranteed not to go on an infinite loop

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• Three basic policy of recovering syntax error:• 1.Deletion of a source symbol• 2.Insertion of a synthetic symbol.• 3.Replacement.

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• The motive behind all these actions is to present a new string to the parser which would lead to bypassing the error situation and continue to parse.• Here multiple recovery possibilities may exists.• We should choose the one which has smallest number of changes – minimum distance recovery

RECOVERY IN TOP DOWN PARSING

• There are two methods.• First is to try and successfully complete the predictions existing in the stack at the error point.Ex: Input string: Aα• Last prediction was W:…ABν• If no other rules exit with A on right hand side then recovery can be effected by inserting B and deleting parts a until a ν is recognized in source string.• Another is unstack certain symbols from parser stack until we have a TOS symbol which can produce one of the synchronizing symbols.• We can skip until we find a synchronising symbol in input stack

OPERATOR PRECEDENCE PARSING

Operator precedence parser uses set of production rules and operator precedence table to parse an arithmetic expression.E → E + E | E – E | E * E | E / E | E ^ E | ( E ) | - E | id

CHARACTER PAIR ERROR RECOVERY

Fill each empty entry with a pointer to an error routine.Example,

E1 – ‘missing operand’ – whole expression is missingE2- ‘unbalanced right parenthesis’E3- ‘missing operator’E4- ‘missing right parenthesis’

REDUCIBILITY ERROR RECOVERY

• Decides the popped handle “looks like” which right hand side. And tries to recover from that situation.• Same like shift-reduce errors

SEMANTIC ERRORS

• Can be detected both at compile time and run tume• Can be both local and global in scope.• Types

– Immediate errors• Can be detected while processing the erroneous statement itself.

– Delayed errors• Can’t be detected while processing the statement.• But can be detected at a later stage when its effect is felt.

EXAMPLES FOR SEMANTIC ERRORS

• Illegal Operator or Operand (immediate)• Control Structure Violation (both)• Missing Labels (delayed at the end of the program)• Duplicate Labels• Most common semantic errors that can be detected at compile time are errors of declaration and scope• Eg type incompatability between operators and operands

THE ERROR PRINT ROUTINE

• Messages have to be displayed for all errors which are detected , or detected and corrected in the source program.• The error print routine is the common agency that is used by all individual compiler routines for this purpose.• The text of the error is normally stored in the table local to this routine• Associated with each message is the numerical value indicating it’s error severity• This value is mainly used for purposes internal to the compiler’s operation

1 Warning and correction. Compilation continues and the compiled program will execute2 Warning only. Compilation continues and compiled program will execute3 Fatal error. Compilation continues but the compiled program will not execute4 Compiler error . Compilation terminated

• For each individual error two items of information need to be passed to this routine• The error number and the statement number• The structure and logic of the routine depends largely on the decisions regarding the place where the message is to be printed.

Desirable place for printing error messages

• The messages are best printed against the erraneous statement itself• Single pass compilers find it difficult to indicate all errors against the offending statement• Multipass compilers can provide such error condition

• The compiler error table will be in the form Error number(Message identifier) Erroneous statement Auxiliary information

Message text

Runtime errors

• The runtime errors are detected by1. The run time control routine which is interfaced with the generated code in standard manner2. The machine hardware3 . Operating system interfaces to I/O

• The agency required to detect particular type of error depends on nature of error and in general varies from machine to machine and compiler to compiler

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Detection of runtime errors

• Arithmetic exceptions Arises because of the violations of semantics of machine computations.Includes frequently occurring error conditions like overflows, underflows, loss of precision etc..Present day architecture detects most of the conditions at the machine hardware level and indicate their presence through interrupts or traps

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