Error Handler

8/8/2019 Error Handler

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Error Handler

Presented by:

Junaid Khan

Department of Computer Science

University of Peshawar


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Compiler Errorsy Errors

y An Error is an abnormal condition in thesource program which either stops the

compilation or results in undesired output.

y Programs submitted to a compiler oftenhave errors of various kinds.

y

With regard to errors, most compilers arenot very effective at communication sincethey deliver their comments to the user

without any knowledge of the user's"intent" for a sentence.


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Compiler Errors(cont..)

y Since no user has a perfect knowledge of syntaxand semantics, errors are inevitable.

y The compiler detects an error, then attempts torecover from the error so that it can detect moreerrors. That is, even in the presence of errors, the

compiler tries to parse the entire program inorder to detect as many errors as possible.


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Error Detectiony Each phase can encountered errors.

y After detecting an error, a phase must somehowdeal with that error so that compilation canproceed ,allowing further errors in the sourceprogram to be detected.

y A compiler that stops when it find the first erroris not as helpful as it could be.


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Error handler

y The compiler has, as one of its parts, an errorhandler.

y Most programming languages specifications donot describes how a compiler should respond toerrors, the response is left to the compilerdesigner.

y Planning the error handling right from the startcan both simplify the structure of a compiler andimprove its response to errors.


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Error handler(cont..)y The error handler is invoked when the symbols

in a sentence do not match the compiler's currentposition in the syntax diagram.

y The error handler warns the programmer byissuing an appropriate message.


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y The two basic task of compiler are

y Error Detection

y Error Recovery.

y During the compilation process each phasemay encounter errors so error handler is used

to handle these errors

y Error handler contains error handlingroutines.

Error handler(cont..)


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The Many Phases of a CompilerSource Program

Lexical Analyzer1

Syntax Analyzer2

Semantic Analyzer3

Intermediate

Code Generator

4

Code Optimizer5

Code Generator6

Target Program

Symbol-table

Manager

Error Handler


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Types Errorsy There are five types of errors:

1. Lexical Errors.

2. Syntax Errors.

3. Semantic Errors.

4. Logical Errors.

5. Spurious Errors.

Note:The first three types of errors arehandle by error handler.


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Lexical Errorsy When the remaining characters in the input do

not found a valid token then it is called lexicalerrors.

y For example

y An identifier names beginning with

digit/integery Misspelling of keywords may or may not be

handled in lexical error handler.


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Lexical Errors (cont..)y Few errors are discernible at the lexical level

alone, because lexical analyzer has a verylocalized view of a source program.

y If the string fi is encountered in a C program for

the first time in the context

Fi(a==f(x))

y A lexical analyzer cannot tell whether fi is amisspelling of the key word if or an undeclaredfunction identifier.


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Lexical Errors (cont..)y Since fi is a valid identifier, the lexical analyzer

must return the token for an identifier and let

some other phase of the compiler handle anyerror.

y [

y But, suppose a situation does arise in which thelexical analyzer is unable to proceed because

none of the patterns for tokens matches a prefixof the remaining input.


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Syntax Errorsy When the stream of tokens violates the grammar

rule of a language then it is a Syntax Errors.

y Often much of the error detection and recoveryin a compiler is centered around the syntaxanalysis phase.

y One reasons for this is that many errors are

syntactic in nature or are exposed when thestream of tokens coming from the lexicalanalyzer disobeys the grammatical rules definingthe programming language.


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Syntax Errors

(cont)

y Another is the precision of modern parsingmethods, they can detect the presence of syntacticerrors in programs very efficiently.

y Example:y Consider the sentence A By The sentence requires an operator (+) between the 2 letters.y The compiler message might be Missing operator


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Syntax Errors (cont)yExample 2:y Consider the sentence A+B+y This sentence is not valid.y The reason for the rejection is an error in

the input sentence.y The sentence needs another letter at the end.y The compiler message might appear as

follows:y program.cpp(5) error c2059: Missing

operand^^file name.................^^the compiler'sexplanation............^^line number within the file


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y What the compiler thinks is that you meant to type anotherletter.

y In the world of compilers, the "things" that you add are

termed "operands"; thus, the error message "missingoperand".

y However, it is just as likely that you made a mistake typingthe extra "+".

y The conclusion is that the message may, or may not, help

you.y The best plan is to use the point of error (line 5) as the

starting point to determine which syntax or semantic rulehas been violated.

Syntax Errors (cont)


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3. Semantic Errorsy When the operation of source program are not

meaningful.

y Remember that a semantic error is a ruleviolation that has nothing to do with syntaxdiagrams.

y For example, numbers in a computer are

normally restricted to a maximum number ofdigits.

y If you type too many digits 871239817231723178,you will get a semantics-based error message.


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Example

y Consider thesentence 57/0

y Thissentenceissyntacticallyvalid, but failssemantic test.

division byzeroisinvalidinmathematicssomost compiler

check for thiscondition whenever possible.y Inmost situation thedividendisnot constant but might

be a number typed byyou at run time

in thiscase division byzeroisillegal .


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Continuey Most computers check for a "division by zero"

error during program execution so you can get

a "division by zero" error message at runtime,which will stop your program,

y to create a correct program, you mustovercome syntax errors, semantic errors,runtime errors and writing a good program to

start with.


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4. Logical Errorsy It is an error in the algorithm of the source

program, due to which an undesired outputresults.

y A compiler can not detect logical errors.

y Accurately detecting the presence of logicalerrors at compile time is much more difficulttask.

yFor exampley Infinite loop

yArray indexes out of bound.


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5. Spurious Errorsy These are compiler generated errors.

y These are those errors made by compiler

during the error recovery.yFor exampleywe are using a function fi( ) in our source

program but during error recovery

compiler automatically correct it as if ( )so it is a spurious error and it willgenerate a problem in program.


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MessageWhen we pray GOD hears more than we

say.

He answer more than we ask .

He gives more than we imagine.

thanks

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Error Handler