UNIT-2-compiler design

18CSC304J-COMPILER DESIGN
UNIT -2: SESSION -1: SLO -2
SYNTAX ERROR HANDLING
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SRM Institute of Science and Technology

Error Handling:
 The tasks of the Error Handling process are to detect each error,
report it to the user, and then make some recovery strategy and
implement them to handle the error.
 During this whole process processing time of the program should not
be slow.
 Functions of Error Handler:
(i)Error Detection
(ii)Error Report
(iii)Error Recovery
 Error handler=Error Detection Error+ Report Error Recovery.
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Cont..
 An Error is the blank entries in the symbol table.
 Errors in the program should be detected and reported by the parser.
 Whenever an error occurs, the parser can handle it and continue to
parse the rest of the input.
 Although the parser is mostly responsible for checking for errors,
errors may occur at various stages of the compilation process.
Types or Sources of Error – There are three types of error: logic,
run-time and compile-time error.
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Types of Errors:
• Logic errors occur when programs operate incorrectly but do not
terminate abnormally (or crash).
Unexpected or undesired outputs or other behaviour may result from
a logic error, even if it is not immediately recognized as such.
• A run-time error is an error that takes place during the execution of
a program and usually happens because of adverse system
parameters or invalid input data.
The lack of sufficient memory to run an application or a memory
conflict with another program and logical error is an example of this.
Logic errors occur when executed code does not produce the
expected result. Logic errors are best handled by meticulous
program debugging.
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Cont..
• Compile-time errors rise at compile-time, before the execution of
the program. Syntax error or missing file reference that prevents the
program from successfully compiling is an example of this.
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Classification of Compile-time error:
Lexical : This includes misspellings of identifiers, keywords or operators
Syntactical : a missing semicolon or unbalanced parenthesis
Semantical : incompatible value assignment or type mismatches between
operator and operand
Logical : code not reachable, infinite loop.
Finding error or reporting an error – Viable-prefix is the property of a
parser that allows early detection of syntax errors.
•Goal detection of an error as soon as possible without further consuming
unnecessary input
•How: detect an error as soon as the prefix of the input does not match a
prefix of any string in the language.
Example: for(;), this will report an error as for having two semicolons inside
braces. 7

Error Recovery :
1. Panic mode recovery :
This is the easiest way of error-recovery and also, it prevents the
parser from developing infinite loops while recovering error.
• The parser discards the input symbol one at a time until one of the
designated (like end, semicolon) set of synchronizing tokens (are
typically the statement or expression terminators) is found. This is
adequate when the presence of multiple errors in the same
statement is rare.
• Example: Consider the erroneous expression- (1 + + 2) + 3. Panic-
mode recovery: Skip ahead to the next integer and then continue.
Bison: use the special terminal error to describe how much input to
skip.
• E->int|E+E|(E)|error int|(error)
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2.Phase level recovery :
When an error is discovered, the parser performs local correction on the
remaining input.
If a parser encounters an error, it makes the necessary corrections on
the remaining input so that the parser can continue to parse the rest of
the statement.
You can correct the error by deleting extra semicolons, replacing
commas with semicolons, or reintroducing missing semicolons. To
prevent going in an infinite loop during the correction, utmost care should
be taken.
Whenever any prefix is found in the remaining input, it is replaced with
some string. In this way, the parser can continue to operate on its
execution.
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3.Error productions :
The use of the error production method can be incorporated if the
user is aware of common mistakes that are encountered in grammar in
conjunction with errors that produce erroneous constructs.
When this is used, error messages can be generated during the
parsing process, and the parsing can continue. Example: write 5x instead
of 5*x
4.Global correction :
In order to recover from erroneous input, the parser analyzes the
whole program and tries to find the closest match for it, which is error-free.
The closest match is one that does not do many insertions, deletions, and
changes of tokens.
This method is not practical due to its high time and space
complexity. 10

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UNIT-2-compiler design

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