GATE CSE 2021 Set 2 | Question: 3

Question

Consider the following $\text{ANSI C}$ program:

int main () {
    Integer x;
    return 0;
}

Which one of the following phases in a seven-phase $C$ compiler will throw an error?

• A. Lexical analyzer
• B. Syntax analyzer
• C. Semantic analyzer
• D. Machine dependent optimizer

Comments

According to GATE final answer key, answer is C: Semantic analyzer

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Is there any deterministic way to know for any given program which phase will detect an error?

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It is a statement similar to a type declaration. Int a, char x etc. But with a unknown type integer which is not defined. So unknown reference to a data type is used in the above statement so it's semantic error. I didn't see it at once but it's visible after watching some references.

Answer 1

Let’s dry run the lexical analysis on this –

Integer is a Lexeme of token type “identifier”, so it x. So, for this line of code, lexical analyzer will generate the token stream – identifier identifier EOS. Here EOS signifies End of Statement.

Now, ask yourself if CFG for C supports such a word? No it doesn’t – it will produce a Syntax Error. However, this doesn’t mean that there won’t be a Semantic Error – Integer is an identifier used before definition. As per Syntax Directed Translation scheme, both these error would be reported. An unambiguous question would have been – Which error is reported first? The answer to this is Syntax Error.

Answer 2

The compiler will consider Integer as a variable so it will be detected in Semantic Analyzer(Since Undeclared Identifier).

Comments

ANSWER: SEMANTIC ERROR

For answering this question u need to know generally which type of grammar is used in which type of phase of compiler and for what purpose:

LET ME GIVE ROUGH IDEA ABOUT IT:

LEXICAL ANALYSIS:

In this mainly DFA is used for pattern searching and token forming which is loaded in the symbol table. Here “Integer” is not a keyword in ANSI C.

*Lexical errors are those illegal strings, unmatched symbols, length of the boundaries are exceeding.

SYNTAX ANALYSIS:

In this we generally use CFG and CFG is context-free (which means what’s written before it can’t be taken into consideration).

*Syntax errors are those missing operand, missing left parenthesis, missing right parenthesis, etc.

SEMANTIC ANALYSIS:

This generally uses PDA which is not context-free.

*Semantic errors are those type checking does not match, the scope of the functions does not found, and type conversion.

Here in this question, “x” is Undeclared(PDA can detect it because of its non-context free nature) as “Integer” is not a data type in ANSI C

Answer 3

It is a statement similar to a type declaration. Int a, char x etc. But with a unknown type integer which is not defined. So unknown reference to a data type is used in the above statement so it's semantic error. I didn't see it at once but it's visible after watching some references.