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

helpful answer @arbpass

---

C

Compiler Phases:

Lexical Analysis

• Tokens are identified correctly.
• "Integer" is treated as an identifier.
• No error.

 

Syntax Analysis

typedef int Integer;

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

• Statement structure is grammatically correct.
• Parser accepts

type identifier ;

• No error.

 

Semantic Analysis

• Compiler checks whether "Integer" is a declared type.
• Since it is not declared anywhere, semantic error occurs.

Final Answer:

Error is detected in the Semantic Analysis phase.

---

Why are you pasting from ChatGPT

Answer 1

This question is difficult to answer from a practical point of view because most of the C compilers (even other language compilers) do not follow the classical ordering of compilation phases. Since this is a one-mark question ignoring the practical implementations and going by just theory answer will be syntax error. Because there are no lexical errors and “Integer” and “x” get read as identifiers as shown in the following output.

arjun@Armi:~$ cat p1.c
int main()
{
	Integer x;
	return 0;
}
arjun@Armi:~$ clang p1.c -c -Xclang -dump-tokens
int 'int'	 [StartOfLine]	Loc=<p1.c:1:1>
identifier 'main'	 [LeadingSpace]	Loc=<p1.c:1:5>
l_paren '('		Loc=<p1.c:1:9>
r_paren ')'		Loc=<p1.c:1:10>
l_brace '{'	 [StartOfLine]	Loc=<p1.c:2:1>
identifier 'Integer'	 [StartOfLine] [LeadingSpace]	Loc=<p1.c:3:2>
identifier 'x'	 [LeadingSpace]	Loc=<p1.c:3:10>
semi ';'		Loc=<p1.c:3:11>
return 'return'	 [StartOfLine] [LeadingSpace]	Loc=<p1.c:4:2>
numeric_constant '0'	 [LeadingSpace]	Loc=<p1.c:4:9>
semi ';'		Loc=<p1.c:4:10>
r_brace '}'	 [StartOfLine]	Loc=<p1.c:5:1>
eof ''		Loc=<p1.c:5:2>

Now, when this stream of tokens get passed to the syntax analyser – we have an identifier followed by another identifier which is not valid in C syntax – so syntax error. And this must be the answer here though we can argue for semantic error as well as follows.

Now consider a typedef usage like “typedef int Integer”. Now, this can be implemented by the compiler in multiple ways. One option is to immediately change the token type of “Integer” from identifier to the given “type”. Otherwise the syntax check can go with the AST generation. But if we go by the classical meaning of the compilation phases here we are matching a string which means it is a semantic phase.

Correct Answer: Syntax Analysis/Semantic analysis

More read: https://stackoverflow.com/questions/66290247/integer-x-is-syntactic-error-or-semantic-error

Official answer given in GATE key is “Semantic analysis” – but even the best compiler professors won’t conclude on that. 

---

Though this was a bad question and even worse answer key, lets use it to learn something useful.

The following three flags will force cc (C compiler) to check that your code complies to the relevant international standard, often referred to as the ANSI standard, though strictly speaking it is an ISO standard.

• -Wall
  Enable all the warnings which the authors of cc believe are worthwhile. Despite the name, it will not enable all the warnings cc is capable of.
• -ansi
  Turn off most, but not all, of the non-ANSI C features provided by cc. Despite the name, it does not guarantee strictly that your code will comply to the standard.
• -pedantic
  Turn off all cc's non-ANSI C features.

Without these flags, cc will allow you to use some of its non-standard extensions to the standard. Some of these are very useful, but will not work with other compilers—in fact, one of the main aims of the standard is to allow people to write code that will work with any compiler on any system. This is known as portable code.

https://docs.freebsd.org/en_US.ISO8859-1/books/developers-handbook/tools-compiling.html

Comments

Apologies for posting a chatgpt answer, but I wanted to present this perspective of seeing a question too

---

@Pranavpurkar Sir, 

https://gateoverflow.in/357537/gate-cse-2021-set-2-question-3?show=365144#c365144  

$\text{you are right, it's a semantic error not syntax error.}$

---

Yes but At Nptel Porter it also gives as syntax error so what to be consiider

Answer 2

Syntax Analyzer throws error for above

Comments

@kshitij86 can you link to that stack overflow thread?

---

https://cs.stackexchange.com/questions/135954/compiler-design-question

---

Can U just tell that, Is gateoverflow Rank predictor is now perfectly set according to gate Answer Key(except Challeged ques) ,ie, will expect challenge ques will there be no more variation in marks,Am I correct?

Answer 3

Answer will be C  i.e. Semantic Analyzer because Integer is not defined so compiler will treat it as unknown reference.

See example no 4

https://www.inf.unibz.it/~calvanese/teaching/04-05-ip/lecture-notes/uni09/node4.html

Comments

I think this is a syntactic error, which compiler can indeed detect and report.

A semantic error is something that compiler is fine with; but does not do what you want. Semantic errors are part of your algorithm more than your actual syntax.

As per the given source – https://www.inf.unibz.it/~calvanese/teaching/04-05-ip/lecture-notes/uni09/node4.html ; It seems to be semantic error also;

But again if this error is caught initially at syntax analysis phase ( at compile time) no need to check next compiler phase.

Answer 4

So By Compiling everyone noticed that there is an error saying undeclared identifier. This error comes under Semantic Error. So the answer Should be Semantic Error

 

Answer :- C

Answer 5

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 6

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