GATE CSE 2005 | Question: 61

Question

Consider line number $3$ of the following C-program.

int main() {                /*Line 1 */
    int I, N;               /*Line 2 */
    fro (I=0, I<N, I++);	/*Line 3 */
}

Identify the compiler’s response about this line while creating the object-module:

• A. No compilation error
• B. Only a lexical error
• C. Only syntactic errors
• D. Both lexical and syntactic errors

Comments

The fro statement will be treated as a function call with three arguments.

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No compiler error but linking error come at runtime.

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Fro(); Is not declared in the given question, is there any reason we should declare Fro ?

Answer 1

C language allows only certain words in it- these are called tokens. If we input any invalid tokens it causes lexical error. 

eg:
$44a44$
causes lexical error as in C as an alphabet cannot come in between digits. 

Syntactic error is caused by bad combination of tokens. For example, we cannot have a constant on the left hand side of an assignment statement, a for loop must have two expressions inside $()$ separated by semi colon etc. 

In the given question, line $3$ won't cause a lexical error or syntactic error. The statement will be treated as a function call with three arguments. Function definition being absent will cause link time error, but the question asks only for compile-time errors. So, $(a)$ must be the answer. 

PS: Implicit function declaration was removed from $C99$ standard onwards. As per current standard, we should not use a function without declaration. Still, we cannot guarantee "compilation error"- just expect compiler warnings in C. In C++ this should produce a compilation (semantic) error. The output of compiling the above code using different standards are given below:

arjun@linux:~$ gcc -c chk.c
chk.c: In function ‘main’:
chk.c:3:2: warning: implicit declaration of function ‘fro’ [-Wimplicit-function-declaration]
  fro (I=0, I<N, I++); /*Line 3 */
  ^
arjun@linux:~$ gcc -c -ansi chk.c
arjun@linux:~$ gcc -c -std=c99 chk.c
chk.c: In function ‘main’:
chk.c:3:2: warning: implicit declaration of function ‘fro’ [-Wimplicit-function-declaration]
  fro (I=0, I<N, I++); /*Line 3 */
  ^
arjun@linux:~$ gcc -c -std=c11 chk.c
chk.c: In function ‘main’:
chk.c:3:2: warning: implicit declaration of function ‘fro’ [-Wimplicit-function-declaration]
  fro (I=0, I<N, I++); /*Line 3 */

http://stackoverflow.com/questions/15570553/lexical-and-semantic-errors-in-c

Comments

@Overflow04 Here is your answer : c - Why local variable is initialized to zero - Stack Overflow

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i think it depends on system to system , os 
do it depend on compiler (since gcc is same for all ) ?

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@Shoto Yes, you're right. And I know why the answer to this specific question is "compiler error". What I meant was if the answer to this is Compiler error than why is the answer to the question under discussion isn't? Anyways, I figured it out later why but forgot to delete my comment :)

P.S.: To anyone having the same doubt as I 

int main() { /*Line 1 */

int I, N; /*Line 2 */

fro (I=0, I<N, I++); /*Line 3 */

}

"fro" isn't a valid C keyword, so the compiler treats it as a function call. In C89, an implicit declaration (int fro();) for "fro" is assumed. Since there is no actual definition of "fro" anywhere, although the compilation phase succeeds (possibly with a warning), it is during the linking phase that the linker won't be able to find the function implementation anywhere and will produce “undefined reference to fro”. Hence, this is a link-time error, not a compile-time error/compilation error.

While on the other hand, if "fro" would have been defined later in the program as "double" (without the declaration or definition prior to the call), it would have been a compiler error (reason already explained by @Shoto)

Answer 2

Answer is A. There is no error in the above code. Actually it is a link error ... Here compiler thinks fro is a function which is not declared , hence it will not produce any error .. It will only throw a warning in C .. 

Lexical analyser error comes when we declare a integer as 123zx . it is invalid to declare a integer like this.

Syntax analyser is related to syntax.

There is hence no error. Only link error because no function declaration has been found with 3 arguments.

Comments

so invalid variable declaration will generate lexical errors always ?

like 

1. 2foo (must not begin with a digit)
2. my foo (spaces not allowed in names)
3. $foo ($ not allowed -- only letters, and _)
4. $foo ($ not allowed -- only letters, and _)
5. 44a44(  alphabet cannot come in between digits )

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Sir, please correct me if I'm wrong , but isn't linker used to link external files and/or libraries? So how can it be a Link error?

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@amitqy 

"my foo" it will not give lexical error..

also check your 3rd and 4th point

Answer 3

Option A is correct.

Option B is wrong. Lexical Analyser would not throw any errors! Even if there were spelling errors, it'd simply correct them. 

Option C is wrong because fro is seen as a function. A function's arguments are separated by commas, which is the case here. There's no syntax error.

Consequently, Option D is wrong, too.

 

However, if Line 3 ran like this:

fro (I=0; I<N; I++);

Then, Option C would be correct, because a function's arguments aren't separated by semi-colons, as per the parsing rules — which gives a syntax error.

Comments

Thank you so much!
Just adding to this...
If it is for(i=0,i<n,i++), C would be correct. since it won't match the structure of the for loop...
A for loop uses semicolons to separate the initialisation, condition, and increment parts.

Answer 4

Answer : It is not a lexical error because lexical analyzer will treat fro as a token . I think it should be a syntactic error because few days ago i was confused between these 2 terms and i have posted this question here and i got answer in this way that syntactic errors can't be caught by c compiler because it does not violates all C -programming rules it is the responsibility of programmer to look at them and fix them and i believe if we write a program and by mistake we had written fro instead of for them we have to fix it .

Reference of the question I asked : https://gateoverflow.in/49019/compiler-phase-semantic-phase

Answer 5

There is no error in the above code. Actually it is a link error. Here compiler fro is a function which is not declared. Hence, it will not produce any error. It will only throw a warning in C.

Answer 6

since lexer can understand and create token for each no lexical error can bound be to happen here
now comes to syntax error,it occurs when such syntax not possible in the code 
for ie in c----> while() this is syntax error since while should have an expression inside ()
now here no such syntax error occuring as 

1. int main() { /*this is fine */
2. int I, N; /*declaring variable in captial letter allowed */
3. fro (I=0, I<N, I++); /*its a function fro which takes three argument  */
4. }
   
   Now compiler runs and execute successfully
   but when it goes for linker it searches fro definition either in this file or another file
   but as we have no other file it will generate linker error which we call link time error