Some Questions Asked in Interview

[Jatinder]

I 'm a professional looking for the job.In interview these questions
were asked with some others which I answered.But some of them left
unanswered.Plz help.

Here are some questions on C/C++, OS internals?
Q1 . What is the use of pointer to an array?
Q2 . What is the use of array of pointers?
Q3 . What is the use of pointer to function ?
Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
?
Q5 . What is IOCTL Explain .
Q6 . How to create an interrupt service routine in C?
Q7 . What are the internals of a schedular ?
Q8 . The static variables are declared in heap or stack ?

 

[Spacen Jasset]

I 'm a professional looking for the job.In interview these questions
were asked with some others which I answered.But some of them left
unanswered.Plz help.

These are not all C questions, and can also have quite lengthy answers.

> Here are some questions on C/C++, OS internals?
Q1 . What is the use of pointer to an array?
Q2 . What is the use of array of pointers?
Q3 . What is the use of pointer to function ?
Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
?
Q5 . What is IOCTL Explain .
Q6 . How to create an interrupt service routine in C?
Q7 . What are the internals of a schedular ?
Q8 . The static variables are declared in heap or stack ?

I won't give any answer becuase I feel the C questions are too vague.
Details on IOCTL can be found on the web, as can information about XON,
XOFF.

 

[Dhananjaya Reddy Eadala]

I 'm a professional looking for the job.In interview these questions
were asked with some others which I answered.But some of them left
unanswered.Plz help.

Here are some questions on C/C++, OS internals?
Q1 . What is the use of pointer to an array? many
Q2 . What is the use of array of pointers? many
Q3 . What is the use of pointer to function ?

Useful to for anonymous reverse invocation. Most useful to invoke
application's function from library.

Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
? It is not c question.
Q5 . What is IOCTL Explain .

man ioctl. You will get nice explantion

Q6 . How to create an interrupt service routine in C?

We can write only software interrupts and they are called signal
handlers. These are like normal c functions those take an integer (sig
num) as argument

Q7 . What are the internals of a schedular ?

not a c question. It requires vague explanation

Q8 . The static variables are declared in heap or stack ?

heap

 

[Lew Pitcher]

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Jatinder wrote: [snip]

Here are some questions on C/C++, OS internals? [snip]
Q5 . What is IOCTL Explain .

man ioctl. You will get nice explantion

Actually, this is not a C question. IOCTLs are outside the scope of the
standard C language. /And/ the unix 'man' command isn't necessarily
available to the OP.

[snip]

heap

Perhaps not. The standard C language neither recognizes the term 'heap',
nor the term 'stack', and thus the question doesn't actually relate to
C. As for specific implementations, both 'heap' and 'stack' are eligible
to store static variables, although IMHO 'stack' would take more work to
implement.




- --

Lew Pitcher, IT Consultant, Enterprise Application Architecture
Enterprise Technology Solutions, TD Bank Financial Group

(Opinions expressed here are my own, not my employer's)
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[Xenos]

I 'm a professional looking for the job.In interview these questions
were asked with some others which I answered.But some of them left
unanswered.Plz help.

Here are some questions on C/C++, OS internals?
Q1 . What is the use of pointer to an array?
Q2 . What is the use of array of pointers?
Q3 . What is the use of pointer to function ?
Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
?
Q5 . What is IOCTL Explain .
Q6 . How to create an interrupt service routine in C?
Q7 . What are the internals of a schedular ?
Q8 . The static variables are declared in heap or stack ?

I find it alarming that this trend of giving ridiculous quizzes and tests to
interviewee is increasing. When I was young and looking for a job,
companies that did this were few. I immediately walked out of any interview
were I was asked to take such a test. Regurgitation of facts does not prove
knowledge or wisdom, and is certainly no indication of skill. For someone
with an encyclopedic memory of the C or C++ standards, I would hire for a
one-time fee of $18--the amount I would need to just purchase a copy of the
standard (or whatever it currently costs).

This topic has come up before, and I think I said pretty much the something.
Someone replied with, "So how do you know they can do the job?" Well, there
are no guarantees, but you could try *talking* to them. I never minded
being asked questions in interviews. Isn't that a major part of the
process? Ask about college courses taken and projects done. If you are
interviewing an experienced professional, ask about previous work done. How
about what problems were encountered and how they were overcome. DON'T try
and sitting me in a room with 50 other nameless applicants and presume to
give me a test. That's a company interested in bodies and tests scores, not
cultivating good people. I'm also turned off by companies that call you up
and ask for college transcripts. Unless they have taken the time to
interview me and show an interest in hiring me, I always refused.

As a result, I love the company I work for. They treat me very well, and
there is always very smart, experienced people to learn from.

DrX

 

[Ben Pfaff]

Q1 . What is the use of pointer to an array?

Such things are rarely useful. It is far more common to point to
the first element of an array.

Q2 . What is the use of array of pointers?

This question is so general that it is absurd. You can use
arrays of pointers for any number of things. For example, you
can store pointers to elements to be sorted within the array, and
then sorting will just require moving pointers instead of the
(presumably larger) elements themselves.

Q3 . What is the use of pointer to function ?
Polymorphism.

Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
?
Q5 . What is IOCTL Explain .
Q6 . How to create an interrupt service routine in C?
Q7 . What are the internals of a schedular ?

Q4 through Q7 are off-topic for comp.lang.c

Q8 . The static variables are declared in heap or stack ?

No.

 

[Malcolm]

Q1 . What is the use of pointer to an array?

To pass the array to a function.

Q2 . What is the use of array of pointers?

To pass a list of arrays to a fucntion.

Q3 . What is the use of pointer to function ?

To pass a function to a function.

Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)

Not a clue. Ports vary from platform to platform.

?
Q5 . What is IOCTL Explain .

An acronym. Not a clue what it means.

Q6 . How to create an interrupt service routine in C?

You can't do this in standard C. Refer to your compiler documentation.

Q7 . What are the internals of a schedular ?

Huh? A clock and a series of function pointers it calls at appropriate
times, I would guess.

Q8 . The static variables are declared in heap or stack ?

Neither. They are usually in their own area of memory which is allocated at
program initialisation.

 

[Guillaume]

I find it alarming that this trend of giving ridiculous quizzes and tests to

interviewee is increasing.

Well, as for me, I don't particularly find it alarming that technical
questions be asked during job interviews - even if it looks like a
formal "test".
After all, education itself is not enough to judge someone's
skills. And related past work may be just lies or edulcorated stories -
unless the human resources staff checks them thoroughly, which is not
always possible nor practical.

What I find alarming in the above questions is that at least half of
them don't make any sense or are so general that they can't be answered
directly. I find alarming that it seems these questions have been
written by someone inept in the technical and software engineering
fields, and may disqualify skilled people and qualify people who can't
write actual software.

 

[Jarno A Wuolijoki]

Little. Usually you don't point to arrays.


Some. You may need to point to more than one thing at a time.


Immense. You can't call a function without pointing to it.


Stick a pen through it. If you make a mess you need to control the ink flow.


Sorry, I've seen some IOCTLs but never that one.


An infinite loop often does the trick.


The concept of an event is quite general. They aren't likely to have
common internals.


False. They are typically declared near the beginning of the
program unless they are local to some function.

 

[Mabden]

I 'm a professional looking for the job.In interview these questions
were asked with some others which I answered.But some of them left
unanswered.Plz help.

Here are some questions on C/C++, OS internals?
Q1 . What is the use of pointer to an array?

TO step through the array, or pass it by reference to another function.

Q2 . What is the use of array of pointers?

A list of strings of various sizes, or it could be an array of function
pointers. If one has 20 field validation functions (check_ssn, check_phone,
check_zip, etc.) you could put them in an array of pointers and attach the
pointer to a field on a form.

Q3 . What is the use of pointer to function ?

See above, you can put them in an array and call them like this: validate
(field, vals[check_zip]);

Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)

That's what parallel ports are for. Who cares. I use USB.

Q5 . What is IOCTL Explain .

A dinosaur. DOS based device-driver accessor: ioctl (handle, cmd, &dx, &cx);
Time to upgrade.

Q6 . How to create an interrupt service routine in C?

signal ();
With this you can capture Ctrl-C, divide by zero, etc.

Q7 . What are the internals of a schedular ?

A state machine?

Q8 . The static variables are declared in heap or stack ?

Undefined, but the answer desired is probably heap.

 

[pete]

TO step through the array,
or pass it by reference to another function.

A pointer to an array is also handy for mallocating
a multidimensional array, using only one pointer.

 

[Thomas stegen]

TO step through the array, or pass it by reference to another function.

You can't step through an array using a pointer to it.
If you increment a pointer to an array by 1 you will point
one past the end of the object (the array in this case) this
is defined, but you cannot dereference this pointer as that
leads to undefined behaviour.

A pointer to an array is rarely useful, I have never had the
need for one.

 

[Mabden]

You can't step through an array using a pointer to it.
If you increment a pointer to an array by 1 you will point
one past the end of the object (the array in this case) this
is defined, but you cannot dereference this pointer as that
leads to undefined behaviour.

A pointer to an array is rarely useful, I have never had the
need for one.

You are assuming I am incompetent. I would either keep a count on members or
add a terminating value to the array. Running off the end, or "off by one"
is a common error that one must plan to handle. Please don't tell me I can't
do something because you see a trivial restriction. When you say "you can't"
I think of going to the moon. You would have said the same thing back then,
"you can't"

I can.

 

[Jens.Toerring]

You are assuming I am incompetent. I would either keep a count on members or
add a terminating value to the array. Running off the end, or "off by one"
is a common error that one must plan to handle. Please don't tell me I can't
do something because you see a trivial restriction. When you say "you can't"
I think of going to the moon. You would have said the same thing back then,
"you can't"

I can.

I guess you're overlooking the difference between "pointer to an array"
and "pointer to the first element of an array". The OP probably meant the
second but when you take it to mean what it really says a pointer to an
array would be something like

int array[ 10 ];
int ( * pointer_to_array_of_ten_ints )[ 10 ] = &array;

And that construct probably would tend to be used not that often;-)

Regrds, Jens

 

[James Kanze]

|> >> Q8 . The static variables are declared in heap or stack ?

|> > heap

|> Perhaps not. The standard C language neither recognizes the term
|> 'heap', nor the term 'stack', and thus the question doesn't actually
|> relate to C. As for specific implementations, both 'heap' and
|> 'stack' are eligible to store static variables, although IMHO
|> 'stack' would take more work to implement.

On the systems I'm familiar with, the answer would be neither.

 

[James Kanze]

|> |> > I 'm a professional looking for the job.In interview these
|> > questions were asked with some others which I answered.But some of
|> > them left unanswered.Plz help.

|> > Here are some questions on C/C++, OS internals?
|> > Q1 . What is the use of pointer to an array?
|> > Q2 . What is the use of array of pointers?
|> > Q3 . What is the use of pointer to function ?
|> > Q4 . How to print through serial port? What is Flow Control(Xon,Xoff)
|> > ?
|> > Q5 . What is IOCTL Explain .
|> > Q6 . How to create an interrupt service routine in C?
|> > Q7 . What are the internals of a schedular ?
|> > Q8 . The static variables are declared in heap or stack ?

|> I find it alarming that this trend of giving ridiculous quizzes and
|> tests to interviewee is increasing.

It depends. The above questions aren't multiple choice, and can lead to
interesting and thoughtful answers. At least, some could.

In my experience, such "quizzes" are most useful if given orally and
interactively by someone familiar with the topic. Less formally, I
suspect that most people called on to vet a number of so-called
specialists will make up a list of questions to start with. After the
first few, of course, you play it by ear, adjusting to the apparent
competence of the person being interviewed.

 

[James Kanze]

|> Mabden wrote:

|> >>Q1 . What is the use of pointer to an array?
|> > TO step through the array, or pass it by reference to another
|> > function.

|> You can't step through an array using a pointer to it. If you
|> increment a pointer to an array by 1 you will point one past the end
|> of the object (the array in this case) this is defined, but you
|> cannot dereference this pointer as that leads to undefined
|> behaviour.

|> A pointer to an array is rarely useful, I have never had the need
|> for one.

Nonsense. If a is a two dimensional array, the expression a returns
a pointer to an array.

 

[Joona I Palaste]

|> Mabden wrote:
|> >>Q1 . What is the use of pointer to an array?
|> > TO step through the array, or pass it by reference to another
|> > function.

|> You can't step through an array using a pointer to it. If you
|> increment a pointer to an array by 1 you will point one past the end
|> of the object (the array in this case) this is defined, but you
|> cannot dereference this pointer as that leads to undefined
|> behaviour.

|> A pointer to an array is rarely useful, I have never had the need
|> for one.

Nonsense. If a is a two dimensional array, the expression a returns
a pointer to an array.

No it doesn't. It returns an array, which "decays" into a pointer when
used as a value. a itself "decays" into a pointer to an array when used
as a value. "Decays" does not mean "is", as Chris Torek will explain at
length given half the chance.

 

[Jens.Toerring]

Thomas stegen <[email protected]> writes:

|> Mabden wrote:

|> >>Q1 . What is the use of pointer to an array?
|> > TO step through the array, or pass it by reference to another
|> > function.

|> You can't step through an array using a pointer to it. If you
|> increment a pointer to an array by 1 you will point one past the end
|> of the object (the array in this case) this is defined, but you
|> cannot dereference this pointer as that leads to undefined
|> behaviour.

|> A pointer to an array is rarely useful, I have never had the need
|> for one.

Nonsense. If a is a two dimensional array, the expression a returns
a pointer to an array.

Well, lets look at that. As you probably know very well there aren't
any true multidimensional arrays in C, they are basically syntactic
sugar, made up from flat, one-dimensional arrays. And the rules for
such arrays, like for example

int a[ N ][ M ];

are to evaluate

a[ i ][ j ] := *( a + i * M + j )

and

a[ i ] := a + i * M

where 'a' itself is a pointer to the first element of that "two-dimen-
sional" array (not a "pointer to a two-dimensional array"). So the type
of 'a' isn't "pointer to array" but (in this case) "pointer to int".

It seems to be a common misconception that with

int b[ 20 ];

'b' would be of type "pointer to array". But 'b' is of type "pointer to
int", pointing to the first element of the array, so you loosely can
call it an "array pointer". But only '&b' is of type "pointer to array
(of 20 ints)". You will see that easily when you create type of "pointer
to array (of 20 ints)" e.g. using a typedef to make things easier to
read (and not to offend Emmanuel sense of beauty;-):

typedef int ( *AP )[ 20 ];

AP a;

If you now try

int b[ M ];

a = b;

you will get a "assignment from incompatible pointer type" error, and
you must change that to

a = &b;

to get it working.
Regards, Jens

 

[James Kanze]

(e-mail address removed)-berlin.de writes:


|> > |> Mabden wrote:

|> > |> >>Q1 . What is the use of pointer to an array?
|> > |> > TO step through the array, or pass it by reference to
|> > |> > another function.

|> > |> You can't step through an array using a pointer to it. If you
|> > |> increment a pointer to an array by 1 you will point one past
|> > |> the end of the object (the array in this case) this is
|> > |> defined, but you cannot dereference this pointer as that leads
|> > |> to undefined behaviour.

|> > |> A pointer to an array is rarely useful, I have never had the
|> > |> need for one.

|> > Nonsense. If a is a two dimensional array, the expression a
|> > returns a pointer to an array.

|> Well, lets look at that. As you probably know very well there aren't
|> any true multidimensional arrays in C, they are basically syntactic
|> sugar, made up from flat, one-dimensional arrays.

Right.

I think you'll agree that if a is declared as a 2 diminsional array, the
expression a + i is a pointer to an array. I think I got carried away
with a -- a is an array, which decays to a pointer to the first
element in certain cases.

|> And the rules for such arrays, like for example

|> int a[ N ][ M ];
|>
|> are to evaluate

|> a[ i ][ j ] := *( a + i * M + j )

Not quite. It evaluates to *(a + i) + j. In this case, a decays to a
pointer to an array (the first element of a, and int[M]); i is added to
this pointer, which gives a pointer to an array, the i'th array in a.
This pointer then decays to a pointer to the first element, to which j
is added.

The expression you wrote will typically invoke undefined behavior
(except for special cases, e.g. where i and j equal 0).

|> and

|> a[ i ] := a + i * M

No. The expression a is explicitly defined by the standard as *(a+i).

You never, never multiply by M. Behind the scenes, of course, the
compiler will multiply by sizeof(a), i.e. the size of an int[M]. But
that is simply the implementation technique, and is no different than
when you have an array of struct's or whatever.

|> where 'a' itself is a pointer to the first element of that
|> "two-dimensional" array (not a "pointer to a two-dimensional
|> array").

Not quite. Read your first sentence: there are no two-dimensional
arrays, just arrays of arrays. So a decays to a pointer to the first
element of a, and the type of that element is an int[M].

|> So the type of 'a' isn't "pointer to array" but (in this case)
|> "pointer to int".

No, the type of a is int[M]. Not pointer to array, since we've
already dereferenced the pointer. But array of int.

Depending on what you do next, that array may decay into a pointer. Or
may not: what does sizeof( a ) return?

|> It seems to be a common misconception that with

|> int b[ 20 ];

|> 'b' would be of type "pointer to array".

Not with anyone who has every used C. The common misconception seems to
be that b is a pointer to int, and that the declarations:
extern int * b ;
and
extern int b[] ;
are equivalent, and can be used interchangeably.

|> But 'b' is of type "pointer to int", pointing to the first element
|> of the array, so you loosely can call it an "array pointer".

No, that is the common misconception. b is of type array[20] of int.

|> But only '&b' is of type "pointer to array (of 20 ints)". You will
|> see that easily when you create type of "pointer to array (of 20
|> ints)" e.g. using a typedef to make things easier to read (and not
|> to offend Emmanuel sense of beauty;-):

|> typedef int ( *AP )[ 20 ];

|> AP a;

|> If you now try

|> int b[ M ];

|> a = b;

|> you will get a "assignment from incompatible pointer type" error, and
|> you must change that to

|> a = &b;

|> to get it working.

But I was talking about 2 dimentional arrays. And
int b[ 10 ][ 20 ] ;
AP a = b ;
should work.

Although my example went a step too far -- I should have said a+i,
rather than a (which is, of course *(a+i)). But the point remains;
you cannot use two dimensional arrays in C without using pointers to
arrays. Even if typically, you don't write them explicitly.

Another obvious case, of course: what is the type of the parameter of:
int f( int a[5][5] ) ;