Re: c interview

Hi Flash,
First of all, thanks a lot for the detailed comments.

You use printf without a function prototype in scope. This invokes
undefined behaviour because printf is a varidac function. ALWAYS include
stdio.h before using printf and other headers as appropriate before
using other functions.
For most of the programs, I have included it. But as you pointed out I
missed out in a few of them. Thanks for pointing it.

In your first CountBits function you assume that int is 32 bits. int
could be as small as 16 bits. If you want a number of at least 32 bits
use long. In addition it is not guaranteed to work properly with
negative numbers, so you should use an unsigned type such as unsigned long.
Would make it a point that the size of int is assumption to be ....

int foobar();
is *not* a function prototype. To be correct I believe you should say:
| Are the following two function declarations same?
Agreed.

..

Your example:
| #include <stdio.h>
| int main()
| {
| float a = 12.5;
| printf("%d\n", a);
| printf("%d\n", *(int *)&a);
| return 0;
| }
may also not behave as *you* expect since it invokes undefined
behaviour. If I recall correctly one implementation I have would print
12 on the first line and other implementations I have definitely would not.
Yes, But most of the questions (including this) are open ended
questions. They are not multiple choice
questions with fixed answers..
And it is also stated on the website (in the introduction before the
programs):
<snip>
Most of the programs are meant to be compiled, run and to be explained
for their behaviour
</snip>


You don't always check the value returned by malloc before using it. You
should.
Yes.

Your example of a definition of the offsetof macro invokes undefined
behaviour. It is *not* possible to implement it portably and this is
probably why it is provided in a standard header.
It may invoke undefined bahaviour. But for the implementation (which is
assumed to work), the reader is expected to find out how it is working.

Some of your other example questions are, in my opinion, plain stupid.
This does not mean that they are never asked of course
Agreed.
But history says something more about the stupid mistakes[1]

Who have expected that the following stupid bug would
have cause the major disruption of AT&T phone services
throughout US. (AT&T's network was unusable for almost
nine hours starting on afternoon of January 15,1990)

<snip>
network code()
{
switch (line) {
case THING1:
doit1();
break;
case THING2:
if (x == STUFF) {
do_first_stuff();
if (y == OTHER_STUFF)
break;
do_later_stuff();
} /* coder meant to break to here... */
initialize_modes_pointer();
break;
default:
processing();
} /* ...but actually broke to here! */
use_modes_pointer();/* leaving the modes_pointer
uninitialized */
}
</snip>

and fingerd code leading to a worm creating havoc in November 1998
<snip>
main(argc, argv)
char *argv[];
{
char line[512];
...
gets(line);
</snip>

And this causing an ANSI C compiler to become very slow:
<snip>

int hashval=0;
/* PJW hash function from "Compilers: Principles, Techniques,
and Tools"
* by Aho, Sethi, and Ullman, Second Edition.
while (cp < bound)
{
unsigned long overflow;
hashval = ( hashval <<4)+*cp++;
if ((overflow = hashval & (((unsigned long) 0xF) << 28)) != 0)
hashval ^= overflow | (overflow >> 24);
}
hashval %= ST_HASHSIZE; /* choose start bucket */
/* Look through each table, in turn, for the name. If we fail,
* save the string, enter the string's pointer, and return it.
*/
for (hp = &st_ihash; ; hp = hp->st_hnext) {
int probeval = hashval; /* next probe value */
...
.....
......
</snip>

You should acknowledge the original author of Duff's device. Tom Duff
deserves recognition for his highly warped thinking. I suggest a link
over to http://www.lysator.liu.se/c/duffs-device.html unless someone can
suggest a better link
I would also suggest you include references to other useful resources,
in particular the comp.lang.c FAQ at http://c-faq.com/

References and credits are badly missing. I would work on it in the
near future.
When I hosted up those questions, I never thought the site would become
so popular that it now ranks in top 20 for the search "c puzzles" in
google. Now I'm making an effort to make the questions more correct and
the material to be more useful.

Regards,
Gowri Kumar
[1] From the book "Expert C programming - Deep C secrets" -by Peter Van
Der Lindenman
In fact many of the examples are taken directly from the above book ( I
do have persmission for it from Peter).

.

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