Set of questions.

**Ron** · 08-07-2008

I have a set of questions that may be asked for my exams. Can anyone tell me the solutions to this?

Q1)

Code:

What is the problem with the following copy?
char *s1 = “hello”;
char *s2;
strcpy(s2,s1);

I know it is a segmentation fault, but why"?

Q2)
What are the two common problems of working with dynamic memory
allocation and pointers?

Q3)
Explain top-down and bottom-up debugging techniques.

Q4)
Assume function static void f(void) whose definition is in body.c
and its prototype is in body.h. If I include body.h in another C file, myprog.c,
having the main function and calls function f, and try to run gcc body.c
myprog.c –o myprog, should I expect any error message (assume that all
programs are syntactically correct)? If yes, the error belongs to which step of
compilation (i.e. preprocessing, linking, etc.)? Why?

I know there will be an error. But why?

Pleas reply.

**valaris** · 08-07-2008

The pointer s2 is just that, a pointer, it has no memory to store anything in.

**rasta_freak** · 08-07-2008

Q1) when you use

Code:

char *junk = "Some text";

compiler will allocate space to hold that text ("Some text") and put address of it into "junk". It can mark this space as const (so program crashes if it modifies it), or it can allocate is as usual (on stack?) so you can modify it, it depends on compiler (options?). But the point is that there's something somewhere allocated so s1 is valid pointer. s2 points to who knows what (probably NULL), and isn't associated with any allocated memory, so using that pointer is "being brave"

**rasta_freak** · 08-07-2008

Q4) when function has "static" 'decoration' it means it's linkage is static to that source/object file - only code in .c file where it's defined can use it, and it's symbols aren't exported to linker so linker can't resolve 'requests' from other object files to call that function. Static function is local to that .c file. Is exists to ensure that locality, so noone else can see/use that function. If you write your version of printf(), and use it in only one .c file, but don't make it static, all other printf()s from other modules might get replaced with that function (depends on linking order, maybe linker will pick printf() from libc, maybe from your module).

**Ron** · 08-07-2008

I did not understand the explanation for the 4th question.

**noops** · 08-07-2008

Originally Posted by Ron

I did not understand the explanation for the 4th question.

Do you know what 'static' means?

**Dogmasur** · 08-07-2008

Originally Posted by Ron

I did not understand the explanation for the 4th question.

I am very new to programming, but I have read a little about static. I believe what rasta_freak is saying is that the static function is usable only by the file it was defined in. Another linked file cannot call for use that function.

Like I said, I am extremely new...so someone please let me know if I am right or wrong as I decided to write simply to see if I have actually understood this concept.

Also, a linked file could still call a function from the first file that used said static function, right?

**tabstop** · 08-07-2008

Originally Posted by Dogmasur

I am very new to programming, but I have read a little about static. I believe what rasta_freak is saying is that the static function is usable only by the file it was defined in. Another linked file cannot call for use that function.

Right.

Originally Posted by Dogmasur

Also, a linked file could still call a function from the first file that used said static function, right?

If you mean that something in file2 called a function in file1 (which itself called a static function in file1), then yes.

**Dogmasur** · 08-07-2008

Yeah...I understood something..hhaha.

**King Mir** · 08-07-2008

Originally Posted by Ron

Q2)
What are the two common problems of working with dynamic memory
allocation and pointers?

Probably checking for NULL and memory leaks. But there are many possible problems.

**swgh** · 08-08-2008

One being forgetting to de-allocate it after you have allocated it the required memory when the program has finished.

**icoolucool** · 08-08-2008

s2 need to be malloc as it need some space to copy s1
For anythin on C programming tutorials find here :
http://c4tutorials.blogspot.com

**Elysia** · 08-08-2008

Using pointers without allocating them first is a common problem. Indeed, this is a very common error.
http://cpwiki.sourceforge.net/Common...llocating_them
There are more too, you can study and learn to avoid.

**matsp** · 08-08-2008

Originally Posted by Elysia

Using pointers without allocating them first is a common problem. Indeed, this is a very common error.
http://cpwiki.sourceforge.net/Common...llocating_them
There are more too, you can study and learn to avoid.

Well, I'd say that's common only in beginners programs. I wouldn't classify any of my colleagues or the software engineers at the third parties that out company contracts to do work as beginners, but they still make occasional mistakes of:
1. Not checking that allocation was successful before using the allocated memory [this usually happens when the scenario is slightly more complex than just calling malloc or new].
2. Not freeing the memory after it is no longer needed.
3. Use of memory after it has been freed.

So whilst beginners may not realize the need for allocation (and thus we see it quite often here), but the three issues listed above are certainly much harder to keep track of and fulfill at all times.

--
Mats

**Elysia** · 08-08-2008

Of course, I was referring to a common error seen on the boards. Yes.

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