Structure having pointers

[sam_cit@yahoo.co.in]

Hi,

I have the following Struct,

Struct Sample
{
int i;
char *p;
};

int main()
{
Sample a;
a.p = malloc(10);
Sample b;
b = a;
}

Now i think a shallow copy is done and if i destry only on the object
there would be a dangling pointer.
How do i overcome this problem as C structures don't support
functions?

Thanks in advance!!!

[AnticitizenOne]

Re: Structure having pointers

Hi,
when you write b=a you copy the memory of the struct instance a in b.
In this way you copy the pointer p of a in b. Now a.p and b.p point to
the same memory.
To make a depth copy of the struct you have to write a function:

struct Sample* SampleCopy(stru ct Sample* src)
{
struct Sample* b=NULL;
b=(struct Sample*)malloc( sizeof(struct Sample));
b.i=src.i;
b.p=(char*)mall oc(sizeof(char) *(strlen(src.p) +1));
memcpy(b.p,src. p);
return b;
}

int main()
{
Sample a;
a.p = (char*)malloc(1 0);
Sample *b;
b = SampleCopy(&a);

free(a.p);
free(a);
printf("%s",b->p); //should work!!
}

I think in this way you may solve your problem... if I've understood.
Probably I've committed some errors in the code.. but I've not a
reference now!

Bye
Gio


On 29 Lug, 12:20, sam_...@yahoo.c o.in wrote:

Hi,
>
I have the following Struct,
>
Struct Sample
{
int i;
char *p;
};
>
int main()
{
Sample a;
a.p = malloc(10);
Sample b;
b = a;
>
}
>
Now i think a shallow copy is done and if i destry only on the object
there would be a dangling pointer.
How do i overcome this problem as C structures don't support
functions?
>
Thanks in advance!!!

[santosh]

Re: Structure having pointers

sam_cit@yahoo.c o.in wrote:

Hi,
>
I have the following Struct,
>
Struct Sample

The keyword is struct, not Struct. C is case sensitive.

{
int i;
char *p;
};
>
int main()
{
Sample a;

The declaration should be:

struct Sample a;

a.p = malloc(10);
Sample b;

As above.

b = a;
}
>
Now i think a shallow copy is done

Since a.i has an indeterminate value, the copy invokes undefined behaviour.

and if i destry only on the object
there would be a dangling pointer.
How do i overcome this problem as C structures don't support
functions?

Deallocate the memory by calling free on either a.p or b.p, and set them
both to NULL.

[santosh]

Re: Structure having pointers

AnticitizenOne wrote:

[top-post corrected]

On 29 Lug, 12:20, sam_...@yahoo.c o.in wrote:

>Hi,
>>
> I have the following Struct,
>>
> Struct Sample
> {
> int i;
> char *p;
> };
>>
>int main()
>{
> Sample a;
> a.p = malloc(10);
> Sample b;
> b = a;
>>
>}
>>
> Now i think a shallow copy is done and if i destry only on the object
>there would be a dangling pointer.
>How do i overcome this problem as C structures don't support
>functions?

>
Hi,
when you write b=a you copy the memory of the struct instance a in b.
In this way you copy the pointer p of a in b. Now a.p and b.p point to
the same memory.
To make a depth copy of the struct you have to write a function:
>
struct Sample* SampleCopy(stru ct Sample* src)
{
struct Sample* b=NULL;
b=(struct Sample*)malloc( sizeof(struct Sample));

The cast isn't recommended in C.

b.i=src.i;
b.p=(char*)mall oc(sizeof(char) *(strlen(src.p) +1));

And sizeof(char) is always one in C.

memcpy(b.p,src. p);
return b;
}
>
int main()
{
Sample a;

It should be struct Sample a;

a.p = (char*)malloc(1 0);
Sample *b;

Mixed code and declarations are not allowed in C89 though they're allowed in
C99.

b = SampleCopy(&a);
>
free(a.p);
free(a);
printf("%s",b->p); //should work!!

return 0;

}

[Malcolm McLean]

Re: Structure having pointers


<sam_cit@yahoo. co.inwrote in message
news:1185704400 .274756.296890@ i38g2000prf.goo glegroups.com.. .

Hi,
>
I have the following Struct,
>
Struct Sample
{
int i;
char *p;
};
>
int main()
{
Sample a;
a.p = malloc(10);
Sample b;
b = a;
}
>
Now i think a shallow copy is done and if i destry only on the object
there would be a dangling pointer.
How do i overcome this problem as C structures don't support
functions?
>
Thanks in advance!!!
>

Do it like this

struct Sample *sample(int N)
{
struct Sample *answer;
answer = malloc(sizeof(s truct Sample));
if(!answer)
goto error_exit;
answer.p = malloc(N);
if(!answer.p)
goto error_exit;
return answer;
error_exit:
killSample(answ er);
return 0;
}

void killSample(stru ct Sample *s)
{
if(s)
{
free(s.p);
free(s);
}
}

struct Sample *Sample_clone(s truct Sample *s)
{
struct Sample *answer;

answer = sample(s.N):
if(!answer)
return 0;
memcpy(answer.p , s.p, s.N);
return answer;
}

All we are doing is replacing member functions with fucntions which take a
struct Sample * as a parameter. For simplicity everything is kept in dynamic
memory - very rarely this will cause a performance problem and need to be
changed - but it simpifies the code management.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Richard Heathfield]

Re: Structure having pointers

sam_cit@yahoo.c o.in said:

Hi,
>
I have the following Struct,
>
Struct Sample
{
int i;
char *p;
};
>
int main()
{
Sample a;
a.p = malloc(10);
Sample b;
b = a;
}

In addition to other people's comments, I would point out that this code
is not legal in either C90 or C99. The absence of a function declarator
for malloc requires a diagnostic message under the rules of either
Standard, and the mixing of declarations with code requires a
diagnostic message under C90.

I suggest you increase your compiler's level of diagnostic checking
until it conforms with the Standard's requirements.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[AnticitizenOne]

Re: Structure having pointers

On 29 Lug, 14:04, santosh <santosh....@gm ail.comwrote:

AnticitizenOne wrote:
>
[top-post corrected]
>
>
>

On 29 Lug, 12:20, sam_...@yahoo.c o.in wrote:

Hi,

>

I have the following Struct,

>

Struct Sample
{
int i;
char *p;
};

>

int main()
{
Sample a;
a.p = malloc(10);
Sample b;
b = a;

>

}

>

Now i think a shallow copy is done and if i destry only on the object
there would be a dangling pointer.
How do i overcome this problem as C structures don't support
functions?

>

Hi,
when you write b=a you copy the memory of the struct instance a in b.
In this way you copy the pointer p of a in b. Now a.p and b.p point to
the same memory.
To make a depth copy of the struct you have to write a function:

>

struct Sample* SampleCopy(stru ct Sample* src)
{
struct Sample* b=NULL;
b=(struct Sample*)malloc( sizeof(struct Sample));

>
The cast isn't recommended in C.
>

b.i=src.i;
b.p=(char*)mall oc(sizeof(char) *(strlen(src.p) +1));

>
And sizeof(char) is always one in C.
>

memcpy(b.p,src. p);
return b;
}

>

int main()
{
Sample a;

>
It should be struct Sample a;
>

a.p = (char*)malloc(1 0);
Sample *b;

>
Mixed code and declarations are not allowed in C89 though they're allowed in
C99.
>

b = SampleCopy(&a);

>

free(a.p);
free(a);
printf("%s",b->p); //should work!!

>
return 0;
>

}

Sorry for my errors... 90% of them are for distraction and cut'n
paste...

b=(struct Sample*)malloc( sizeof(struct Sample));

>
The cast isn't recommended in C.
>

This is really new for me o_O
I use the cast a lot... why in C is not recommended?

thanks!
bye
Gio

[Richard Heathfield]

Re: Structure having pointers

AnticitizenOne said:

On 29 Lug, 14:04, santosh <santosh....@gm ail.comwrote:

>AnticitizenO ne wrote:

<snip>

b=(struct Sample*)malloc( sizeof(struct Sample));

>>
>The cast isn't recommended in C.
>>

This is really new for me o_O
I use the cast a lot...

Why?

why in C is not recommended?

Several reasons, but before we get into those reasons, let's find out
your reasons for using the cast.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Richard]

Re: Structure having pointers

santosh <santosh.k83@gm ail.comwrites:

sam_cit@yahoo.c o.in wrote:
>

>Hi,
>>
> I have the following Struct,
>>
> Struct Sample

>
The keyword is struct, not Struct. C is case sensitive.
>

> {
> int i;
> char *p;
> };
>>
>int main()
>{
> Sample a;

>
The declaration should be:
>
struct Sample a;
>

> a.p = malloc(10);
> Sample b;

>
As above.
>

> b = a;
>}
>>
> Now i think a shallow copy is done

>
Since a.i has an indeterminate value, the copy invokes undefined
behaviour.

How?

>

> and if i destry only on the object
>there would be a dangling pointer.
>How do i overcome this problem as C structures don't support
>functions?

>
Deallocate the memory by calling free on either a.p or b.p, and set them
both to NULL.
>

--

[AnticitizenOne]

Re: Structure having pointers

On 29 Lug, 14:39, Richard Heathfield <r...@see.sig.i nvalidwrote:

AnticitizenOne said:
>

On 29 Lug, 14:04, santosh <santosh....@gm ail.comwrote:

AnticitizenOne wrote:

>
<snip>
>

b=(struct Sample*)malloc( sizeof(struct Sample));

>

The cast isn't recommended in C.

>

This is really new for me o_O
I use the cast a lot...

>
Why?
>

why in C is not recommended?

>
Several reasons, but before we get into those reasons, let's find out
your reasons for using the cast.
>
--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

I use the cast as referenced in C-Language (Kerningan & Ritchie)

[santosh]

Re: Structure having pointers

AnticitizenOne wrote:

On 29 Lug, 14:39, Richard Heathfield <r...@see.sig.i nvalidwrote:

>AnticitizenO ne said:
>>

On 29 Lug, 14:04, santosh <santosh....@gm ail.comwrote:
>AnticitizenO ne wrote:

>>
><snip>
>>

b=(struct Sample*)malloc( sizeof(struct Sample));

>>

>The cast isn't recommended in C.

>>

This is really new for me o_O
I use the cast a lot...

>>
>Why?
>>

why in C is not recommended?

>>
>Several reasons, but before we get into those reasons, let's find out
>your reasons for using the cast.

>
I use the cast as referenced in C-Language (Kerningan & Ritchie)

That book was written before the first Standard for C was published. At that
point, there was no void * as a generic pointer type, but instead that
purpose was served by the char * type. Since then it's not necessary in C
to cast between a void * and another pointer type. Any pointer can be
converted to a void * and back again without any loss of information.

Infact casting the return value of malloc can prevent your compiler from
warning you when you fail to include stdlib.h, which declares malloc's
prototype. This can lead to possible undefined behaviour.

The only real reason for casting the return of malloc in C is if you're
forced to compile it under a C++ compiler or in a mixed C and C++
environment. That's pretty rare.

[AnticitizenOne]

Re: Structure having pointers

On 29 Lug, 14:53, santosh <santosh....@gm ail.comwrote:

AnticitizenOne wrote:

On 29 Lug, 14:39, Richard Heathfield <r...@see.sig.i nvalidwrote:

AnticitizenOne said:

>

On 29 Lug, 14:04, santosh <santosh....@gm ail.comwrote:
AnticitizenOne wrote:

>

<snip>

>

b=(struct Sample*)malloc( sizeof(struct Sample));

>

The cast isn't recommended in C.

>

This is really new for me o_O
I use the cast a lot...

>

Why?

>

why in C is not recommended?

>

Several reasons, but before we get into those reasons, let's find out
your reasons for using the cast.

>

I use the cast as referenced in C-Language (Kerningan & Ritchie)

>
That book was written before the first Standard for C was published. At that
point, there was no void * as a generic pointer type, but instead that
purpose was served by the char * type. Since then it's not necessary in C
to cast between a void * and another pointer type. Any pointer can be
converted to a void * and back again without any loss of information.
>
Infact casting the return value of malloc can prevent your compiler from
warning you when you fail to include stdlib.h, which declares malloc's
prototype. This can lead to possible undefined behaviour.
>
The only real reason for casting the return of malloc in C is if you're
forced to compile it under a C++ compiler or in a mixed C and C++
environment. That's pretty rare.

uh! Thanks! I've understood.

[pete]

Re: Structure having pointers

Richard wrote:

>
santosh <santosh.k83@gm ail.comwrites:
>

sam_cit@yahoo.c o.in wrote:

Hi,
>
I have the following Struct,
>
Struct Sample

The keyword is struct, not Struct. C is case sensitive.

{
int i;
char *p;
};
>
int main()
{
Sample a;

The declaration should be:

struct Sample a;

a.p = malloc(10);
Sample b;

As above.

b = a;
}
>
Now i think a shallow copy is done

Since a.i has an indeterminate value, the copy invokes undefined
behaviour.

>
How?

Simply and obviously.

--
pete

[Ben Bacarisse]

Re: Structure having pointers

"Malcolm McLean" <regniztar@btin ternet.comwrite s:

<sam_cit@yahoo. co.inwrote in message
news:1185704400 .274756.296890@ i38g2000prf.goo glegroups.com.. .

>Hi,
>>
>I have the following Struct,
>>
>Struct Sample
>{
> int i;
> char *p;
>};
>>
>int main()
>{
>Sample a;
>a.p = malloc(10);
>Sample b;
>b = a;
>}
>>
>Now i think a shallow copy is done and if i destry only on the object
>there would be a dangling pointer.
>How do i overcome this problem as C structures don't support
>functions?
>>

Do it like this
>
struct Sample *sample(int N)
{
struct Sample *answer;
answer = malloc(sizeof(s truct Sample));
if(!answer)
goto error_exit;
answer.p = malloc(N);
if(!answer.p)
goto error_exit;
return answer;
error_exit:
killSample(answ er);
return 0;
}
>
void killSample(stru ct Sample *s)
{
if(s)
{
free(s.p);
free(s);
}
}
>
struct Sample *Sample_clone(s truct Sample *s)
{
struct Sample *answer;
>
answer = sample(s.N):
if(!answer)
return 0;
memcpy(answer.p , s.p, s.N);
return answer;
}

Depending on how you count them, you have about 8 errors and a rather
complex way of doing things. I would write:

struct Sample *sample_clone(s truct Sample *sp)
{
struct Sample *answer = malloc(sizeof *answer);
if (answer && (answer->p = malloc(sp->i)))
memcpy(answer->p, sp->p, sp->i);
else {
free(answer);
answer = NULL;
}
return answer;
}

void sample_kill(str uct Sample *sp)
{
if (sp) free(sp->p);
free(sp);
}

I know that many people don't like this style, but I think C works
that way. I am sure that it is not accidental that things like short
circuit &&, assignments in conditionals, and being able to free NULL all
work together to make compact, idiomatic code.

--
Ben.

[Richard]

Re: Structure having pointers

pete <pfiland@mindsp ring.comwrites:

Richard wrote:

>>
>santosh <santosh.k83@gm ail.comwrites:
>>

sam_cit@yahoo.c o.in wrote:
>
>Hi,
>>
> I have the following Struct,
>>
> Struct Sample
>
The keyword is struct, not Struct. C is case sensitive.
>
> {
> int i;
> char *p;
> };
>>
>int main()
>{
> Sample a;
>
The declaration should be:
>
struct Sample a;
>
> a.p = malloc(10);
> Sample b;
>
As above.
>
> b = a;
>}
>>
> Now i think a shallow copy is done
>
Since a.i has an indeterminate value, the copy invokes undefined
behaviour.

>>
>How?

>
Simply and obviously.

I think I have my dumb head on today Aunt Sally.

Really, how does the copy invoke undefined behaviour?

An undefined value is copied, but how does the invoke undefined
behaviour at the point of the copy?