Pointer problem

[seema]

I have a struct and I want to access its values, I am new to pointers
so I have written something like this,
# define COLLATE "Beetle"
struct mytips
{
short inverse;
long mass;
char *c;
char *m;
};
int main(){
struct mytips store[10] = {
{0,0x00000001L, COLLATE,"playin g one"},
{0,0x00000002L, COLLATE,"playin g two"},
{0,0x00000004L, COLLATE,"playin g three"},
{0,0x00000008L, COLLATE,"playin g four"},
{0,0x00000008L, COLLATE,"playin g five"},
{0,0x00000008L, COLLATE,"playin g six"},
};
void *dinku;
dinku=(struct mytips*)malloc( (sizeof(char)*s izeof(store))+1 );
}
Now how can I access the values of store using void pointer? Can
somebody explaing
how to do it
Thanks in advance,
Seema

[Michael Mair]

Re: Pointer problem

seema wrote:[color=blue]
> I have a struct and I want to access its values, I am new to pointers
> so I have written something like this,[/color]

No. Nononononono!
Please copy and paste the actual code.
"something like this" may differ in important points
from the posted code. This makes it unnecessarily hard
to help you.
BTW: Creating a minimal example helps people see your
problem at a glance...

Your code lacks

#include <stdlib.h>
[color=blue]
> # define COLLATE "Beetle"
> struct mytips
> {
> short inverse;
> long mass;
> char *c;
> char *m;
> };
> int main(){[/color]
make that
int main (void)[color=blue]
> struct mytips store[10] = {
> {0,0x00000001L, COLLATE,"playin g one"},
> {0,0x00000002L, COLLATE,"playin g two"},
> {0,0x00000004L, COLLATE,"playin g three"},
> {0,0x00000008L, COLLATE,"playin g four"},
> {0,0x00000008L, COLLATE,"playin g five"},[/color]
You probably wanted 0x00000010L[color=blue]
> {0,0x00000008L, COLLATE,"playin g six"},
> };
> void *dinku;
> dinku=(struct mytips*)malloc( (sizeof(char)*s izeof(store))+1 );[/color]

Wrong idiom.
Usually, you do
struct mytips *dinku;
dinku = malloc(NumberOf Mytips * sizeof *dinku);

Apart from that: sizeof(char) == 1, always.
I do not understand in the least what you are trying to achieve
with the above.
You forgot:
if (dinku == NULL) {
/* error handling */
}
.... /* use dinku */
free(dinku);

and finally
return 0;[color=blue]
> }
> Now how can I access the values of store using void pointer? Can
> somebody explaing
> how to do it[/color]

No. Yes.
If you passed a pointer to an object as a void *, access
it as the right type:
int ireceivedinkus( void *pData)
{
struct mytips *pDinku = pData;
/* access data via pDinku */
....
}

You can convert between void* and other pointer types without
explicit cast.
Best, you just read the comp.lang.c FAQ (google will point you
there). There is enough background about pointers.

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Kleuskes & Moos]

Re: Pointer problem

On Thu, 08 Dec 2005 23:25:38 -0800, seema wrote:
[color=blue]
> I have a struct and I want to access its values, I am new to pointers
> so I have written something like this,
> # define COLLATE "Beetle"
> struct mytips
> {
> short inverse;
> long mass;
> char *c;
> char *m;
> };
> int main(){
> struct mytips store[10] = {
> {0,0x00000001L, COLLATE,"playin g one"},
> {0,0x00000002L, COLLATE,"playin g two"},
> {0,0x00000004L, COLLATE,"playin g three"},
> {0,0x00000008L, COLLATE,"playin g four"},
> {0,0x00000008L, COLLATE,"playin g five"},
> {0,0x00000008L, COLLATE,"playin g six"},
> };
> void *dinku;
> dinku=(struct mytips*)malloc( (sizeof(char)*s izeof(store))+1 );[/color]

Casting after malloc is unneccesary. Include <stdlib.h> instead.
[color=blue]
> Now how can I access the values of store using void pointer?[/color]

You must cast the void pointer, otherwise there's no way to access any of
the fields, since the compiler does not know what is actually referenced.

[Richard Heathfield]

Re: Pointer problem

Kleuskes & Moos said:
[color=blue]
> On Thu, 08 Dec 2005 23:25:38 -0800, seema wrote:
>[color=green]
>> I have a struct and I want to access its values, I am new to pointers
>> so I have written something like this,
>> # define COLLATE "Beetle"
>> struct mytips
>> {
>> short inverse;
>> long mass;
>> char *c;
>> char *m;
>> };
>> int main(){
>> struct mytips store[10] = {
>> {0,0x00000001L, COLLATE,"playin g one"},
>> {0,0x00000002L, COLLATE,"playin g two"},
>> {0,0x00000004L, COLLATE,"playin g three"},
>> {0,0x00000008L, COLLATE,"playin g four"},
>> {0,0x00000008L, COLLATE,"playin g five"},
>> {0,0x00000008L, COLLATE,"playin g six"},
>> };
>> void *dinku;
>> dinku=(struct mytips*)malloc( (sizeof(char)*s izeof(store))+1 );[/color]
>
> Casting after malloc is unneccesary. Include <stdlib.h> instead.
>[color=green]
>> Now how can I access the values of store using void pointer?[/color]
>
> You must cast the void pointer, otherwise there's no way to access any of
> the fields, since the compiler does not know what is actually referenced.[/color]

{
struct mytips *p = dinku; /* cast? what cast? */
}

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at above domain (but drop the www, obviously)

[Kleuskes & Moos]

Re: Pointer problem

On Fri, 09 Dec 2005 17:04:13 +0000, Richard Heathfield wrote:

<snip>[color=blue][color=green][color=darkred]
>>> Now how can I access the values of store using void pointer?[/color]
>>
>> You must cast the void pointer, otherwise there's no way to access any of
>> the fields, since the compiler does not know what is actually referenced.[/color]
>
> {
> struct mytips *p = dinku; /* cast? what cast? */
> }[/color]

Me and my big mouth. :-).

This is better, of course. Use 'void *' only if you have no choice.

[Richard Heathfield]

Re: Pointer problem

Kleuskes & Moos said:
[color=blue]
> On Fri, 09 Dec 2005 17:04:13 +0000, Richard Heathfield wrote:
>[color=green]
>> {
>> struct mytips *p = dinku; /* cast? what cast? */
>> }[/color]
>
> Me and my big mouth. :-).
>
> This is better, of course. Use 'void *' only if you have no choice.[/color]

....or rather, use it when it's the Right Thing. Sometimes void * is not the
*only* choice, but is nevertheless the most natural choice that leads to
the most elegant code. The only real problem with it is that, by its very
nature, it sidesteps type-checking. That, of course, is also its great
asset.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at above domain (but drop the www, obviously)