error when allocating memory using a pointer inside a struct via a function

[elmafiacs@yahoo.es]

I need some help with an error I'm getting...
Sorry if this is basic stuff, but I'm still a newbie on C programming
(working up, though).
Also, I'm using gcc as my compiler ('gcc t.c -o t', no strange stuff).
Sorry about the long subject, I didn't know how to explain it better...

struct small {
int num;
};

struct big {
struct small *small_thing;
};

void f(struct small **s)
{
s = malloc(sizeof(s truct small));
memset(s, 0, sizeof(struct small));
s->num = 42;
}

main()
{
struct big *big_thing;

big_thing = malloc(sizeof(s truct big));
memset(&big_thi ng, 0, sizeof(struct big));

f(&big_thing->small_thing) ;
printf("%i\n", big_thing->small_thing->n);
}

But when I do:
s->num = 42;
the compiler fails with error 'request for member 'num' in something
not an structure or union', and I don't know why.

I tried this way:

void f(struct small **s)
{
struct small *s_ptr;
s_ptr->num = 42;
memcpy(s, &s_ptr, sizeof(struct small));
}

I'm not sure, but I think that the second function is unnecessary. So,
how to fix the first function?

Thanks.

[David Resnick]

Re: error when allocating memory using a pointer inside a structvia a function

elmafiacs@yahoo .es wrote:
[color=blue]
> I need some help with an error I'm getting...
> Sorry if this is basic stuff, but I'm still a newbie on C programming
> (working up, though).
> Also, I'm using gcc as my compiler ('gcc t.c -o t', no strange stuff).
> Sorry about the long subject, I didn't know how to explain it better...
>
> struct small {
> int num;
> };
>
> struct big {
> struct small *small_thing;
> };
>
> void f(struct small **s)
> {
> s = malloc(sizeof(s truct small));
> memset(s, 0, sizeof(struct small));
> s->num = 42;
> }
>[/color]

s is a pointer to a pointer to a struct small, not a pointer
to a struct small. Accordingly, you want this:

*s = malloc(sizeof **s);
memset(*s, 0, sizeof **s);
(*s)->num = 42;

And you should check the return value of malloc...

-David



-David

[Martin Ambuhl]

Re: error when allocating memory using a pointer inside a structvia a function

elmafiacs@yahoo .es wrote:[color=blue]
> I need some help with an error I'm getting...
> Sorry if this is basic stuff, but I'm still a newbie on C programming
> (working up, though).
> Also, I'm using gcc as my compiler ('gcc t.c -o t', no strange stuff).
> Sorry about the long subject, I didn't know how to explain it better...
>
> struct small {
> int num;
> };
>
> struct big {
> struct small *small_thing;
> };
>
> void f(struct small **s)
> {
> s = malloc(sizeof(s truct small));
> memset(s, 0, sizeof(struct small));
> s->num = 42;
> }
>
> main()
> {
> struct big *big_thing;
>
> big_thing = malloc(sizeof(s truct big));
> memset(&big_thi ng, 0, sizeof(struct big));
>
> f(&big_thing->small_thing) ;
> printf("%i\n", big_thing->small_thing->n);
> }
>
> But when I do:
> s->num = 42;
> the compiler fails with error 'request for member 'num' in something
> not an structure or union', and I don't know why.
>
> I tried this way:
>
> void f(struct small **s)
> {
> struct small *s_ptr;
> s_ptr->num = 42;
> memcpy(s, &s_ptr, sizeof(struct small));
> }
>
> I'm not sure, but I think that the second function is unnecessary. So,
> how to fix the first function?
>
> Thanks.
>[/color]

Please don't post code that is not that which you have actually
compiled. Here is a start toward fixing your code:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

struct small
{
int num;
};

struct big
{
struct small *small_thing;
};

void f(struct small **s)
{
*s = malloc(sizeof **s);
/* add code to check that *s != 0 */
memset(*s, 0, sizeof **s);
(*s)->num = 42;
}

int main(void)
{
struct big *big_thing;

big_thing = malloc(sizeof *big_thing);
/* add code to check that big_thing != 0 */
memset(&big_thi ng, 0, sizeof big_thing);

f(&big_thing->small_thing) ;
printf("%i\n", big_thing->small_thing->num);
free(big_thing->small_thing) ;
free(big_thing) ;
return 0;
}

[elmafiacs@yahoo.es]

Re: error when allocating memory using a pointer inside a struct via a function

Thanks for the help.

[Al Bowers]

Re: error when allocating memory using a pointer inside a structvia a function

Martin Ambuhl wrote:
[color=blue]
>
> Please don't post code that is not that which you have actually
> compiled. Here is a start toward fixing your code:[/color]

In addition to compiling, run and testing the code before posting can be
helpful
(but not final) in catching additional flaws. For example....
[color=blue]
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
>
> struct small
> {
> int num;
> };
>
> struct big
> {
> struct small *small_thing;
> };
>
> void f(struct small **s)
> {
> *s = malloc(sizeof **s);
> /* add code to check that *s != 0 */
> memset(*s, 0, sizeof **s);
> (*s)->num = 42;
> }
>
> int main(void)
> {
> struct big *big_thing;
>
> big_thing = malloc(sizeof *big_thing);
> /* add code to check that big_thing != 0 */
> memset(&big_thi ng, 0, sizeof big_thing);
>[/color]
Running and testing might identify the memset statement flaw
above. It should be:
memset(big_thin g, 0 , sizeof big_thing);

And, as you suggested, code to catch dynamic memory allocations
with function malloc needs to be added to make execution of the code
safe. See the example below:
[color=blue]
> f(&big_thing->small_thing) ;
> printf("%i\n", big_thing->small_thing->num);
> free(big_thing->small_thing) ;
> free(big_thing) ;
> return 0;
> }
>[/color]
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct small {
int num;
};

struct big {
struct small *small_thing;
};

void f(struct small **s)
{
*s = malloc(sizeof(s truct small));
if(*s)
{
memset(*s, 0, sizeof(struct small));
(*s)->num = 42;
}
}

int main(void)
{
struct big *big_thing;

big_thing = malloc(sizeof(s truct big));
if(big_thing)
{
memset(big_thin g, 0, sizeof(struct big));
f(&big_thing->small_thing) ;
if(big_thing->small_thing)
printf("big_thi ng->small_thing->num = %i\n",
big_thing->small_thing->num);
free(big_thing->small_thing) ;
}
free(big_thing) ;
return 0;
}

Al Bowers

[Old Wolf]

Re: error when allocating memory using a pointer inside a struct via a function

Al Bowers wrote:[color=blue]
> Martin Ambuhl wrote:[color=green]
> >
> > Please don't post code that is not that which you have actually
> > compiled. Here is a start toward fixing your code:[/color]
>[color=green]
> > struct big *big_thing;
> >
> > big_thing = malloc(sizeof *big_thing);
> >
> > memset(&big_thi ng, 0, sizeof big_thing);[/color]
>
> Running and testing might identify the memset statement flaw
> above. It should be:
> memset(big_thin g, 0 , sizeof big_thing);
>[/color]

1) Running and testing isn't a great way of checking for
undefined behaviour

2) A statement: memset(foo, bar, sizeof foo); is wrong, unless
foo and &foo are the same address (ie. for an array).
It should be:

memset(big_thin g, 0, sizeof *big_thing);

The OP (who has been snipped) had it correct.
IMHO, even better would be to use calloc instead of malloc.

[Al Bowers]

Re: error when allocating memory using a pointer inside a structvia a function

Old Wolf wrote:
[color=blue]
>Al Bowers wrote:
>
>[color=green]
>>Martin Ambuhl wrote:
>>
>>[color=darkred]
>>>Please don't post code that is not that which you have actually
>>>compiled. Here is a start toward fixing your code:
>>>
>>>
>>> struct big *big_thing;
>>>
>>> big_thing = malloc(sizeof *big_thing);
>>>
>>> memset(&big_thi ng, 0, sizeof big_thing);
>>>
>>>[/color]
>>Running and testing might identify the memset statement flaw
>>above. It should be:
>>memset(big_th ing, 0 , sizeof big_thing);
>>
>>
>>[/color]
>
>
>2) A statement: memset(foo, bar, sizeof foo); is wrong, unless
>foo and &foo are the same address (ie. for an array).
>It should be:
>
> memset(big_thin g, 0, sizeof *big_thing);
>
>The OP (who has been snipped) had it correct.
>
>
>[/color]
I don't think so. The op code was:
[color=blue]
> struct big *big_thing;[/color]
[color=blue]
> big_thing = malloc(sizeof(s truct big));
> memset(&big_thi ng, 0, sizeof(struct big));[/color]

Is that correct?

I believe it should be:
memset(big_thin g, 0, sizeof *big_thing);
or
memset(big_thin g, 0, sizeof(struct big));

[Old Wolf]

Re: error when allocating memory using a pointer inside a struct via a function

Al Bowers wrote:[color=blue]
> Old Wolf wrote:[color=green]
> >
> >It should be:
> >
> > memset(big_thin g, 0, sizeof *big_thing);
> >
> >The OP (who has been snipped) had it correct.
> >[/color]
> I don't think so. The op code was:
>[color=green]
> > struct big *big_thing;
> > big_thing = malloc(sizeof(s truct big));
> > memset(&big_thi ng, 0, sizeof(struct big));[/color]
>
> Is that correct?
>
> I believe it should be:
> memset(big_thin g, 0, sizeof *big_thing);
> or
> memset(big_thin g, 0, sizeof(struct big));[/color]

You're right. I think we got there in the end :)