Question about Malloc

Question about Malloc

[Edward Parrilla]

Hi all,
I got the following structure:
typedef struct  {
 	char *file;
	char *ip_addres;
	}TABLE;

then define a pointer to it:

typedef TABLE *Recpointer;
Recpointer r;


I'm trying to use malloc as:

r=malloc(Recpointer);
but it gives me error.

What should be the right sintax?

Any help would be appreciated.
Thanks in advance

Ed

Re: Question about Malloc

[Jeff Woods]

At 10/10/2004 12:31 AM -0500, Edward Parrilla wrote:
>I got the following structure:
>typedef struct  {
>         char *file;
>         char *ip_addres;
>         }TABLE;
>
>then define a pointer to it:
>
>typedef TABLE *Recpointer;
>Recpointer r;
>
>I'm trying to use malloc as:
>
>r=malloc(Recpointer);
>but it gives me error.
>
>What should be the right sintax?

The word in "syntax".  (A "sin tax" would be too expensive.)

"man malloc" says (in part):

>>         void *calloc(size_t nmemb, size_t size);
>>         void *malloc(size_t size);
>>
>>         calloc()  allocates memory for an array of nmemb elements of 
>> size bytes each and returns a pointer to the allocated memory.  The 
>> memory is  set to zero.
>>
>>         malloc()  allocates  'size'  bytes and returns a pointer to the 
>> allocated memory.  The memory is not cleared.

If you want to allocate exactly one TABLE struct, it should be something 
more like:

         r=malloc(sizeof(TABLE));
         assert(NULL != r);

If you want to allocate an array of TABLE structs (which the name implies), 
use something like:

         #define TABLE_MAX       100
         r = calloc(TABLE_MAX, sizeof(TABLE));
         assert(NULL != r);

>Any help would be appreciated.

RTFM:  Read the manpage.  Or, "Use the manpage, Luke!"  It wouldn't hurt to 
read K&R 2nd edition either.

--
Jeff Woods <kazrak+kernel@cesmail.net> 

Re: Question about Malloc

[Jan-Benedict Glaw]

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On Sun, 2004-10-10 00:31:05 -0500, Edward Parrilla <eparrilla@comcast.net>
wrote in message <1097386256.6100.16.camel@localhost.localdomain>:
> Hi all,
> I got the following structure:
> typedef struct  {
>  	char *file;
> 	char *ip_addres;
> 	}TABLE;
> 
> then define a pointer to it:
> 
> typedef TABLE *Recpointer;
> Recpointer r;
> 
> I'm trying to use malloc as:
> 
> r=malloc(Recpointer);
> but it gives me error.

...which is correct. malloc() allocates memory, byte-wise, not complex
types. So you tell it how many bytes you want and it tries to do exactly
that. It's not C++! Additionally, your newly created Recpointer type is
a pointer, so if you'd do exactly the above correct, you'd not need to
malloc sizeof(Recpointer), but sizeof(TABLE).

But do you see the general mistake here? You typedef'ed your structures
twice, which completely hides you all the details. _This_ is what leads
you to wrong programming in just this case.

If you'd done it like this

struct table_entry {
	char *file;
	char *ip_address;
};
...
struct table_entry *entry;
entry = malloc (sizeof (struct table_entry));

you'd easily see the flow of types here. It doesn't hide the defails
from your eyes, thus gives you the chance to see what's happening. After
a number of typedefs, you usually don't know any more what is what...

MfG, JBG

-- 
Jan-Benedict Glaw       jbglaw@lug-owl.de    . +49-172-7608481             _ O _
"Eine Freie Meinung in  einem Freien Kopf    | Gegen Zensur | Gegen Krieg  _ _ O
 fuer einen Freien Staat voll Freier Bürger" | im Internet! |   im Irak!   O O O
ret = do_actions((curr | FREE_SPEECH) & ~(NEW_COPYRIGHT_LAW | DRM | TCPA));

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Re: Question about Malloc

[Matías Aguirre]

It's not safer to make:

Recpointer r;
r=(Recpointer)malloc(Recpointer);


On Sun, 10 Oct 2004 13:13:16 +0200, Jan-Benedict Glaw <jbglaw@lug-owl.de> wrote:
> On Sun, 2004-10-10 00:31:05 -0500, Edward Parrilla <eparrilla@comcast.net>
> wrote in message <1097386256.6100.16.camel@localhost.localdomain>:
> > Hi all,
> > I got the following structure:
> > typedef struct  {
> >       char *file;
> >       char *ip_addres;
> >       }TABLE;
> >
> > then define a pointer to it:
> >
> > typedef TABLE *Recpointer;
> > Recpointer r;
> >
> > I'm trying to use malloc as:
> >
> > r=malloc(Recpointer);
> > but it gives me error.
> 



-- 
Matías Aguirre
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Re: Question about Malloc

[Jan-Benedict Glaw]

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On Tue, 2004-10-19 09:41:49 -0300, Matías Aguirre <matiasaguirre@gmail.com>
wrote in message <a3c4234104101905413e127d3e@mail.gmail.com>:
> It's not safer to make:
> 
> Recpointer r;
> r=(Recpointer)malloc(Recpointer);

malloc is void, while r isn't. So you don't need the cast here. However,
notice that your malloc call is actually wrong: the argument isn't a
number (or sizeof(something)), but only a type.

General rule of thumb:

	The "output" of malloc should be assigned to a variable whose
	type is a pointer to whatever you give to sizeof(xxx) as the
	malloc argument.

or in short:

	One star more on the left than on the right.

Of course, if you play with types, this doesn't apply any longer...

MfG, JBG

-- 
Jan-Benedict Glaw       jbglaw@lug-owl.de    . +49-172-7608481             _ O _
"Eine Freie Meinung in  einem Freien Kopf    | Gegen Zensur | Gegen Krieg  _ _ O
 fuer einen Freien Staat voll Freier Bürger" | im Internet! |   im Irak!   O O O
ret = do_actions((curr | FREE_SPEECH) & ~(NEW_COPYRIGHT_LAW | DRM | TCPA));

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Re: Question about Malloc

[Matías Aguirre]

i agree with that, i didn't fix inside de malloc, i only suggested
about the cast

but my big error was speak about c++ and not about c, in c++ the
compiler (g++ in this case) warn you about that conversion....

sorry

On Tue, 19 Oct 2004 15:27:38 +0200, Jan-Benedict Glaw <jbglaw@lug-owl.de> wrote:
> On Tue, 2004-10-19 09:41:49 -0300, Matías Aguirre <matiasaguirre@gmail.com>
> wrote in message <a3c4234104101905413e127d3e@mail.gmail.com>:
> > It's not safer to make:
> >
> > Recpointer r;
> > r=(Recpointer)malloc(Recpointer);
> 
> malloc is void, while r isn't. So you don't need the cast here. However,
> notice that your malloc call is actually wrong: the argument isn't a
> number (or sizeof(something)), but only a type.
> 
> General rule of thumb:
> 
>         The "output" of malloc should be assigned to a variable whose
>         type is a pointer to whatever you give to sizeof(xxx) as the
>         malloc argument.
> 
> or in short:
> 
>         One star more on the left than on the right.
> 
> Of course, if you play with types, this doesn't apply any longer...
> 
> 
> 
> MfG, JBG
> 
> --
> Jan-Benedict Glaw       jbglaw@lug-owl.de    . +49-172-7608481             _ O _
> "Eine Freie Meinung in  einem Freien Kopf    | Gegen Zensur | Gegen Krieg  _ _ O
>  fuer einen Freien Staat voll Freier Bürger" | im Internet! |   im Irak!   O O O
> ret = do_actions((curr | FREE_SPEECH) & ~(NEW_COPYRIGHT_LAW | DRM | TCPA));
> 
> 
> 


-- 
Matías Aguirre
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