Re: Question about Malloc

[Jan-Benedict Glaw <jbglaw@lug-owl.de>]

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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

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