Re: Antwort: Re: -EFAULT during freeing a pointer to a structure

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

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On Fri, 2004-10-08 11:03:55 +0200, p.boehm@d-trust.net <p.boehm@d-trust.net>
wrote in message <OFFFC2B2B9.78606041-ONC1256F27.002EEFD1@bln.d-trust.de>:
> (it's written from mind, hope there are no spelling mistakes)

There's one, but not that bad...

> # --------------------------------------------------------- #
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> 
> #define MAXNUM 7
> 
> struct xy {
>      int x;
>      char *y;
> };
> 
> struct abc {
>      struct xy *next[MAXNUM];

So here you're creating an array of pointers to "struct xy". This array
has got MAXNUM elements, this is currently 7. Note that the correct
array subscripts are from 0 to 6!

> };
> 
> void init_abc(struct abc *pt, int index) {
>      pt->next[index]=malloc(sizeof(struct xy));
>      memset(pt->next[index],0,sizeof(struct xy));
> }
> 
> void free_abc(struct abc *pt, int index) {
>      free(pt->next[index]);
>      pt->next[index]=NULL;
> }
> 
> int main(int argc, char *argv[]) {
>      struct abc *ptr;
>      int index=-1;
> 
>      ptr=malloc(sizeof(struct abc));
>      memset(ptr, 0, sizeof((struct abc));

Here's a '(' too much, that's the typo.

> 
>      while(index++<MAXNUM) {
>           init_abc(ptr,index);
>           printf("ptr->next[index] = %p\n", ptr->next[index]);
>      }

...and now, think about this loop. Think hard. Got the point? It's
executed for index=0 to index=7! That's one too much! I suggest you
better write it like

	for (index = 0; index < MAXNUM; index++) {
		init_abc (ptr, index);
		printf ("ptr->next[%d] = %p\n", index, ptr->next[index]);
	}

>      while(index-->0) {
>           free_abc(ptr,index);
>           printf("ptr->next[index] = %p\n", ptr->next[index]);
>      }


And I'd write this as

	for (index = MAXNUM - 1; index >= 0; index--) {
		free_abc (ptr, index);
		printf ("ptr->next[%d] = %p\n", index, ptr->next[index]);
	}

Additionally, I've also made it print out it's index it's actually
working on. If you had done that on the first hand, you'd for sure seen
your error in no time:-)

>      free(ptr);
>      return 0;
> }
> 
> # ---------------------------------------------------------------------- #

MfG, JBG

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