use of uninitialized value of size 4.

Hi, I keep getting this error message for my code even though I initialized it already.

Code:

---

= Use of uninitialised value of size 4
=    at 0x80484D9: irest_destroy (ilist_destructive.c:55)
=    by 0x804865B: iprint (ilist_destructive.c:115)
=    by 0x80486F7: main (ilist_destructive.c:128)
=  Uninitialised value was created by a stack allocation
=    at 0x8048679: main (ilist_destructive.c:122)
= Use of uninitialised value of size 4
=    at 0x80484F4: irest_destroy (ilist_destructive.c:58)
=    by 0x804865B: iprint (ilist_destructive.c:115)
=    by 0x80486F7: main (ilist_destructive.c:128)
=  Uninitialised value was created by a stack allocation
=    at 0x8048679: main (ilist_destructive.c:122)
= Invalid read of size 4
=    at 0x80484D9: irest_destroy (ilist_destructive.c:55)
=    by 0x804865B: iprint (ilist_destructive.c:115)
=    by 0x80486F7: main (ilist_destructive.c:128)
=  Address 0x4 is not stack'd, malloc'd or (recently) free'd
= Process terminating with default action of signal 11 (SIGSEGV)
=  Access not within mapped region at address 0x4
=    at 0x80484D9: irest_destroy (ilist_destructive.c:55)
=    by 0x804865B: iprint (ilist_destructive.c:115)
=    by 0x80486F7: main (ilist_destructive.c:128)
=  If you believe this happened as a result of a stack
=  overflow in your program's main thread (unlikely but
=  possible), you can try to increase the size of the
=  main thread stack using the --main-stacksize= flag.
=  The main thread stack size used in this run was 16777216.

---

why is it doing that???!?

following is my code. this code should start at line 46.

Code:

---

ilist irest_destroy(ilist il){
  // modifies il to remove the first element, and returns the modified ilist
  // frees the memory associated with the first element
  // references to il cease to be valid ilists
  // the result (if non-empty) must eventually be consumed by one of:
  //    icons_destroy, irest_destroy, idelete
  if (il == NULL) return NULL;
  ilist r = il->rest;
  ilist head = malloc(sizeof(struct ilist_ADT));
  head->first = r->first;
  head->rest = NULL;
  ilist l_end = head;
  for (ilist a = r->rest; a != NULL; a=a->rest){
  ilist tmp = malloc(sizeof(struct ilist_ADT));
  tmp->first = a->first;
  tmp->rest=NULL;
  l_end->rest = tmp;
  l_end=tmp;
  }
  free(il);
  return head;
}

---

this is the structure definition used in the above code:

Code:

---

struct ilist_ADT{
  struct ilist_ADT *rest;
  int first; 
};

typedef ilist_ADT *ilist;

---

It might help if you showed the exact error message and pointed out the line that the line number refers to.

please refresh. the position of my code should start at line 46.

Code:

---

struct ilist_ADT{
  struct ilist_ADT *rest;
  int first; 
};

typedef ilist_ADT *ilist;

---

Is this a typo? The last line should be

Code:

---

typedef struct ilist_ADT *ilist;

---

Code:

---

  ilist r = il->rest;
  ilist head = malloc(sizeof(struct ilist_ADT));
  head->first = r->first;

---

Have you run the code through a debugger, so you can tell when the segfault comes up? Could it be that there is only one element left in "il" and "il->rest" points to NULL?

BTW if you want to remove the first element of the list (as your comments suggest), you don't need 5 pointers and 15 lines.

Bye, Andreas

> = Address 0x4 is not stack'd, malloc'd or (recently) free'd
Yes, your pointer is NULL, and the member offset inside your struct is 4 bytes.

Which looking at your struct, corresponds exactly to "int first"

> = Invalid read of size 4
> = at 0x80484D9: irest_destroy (ilist_destructive.c:55)
> = by 0x804865B: iprint (ilist_destructive.c:115)
> = by 0x80486F7: main (ilist_destructive.c:128)
Start with line 55 of ilist_destructive.c