Wild Pointer

Examples

  #include   #include       int main(void)  {      char *p1 = (char *)malloc(sizeof(char *));// initialized pointer      printf("Address of p1: 0x%x\n",  p1);     // points to some place in the heap      printf("Value of  *p1: 0x%x\n", *p1);     // (undefined) value of some place in the heap         char *p2;                                 // wild (uninitialized) pointer      printf("Address of p2: 0x%x\n",  p2);     // undefined value, may not be a valid address         // if you are LUCKY, this will cause an addressing exception      printf("Value of  *p2: 0x%x\n", *p2);     // random value of random address        //the program probably already crashed before it reaches return        return 0;  } 

  #include   #include       int badIdea(int **p)  // p is a pointer to a pointer to an int  {      int x = 1;           // allocate an int on the stack      **p = x;             // assign value of x to int that pointer p points to      *p = &x;             // make the pointer that p points to point to x      return x;            // after return x is out of scope and undefined  }     int main(void)  {      int y = 0;      int *p1 = &y;                             // initialized pointer to y      int *p2 = NULL;                           // a good habit to form      printf("Address of p1: 0x%x\n",  p1);     // prints address of y      printf("Value of  *p1: 0x%x\n", *p1);     // prints value of y      y = badIdea(&p1);                         // changes y and changes p1                    // p1 now points to where x was      // The place where x was will get clobbered,       // for instance, on the next interrupt, or on      // the next subroutine call, as below....         // some other code that also uses the stack      p2 = (int *)malloc(5*sizeof(int));                   // this probably will NOT crash, but value printed is unpredictable      printf("Value of  *p1: 0x%x\n", *p1);     // prints value of where x was          return 0;  } 

  #include   #include      int main(void)  {      int *p1 = (int *)malloc(sizeof(int));     // initialized pointer to heap      int *p2 = p1;                             // make a copy      *p1 = 0;                                  // initialize the heap      printf("Address of p2: 0x%x\n",  p2);     // points into the heap      printf("Value of  *p2: 0x%x\n", *p2);     // should print zero      free(p1);                                 // deallocate the memory      ....     // other code, possibly using the heap          // p2 still points to the original allocation, but who knows what is there      printf("Value of  *p2: 0x%x\n", *p2);     // invalid use of p2           return 0;  } 

  #include   #include     int main(void)  {      int y = 5;                                // create a variable       int *p1 = &y;                             // initialized pointer to y      printf("Address of p1: 0x%x\n",  p1);     // address of y      printf("Value of  *p1: 0x%x\n", *p1);     // value of y      p1 = p1 + y;                              // allowed pointer arithmetic      printf("Value of  *p1: 0x%x\n", *p1);     // p1 no longer points to y      return 0;  } 

  #include    #include     // copy source to destination, no check on sizes  void strcopy(char *d, char *s)  {      while (*d++ = *s++) // copy until '\0' encountered        ;  }       int main(void)  {      char y3;                                  // create a local buffer      char *p1 = (char *)malloc(10*sizeof(char)); // another buffer on heap      p19 = '\0';                               // terminate the larger buffer      strcopy( y, p1);                            // overflow the local buffer      free(p1);      return 0;                                   // now bad stuff happens  } 

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