Question

If for example I have a ptr to a string and move ptr to last character in string and iterate backwards to beginning of string using *p– and I iterate to position one before start of array is this OK? Or will I get an access violation? I am only moving pointer – not accessing. It seems to work in my code so wondering if it is bad practice or not?

Here is a sample – line with *next– = rem + ‘A’; is one I am questioning if ok???

#include <stdio.h>     /* printf */
#include <string.h>    /* strlen, strcpy */
#include <stdlib.h>    /* malloc/free */
#include <math.h>      /* pow */

/* AAAAA (or whatever length) = 0, to ZZZZZ.  base 26 numbering system */
static void getNextString(const char* prev, char* next) {
   int count = 0;
   char tmpch = 0;

   int length = strlen(prev);
   int i = 0;
   while((tmpch = *prev++) != 0) {
      count += (tmpch - 'A') * (int)pow(26.0, length - i - 1);
      ++i;
   }

   /* assume all strings are uppercase eg AAAAA */
   ++count;

   /*if count above ZZZ... then reset to AAA... */
   if( count >= (int)pow(26.0, length))
      count = 0;

   next += (length-1);  /* seek to last char in string */
   while(i-- > 0) {
      int rem = count % 26;
      count /= 26;
      *next-- = rem + 'A';   /*pntr positioned on 1 before array on last iteration - is OK? */
   }
}

int main(int argc, char* argv[])
{
   int buffsize = 5;
   char* buff = (char*)malloc(buffsize+1);

   strcpy(buff, "AAAAA");
   int iterations = 100;

   while(--iterations){
      getNextString(buff, buff);
      printf("iteration: %d buffer: %s\n", iterations, buff);
   }

   free(buff);
    return 0;
}

Answer 1

According to the following C-FAQ question\answer, and I quote:

Pointer arithmetic is defined only as long as the pointer points
within the same allocated block of memory, or to the imaginary
“terminating” element one past it; otherwise, the behavior is
undefined, even if the pointer is not dereferenced.

So my answer would be no, it is not OK to iterate before the beginning of an array.

There are references to the C standards as well:

• K&R2 Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7 pp.
  205-6
• ISO Sec. 6.3.6 (C89) or 6.5.6/8 (C99)
• Rationale Sec. 3.2.2.3