Q58987: Pointer Arithmetic Wraps Around Segment Ends

Article: Q58987
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 1-MAR-1990

When you increment or decrement a pointer beyond a segment boundary,
the offset of the pointer will wrap around the end. For example, if
the pointer is sitting at FFFF and you increment it by 1 (one), the
resulting value of the offset is 0000. This wraparound behavior is
expected behavior in all memory models except huge. The example below
demonstrates the "hidden" wraparound.

The C language permits writing beyond array boundaries and heap
allocations. Consequently, C compilers do not generate warning or
error messages if an index or pointer goes out of bounds. It is up to
the programmer to monitor indices and pointers.

If you have an array that is larger than 64K, use the huge keyword or
compile in the huge memory model. Pointer arithmetic for huge data is
performed on the full 32 bits of segment and offset address. For
complete information on huge model programming, refer to Chapter 6 in
the "Microsoft C Optimizing Compiler User's Guide."

Sample Code:

/* wrap.c */
#include <stdio.h>
#include <dos.h>
#include <malloc.h>

void main (void)
{
 char *ptr;

 ptr = (char*) malloc (100);
 printf ("\nSegment is %u, offset is %u\n", FP_SEG(ptr), FP_OFF(ptr));

 FP_OFF(ptr) = 0x0000;
 printf ("\nSegment is %u, offset is %u\n", FP_SEG(ptr), FP_OFF(ptr));
 ptr--;
 printf ("\nSegment is %u, offset is %u\n", FP_SEG(ptr), FP_OFF(ptr));

 FP_OFF(ptr) = 0xFFFF;
 printf ("\nSegment is %u, offset is %u\n", FP_SEG(ptr), FP_OFF(ptr));
 ptr++;
 printf ("\nSegment is %u, offset is %u\n", FP_SEG(ptr), FP_OFF(ptr));
}

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