C251: Pointer Arithmetic Delivers Unexpected Results

Information in this article applies to:

• C251 Version 2, or higher

QUESTION

I have found a serious problem when I use pointer arithmetic with a generic memory allocation function. The results generated seem to consider only the 16-bit offset rather than the full address space of the 251 architecture (which is up to 16MB).

Why does that happen?

Example:

typedef unsigned char BYTE;
typedef unsigned long DWORD;

BYTE  *MemStart;                // first available memory location
BYTE  *GetLastAddress (void);   // returns last available memory location
DWORD freeSpace;

// this generates a incorrect result:
  freeSpace=(DWORD) GetLastAddress() - MemStart);   // generates incorrect result

ANSWER

A pointer subtraction delivers the number of elements within an array that is addressed by these pointers. By default the C251 uses far pointers. This implies that the pointers point to the same elements within a single 64KB segment.

To get the total memory that is between unrelated pointers (that point to any memory location) use an explicit cast to huge * as shown below:

  freeSpace=(DWORD)((BYTE huge *)GetLastAddress() - (BYTE huge *)MemStart);

Of course, a cast to DWORD will work too and is maybe more transparent to understand:

  freeSpace= (DWORD)GetLastAddress() - (DWORD)MemStart;

MORE INFORMATION

• You may wish to refer to a good General Programming Book like The C Programming Language by Kernigan & Ritchie.

SEE ALSO

• GENERAL: #DEFINEs With Arithmetic Don't Work
• GENERAL: Multiplying Two Ints Gives Incorrect Result
• GENERAL: Logical Not ('~') Gives Incorrect Results
• C166: Multiplication Bug
• ARM: Wrong Result With Binary NOT and UNSIGNED CHAR
• GENERAL: #DEFINE Generates Wrong Results

Last Reviewed: Thursday, February 25, 2021