AIX

i read that an application can remain as a 32-bit applicarion and still run on a 64-bit operating system without any code changes.

But some documents also suggest that during porting 32 bit applications need to have code changes especially pointers and int and long and that they should be replaced with derived data types so that it enables them to compile on both 32-bit and 64-bit mode.

can i have more clarifications on this as this seems to be contradicting and confusing?

This will be far from a complete answer as there are many subtleties but I can at least cite my own experiences which may not be true for all compilers and architectures. That said, in a nutshell...

In most 32-bit environments, the data type int and all pointers were 4-bytes. On some systems, a long was also 4-bytes with a long long being 8-bytes. In most 64-bit environments, the int remains 4-bytes, the long is 8-bytes (as is the long long defined only for backward compatibility), and the pointer data type increased to 8-bytes.

Problems arise in code that ASSUME the size of each data type specifically that an int and a pointer are exactly the same length. As stated, in 32-bit environments, that is a safe bet however when moving to 64-bit compilers that is not necessarily true. As a result, programs that rely on a specific size of, say, a structure containing integers used as pointers are now in error as they have different memory allocations.

Compilers come with derived data types to be used when working with pointers and integers of varying size but most people -- myself included -- use the standard C datatypes. Those work fine, of course, but require extra caution especially if working with unions, bit-mapped data types, implicit datatype conversion, and such. Often inconsistencies can be overcome with explicit casting.

I hope my explanation has helped.

Hi...

I have to move my 32 bit application code on 64 bit AIX environment. Will I have to make changes in code for the datatypes? Please suggest.

Thanks.
Megha