Question

I am presently working on converting a 32-bit application into a 64-bit application in C. This application is currently working on x86 architecture (Windows, osx, Unix, Linux). So, before starting coding, I wanted to know what do I need to consider while converting the application.

Answer 1

1. Find out who wrote it. Are they an idiot? Are they you from a few years ago? Can you ask them questions? Are they familiar with the existence of multiple platforms and systems? Knowing the mind-set of the author(s) of the program will help you understand problems when you run into them.
2. Get a 64-bit machine/build environment up and running.
3. Replace long with int. Be perfectly aware that LONG is not long.
4. Replace (int)&x casts and typing with intptr_t and (unsigned int)&x with uintptr_t
5. Audit anything that relies on casting structures to char* to do pointer arithmetic with it.
6. Regex search for \<4\> in case you assumed 4 = sizeof(void*)
7. Be patient. When you find a problem, look elsewhere if the same problem exists, and wrap the solution in a macro.
8. Try not to use #ifdef RUN64 or anything similar. You’ll regret it if 128-bit platforms ever go into vogue.
9. Encapsulate all of your changes in terms of some centralized macros that’ll hide the portability differences elsewhere in your program.
10. Use a coverage tester to help make sure you’ve covered everything (if appropriate)

EDIT added uintptr_t note as suggested by comment.