Assuming you need to convert your code to 64-bit, the following checklist might be helpful:
Read this entire document with an emphasis on the Guidelines for Converting to LP64.
Review all data structures and interfaces to verify that these are still valid in the 64-bit environment.
Include <sys/types.h> in your code to pull in the _ILP32 or _LP64 definitions as well as many basic derived types.
Move function prototypes and external declarations with non-local scope to headers and include these headers in your code.
Run lint(1) using the -errchk=longptr64 and review each warning individually, being aware that not all warnings require a change to the code. Depending on the resulting changes, you might also want to run lint(1) again, both in 32–bit and 64–bit modes.
Compile code as both 32-bit and 64-bit, unless the application is being provided only as 64-bit.
Test the application by executing the 32-bit version on the 32-bit operating system, and the 64-bit version on the 64-bit operating system. You could include testing the 32-bit version on the 64-bit operating system, but this is not necessary.