Target 32- and 64-bit Platforms Together with a Few Simple Datatype Changes : Page 2

• Large file support: a large file is one that contains 2GB of data or more.
• 64-bit addressing: a 64-bit process has a virtual address space of 16 exabytes; that's about four billion times larger than the current upper limit of 4GB.
• Breaking the 4GB RAM barrier: on 64-bit systems, the theoretical upper limit of physical RAM is 16 exabytes.

To benefit from these advantages (and others) you must recompile and relink the original source files using a 64-bit compiler and 64-bit libraries, respectively. The good news is that existing 32-bit executables will still run on a 64-bit kernel. The converse isn't true; 64-bit binaries will only run on a 64-bit kernel.

ILP32 Versus LP64
The problems associated with writing 64-bit compliant programs stem from the different datatype models of 32-bit and 64-bit systems. Under the 32-bit datatype model known as ILP32, int, long, and pointers all have the same size of 32-bits. Under the 64-bit datatype model which is called LP64, int still occupies 32-bits, whereas long and pointers occupy 64-bits. The following table summarizes the differences between the two models with respect to the number of bits in each fundamental type:

Platform-independent Datatype Widths
When writing dually-targeted code, it is important to clearly define which objects should have a fixed width, regardless of the target architecture. Fixed-width datatypes may be required in the following cases:

• Reading data from a network connection
• On-disk data that was written by a 32-bit application
• Interfacing with certain binary standards

int8_t   int16_t   int32_t   int64_t //signed 

uint8_t   uint16_t   uint32_t   uint64_t //unsigned