A data model defines the sizes assigned to standard data types. These data models are typically named using a XXXn pattern where X referes to a C type and n refers to a size (typically 32 or 64). ILP64, for example, means that int, long and pointer types are 64 bits wide, LP32 means that long and pointer are 32 bits wide, and LLP64 means that long long and pointer are 64 bits wide.

The following observations are derived from the Development Tutorial by Marco van de Voort \[van de Voort 07\]:

• Standard programming model for current (Intel family) PC processors is ILP32.
• The problem with long in C was that there are both codebases that expect pointer and long to have the same size, while
  there are also large codebases that expect int and long to be the same size. The compability model LLP64 was designed to
  keep the long<->{{int}}compability by introducing a new type to remain compatible with pointer (long long)
  ? long long is only defined for LLP64.
  ? LP32 is used as model for the win-16 APIs of Windows 3.1
  ? Most Unixes use LP64, primarily to conserve memory space compared to ILP64. Examples: 64-bit Linux, FreeBSD, NetBSD, OpenBSD
  ? Win64 uses the LLP64 model (also known as P64). This model conserves type compability between long and int, but looses type compability between long and pointer types. Any cast between a pointer and an existing type requires modification.
  ? ILP64 is the easiest model to work with, since it retains all compability with the much used ILP32 model, except specific assumptions that the core types are 32-bit. However this model requires significant memory, and both code and data size significantly increase.

References

van de Voort 07 Marco van de Voort. Development Tutorial (a.k.a Build FAQ). January 29, 2007. http://www.stack.nl/~marcov/buildfaq.pdf

Open Group 97 Go Solo 2 - The Authorized Guide to Version 2 of the Single UNIX Specification. ISBN 0-13-575689-8. May 1997. http://www.unix.org/whitepapers/64bit.html