LLP64

LLP64 is a data model used by 64-bit Windows environments to define the sizes of basic C/C++ data types. The acronym stands for Long, Long, and Pointer 64-bit, reflecting that in this model the long type is 32 bits, while both long long and pointers are 64 bits. This arrangement is used by Windows 64-bit (Win64) compilers such as the Microsoft Visual C++ toolchain.

Under LLP64, the typical sizes are: int is 32 bits, long is 32 bits, long long is 64 bits, and pointers are 64 bits. The same pattern keeps the size of sizes and pointers large for memory addressing: size_t, intptr_t, and uintptr_t are 64 bits wide. In practice, a 64-bit Windows program using LLP64 has 8-byte pointers and 8-byte 64-bit integers for long long, while 4-byte long remains common for compatibility with 32-bit Windows code.

LLP64 contrasts with LP64, the data model used by most Unix-like systems such as Linux and macOS, where long and pointers are both 64 bits (and long long is also 64 bits). It also contrasts with ILP32, where int, long, and pointers are 32 bits (typical of 32-bit systems). The Windows LLP64 model affects API contracts and cross-platform porting, as code that assumes long is 64 bits would fail on Windows, and portable code often needs conditional typedefs or explicit width types.

In practice, LLP64 influences type definitions in Windows headers and ABIs, and is a key consideration for developers writing portable C/C++ code across Windows and non-Windows platforms.