CMAKE_<LANG>_BYTE_ORDER

Added in version 3.20.

Byte order of <LANG> compiler target architecture, if known. If defined and not empty, the value is one of:

BIG_ENDIAN

The target architecture is Big Endian.

LITTLE_ENDIAN

The target architecture is Little Endian.

This is defined for languages C, CXX, OBJC, OBJCXX, and CUDA.

If CMAKE_OSX_ARCHITECTURES specifies multiple architectures, the value of CMAKE_<LANG>_BYTE_ORDER is non-empty only if all architectures share the same byte order.

Examples

Example: Checking Endianness

Checking endianness (byte order) of the target architecture in a CMake project, where C language is one of the enabled languages, and storing the result in a variable WORDS_BIGENDIAN:

if(CMAKE_C_BYTE_ORDER STREQUAL "BIG_ENDIAN")
  set(WORDS_BIGENDIAN TRUE)
elseif(CMAKE_C_BYTE_ORDER STREQUAL "LITTLE_ENDIAN")
  set(WORDS_BIGENDIAN FALSE)
else()
  set(WORDS_BIGENDIAN FALSE)
  message(WARNING "Endianness could not be determined.")
endif()

Or, if the project doesn't have C language enabled, it can be replaced with some other enabled language. For example, if CXX is enabled:

if(CMAKE_CXX_BYTE_ORDER STREQUAL "BIG_ENDIAN")
  set(WORDS_BIGENDIAN TRUE)
elseif(CMAKE_CXX_BYTE_ORDER STREQUAL "LITTLE_ENDIAN")
  set(WORDS_BIGENDIAN FALSE)
else()
  set(WORDS_BIGENDIAN FALSE)
  message(WARNING "Endianness could not be determined.")
endif()

Note, that in most cases this can be simplified by only checking for a big-endian target:

if(CMAKE_C_BYTE_ORDER STREQUAL "BIG_ENDIAN")
  set(WORDS_BIGENDIAN TRUE)
else()
  set(WORDS_BIGENDIAN FALSE)
endif()

Example: Per-language Endianness Check

Most of the time, architectures used today are consistent in endianness across compilers. But here's when per-language endianness check can matter:

• Cross-compilation to different architectures (e.g., big-endian embedded system).

• Heterogeneous toolchains where one target architecture is for C language and another target is for different language.

• Static libraries or binaries reused across platforms (e.g., distributing precompiled CUDA kernels).

if(CMAKE_C_BYTE_ORDER)
  message(STATUS "C byte order: ${CMAKE_C_BYTE_ORDER}")
endif()

if(CMAKE_CXX_BYTE_ORDER)
  message(STATUS "C++ byte order: ${CMAKE_CXX_BYTE_ORDER}")
endif()

if(CMAKE_CUDA_BYTE_ORDER)
  message(STATUS "CUDA byte order: ${CMAKE_CUDA_BYTE_ORDER}")
endif()