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c++c++11endianness

Define macros based on endianness of machine

I want to define variables in structure based on endianness of machine , This is what I tried

#define IS_BIG_ENDIAN (!(union { uint16_t u16; unsigned char c; }){ .u16 = 1 }.c)
//#define IS_BIG_ENDIAN (!*(unsigned char *)&(uint16_t){1})
struct A {
    #if IS_BIG_ENDIAN
    char a:4;
    char b:4;
    #else
    char b:4;
    char a:4;
    #endif
}

The above code is giving error , is this possible to achieve. My Main aim is to achieve reverse order of structure variables.

Solution

  • From https://sourceforge.net/p/predef/wiki/Endianness/

    You don't have standard way to have it.

    but custom headers <sys/param.h> or <endian.h> might provide MACRO such as:

    Type                            Macro           Value
Big endian                      __BYTE_ORDER    __BIG_ENDIAN
Little endian                   __BYTE_ORDER    __LITTLE_ENDIAN
Little endian (word-swapped)    __BYTE_ORDER    __PDP_ENDIAN