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    __STDC_ENDIAN_LITTLE__, __STDC_ENDIAN_BIG__, __STDC_ENDIAN_NATIVE__

    From cppreference.com
     
    C
     
    Numerics
     
    Bit manipulation
     
    Defined in header <stdbit.h> 
    #define __STDC_ENDIAN_LITTLE__ /* implementation-defined */
    		(1) (since C23) 
    #define __STDC_ENDIAN_BIG__    /* implementation-defined */
    		(2) (since C23) 
    #define __STDC_ENDIAN_NATIVE__ /* implementation-defined */
    		(3) (since C23)

    Indicates the endianness of all scalar types:

    • If all scalar types are little-endian, __STDC_ENDIAN_NATIVE__ equals __STDC_ENDIAN_LITTLE__.
    • If all scalar types are big-endian, __STDC_ENDIAN_NATIVE__ equals __STDC_ENDIAN_BIG__.
    • If the platform uses neither little-endian nor big-endian, __STDC_ENDIAN_NATIVE__ equals neither __STDC_ENDIAN_BIG__ nor __STDC_ENDIAN_LITTLE__.
    • The values of the integer constant expressions for __STDC_ENDIAN_BIG__ and __STDC_ENDIAN_LITTLE__ are not equal.

    Example

    Run this code
    #include <stdbit.h>
#include <stdio.h>

int main()
{
    switch(__STDC_ENDIAN_NATIVE__)
    {
        case __STDC_ENDIAN_LITTLE__:
            printf("__STDC_ENDIAN_LITTLE__\n");
            break;
        case __STDC_ENDIAN_BIG__:
            printf("__STDC_ENDIAN_BIG__\n");
            break;
        default:
            printf("mixed-endian\n");
    }
    return __STDC_ENDIAN_NATIVE__;
}

    Possible output: 

    mixed-endian

    See also

    C++ documentation for endian