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__STDC_ENDIAN_LITTLE__, __STDC_ENDIAN_BIG__, __STDC_ENDIAN_NATIVE__

From cppreference.net
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