"Trick" to use union to verify whether the machine is big or small end

Union u{Short
     A;
     char c;
};
u u;

In the book C programming Language is described in this way:

1) A consortium is a structure;

2 The offset of all its members relative to the base address is 0;

3 This structure space to be large enough to accommodate the most "wide" members;

4 its alignment should be suitable for all the members;

So a federation in memory stored in the form of the following figure:

From the diagram, it's easy to see that the address of variable A, C, and U is 0x20000000

u.a=0x1234;

The small-end mode (the high byte of the data is stored in a high memory address) is stored as follows:

In big-endian mode (the high byte of the data is kept in the low memory address), the storage is as follows:

So, as long as we take out the value of variable C, to compare, if its value is 34h, then the machine is a small-end mode, if its value is 12h, then the machine is big-endian mode.

The implementation code is as follows:

#include <stdio.h>

Union u{
    int A;
    char c;
} u;

int main ()
{
    u.a=0x1234;
    if (0x34 = = u.c)
    {
        printf ("The Machine is little-end.\n");
    }
    else if (0x12 = = u.c)
    {
        printf ("The Machine is big-end.\n");
    }
    return 0;
}