#include <stdio.h>

union{
char arr[4];
long data;
} test;

int main() {
char c = 'a';

/* Test platform Endianness */
for(int x = 0; x < 4; x++)
test.arr[x] = c++;
if ( test.data == 0x61626364 )
/* It’s big endian and you do your stuff */
{
printf("Big endian");
} else {
printf("Little endian");
}
}