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Monday, December 22, 2014

Little Endian and Big Endian

Little and big endian are two ways of storing multibyte data-types ( int, float, etc). In little endian machines, last byte of binary representation of the multibyte data-type is stored first. On the other hand, in big endian machines, first byte of binary representation of the multibyte data-type is stored first.

Suppose integer is stored as 4 bytes (For those who are using DOS based compilers such as C++ 3.0 , integer is 2 bytes) then a variable x with value 0×01234567 will be stored as following.
Memory representation of integer ox01234567 inside Big and little endian machines


How to see memory representation of multibyte data types on your machine?

Here is a sample C code that shows the byte representation of int, float and pointer.
#include <stdio.h>
/* function to show bytes in memory, from location start to start+n*/
void show_mem_rep(char *start, int n)
{
    int i;
    for (i = 0; i < n; i++)
         printf(" %.2x", start[i]);
    printf("\n");
}
/*Main function to call above function for 0x01234567*/
int main()
{
   int i = 0x01234567;
   show_mem_rep((char *)&i, sizeof(i));
   getchar();
   return 0;
}
When above program is run on little endian machine, gives “67 45 23 01″ as output , while if it is run on endian machine, gives “01 23 45 67″ as output.


Is there a quick way to determine endianness of your machine?
There are n no. of ways for determining endianness of your machine. Here is one quick way of doing the same.
#include <stdio.h>
int main()
{
   unsigned int i = 1;
   char *c = (char*)&i;
   if (*c)   
       printf("Little endian");
   else
       printf("Big endian");
   getchar();
   return 0;
}
In the above program, a character pointer c is pointing to an integer i. Since size of character is 1 byte when the character pointer is de-referenced it will contain only first byte of integer. If machine is little endian then *c will be 1 (because last byte is stored first) and if machine is big endian then *c will be 0.


#include <stdint.h>


//! Byte swap unsigned short
uint16_t swap_uint16( uint16_t val ) 
{
    return (val << 8) | (val >> 8 );
}

//! Byte swap short
int16_t swap_int16( int16_t val ) 
{
    return (val << 8) | ((val >> 8) & 0xFF);
}

//! Byte swap unsigned int
uint32_t swap_uint32( uint32_t val )
{
    val = ((val << 8) & 0xFF00FF00 ) | ((val >> 8) & 0xFF00FF ); 
    return (val << 16) | (val >> 16);
}

//! Byte swap int
int32_t swap_int32( int32_t val )
{
    val = ((val << 8) & 0xFF00FF00) | ((val >> 8) & 0xFF00FF ); 
    return (val << 16) | ((val >> 16) & 0xFFFF);
}
int64_t swap_int64( int64_t val )
{
    val = ((val << 8) & 0xFF00FF00FF00FF00ULL ) | ((val >> 8) & 0x00FF00FF00FF00FFULL );
    val = ((val << 16) & 0xFFFF0000FFFF0000ULL ) | ((val >> 16) & 0x0000FFFF0000FFFFULL );
    return (val << 32) | ((val >> 32) & 0xFFFFFFFFULL);
}

uint64_t swap_uint64( uint64_t val )
{
    val = ((val << 8) & 0xFF00FF00FF00FF00ULL ) | ((val >> 8) & 0x00FF00FF00FF00FFULL );
    val = ((val << 16) & 0xFFFF0000FFFF0000ULL ) | ((val >> 16) & 0x0000FFFF0000FFFFULL );
    return (val << 32) | (val >> 32);
}