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Littleendian and Bigendian byte order illustrated

Due to extremely high loads on the server because of the popularity of these two pictures, they have been moved to a page of their own.

A pedagogically correct picture describing little endian byte ordering drawn by me:

0 is the least significant bit, as any reasonable person would think.

The followup to the successful Little Endian drawing: Big Endian

When do I have to care about endianess?

If you program C/C++ and you don't do any special tricks, then you almost never have to care about the endianess of the target processor. All the arithmetic funtions work the same, e.g 1 << 16 will yield the same results on a 32-bit number on a PowerPC and an Intel processor, C would be useless otherwise.

But you will have to think about the endianess when you deal directly with the memory where 32-bit and 16-bit numbers are stored. E.g. this program:

#include<inttypes.h>
#include<stdio.h>


int main(int argc, char** argv )
{
   uint32_t nbr32 = 0x12345678;
   uint16_t* p16 = (uint16_t*)&nbr32;
   printf("%x\n", *p16 );
}

A more realistic example would be writing binary data to a file. If you have an uint32_t you would like to write to a file, it could be tempting to write

size_t write_uint32_to_file( FILE* file, uint32_t nbr )
{
   return fwrite( file, &nbr, 4, 1 );
}

size_t read_uint32_from_file( FILE* file, uint32_t* nbr )
{
   return fread( file, nbr, 4, 1 );
}

#include <sys/types.h>
#include <netinet/in.h>
#include <inttypes.h>

size_t write_uint32_to_file( FILE* file, uint32_t nbr )
{
   nbr = htonl( nbr ); // Host to network long
   return fwrite( file, &nbr, 4, 1 ); 
}

size_t read_uint32_from_file( FILE* file, uint32_t* nbr )
{
   size_t res = fread( file, nbr, 4, 1 );
   *nbr = ntohl( *nbr ); // Network to host long
   return res;
}