Little and Big Endian Ordering

Byte Order:

  • Byte Ordering is an attribute of the processor which defines how data are arranged or ordered.
  • Byte Ordering is important when data is transferred across a network or between system using different Ordering.

Network Order:
Network Byte Order refers to how bytes are arranged when sending data over a network.
Host Byte Order:
This refers to how bytes are arranged on a computer system.

Significance of Byte Ordering:

  • A byte is of 8 bits and has a limited range of 256 values.
  • When the value is beyond this range, it has to be stored in multiple bytes.
  • A number such as 258 in hexadecimal format is 0x0102 and required minimum 2 bytes.
  • The order in which these two bytes are stored in memory can be different depending upon the processor.
  • Some processor stores the values in lower memory address followed by higher memory address.

Decimal Values : 258
Hex : 0x0102

  • Lower memory address followed by Higher memory : 0x01 followed by 0x02
  • Higher memory address followed by Lower memory : 0x02 followed by 0x01

Thus if Byte Ordering is not appropriate according to the processor, the 0x0102 can be wrongly interpreted as 0x0201 which is 513 in decimal system.

Little-Endian and Big-Endian:
There is two common Ordering System:

  • Little Endian and
  • Big Endian

Little Endian:

  • In this least-significant(Low-order) byte is stored at a lower address.
  • Example: Intel Processor

Big Endian:

  • In this the most significant(High-order) byte is stored at lower address.
  • Example: IBM PowerPC or Motorola PowerPC Processor

Some Processor support both ordering and therefore Bi-Endian and are configurable. MostRISC architectures(SPARC, MIPS) are configurable now.

Diag-1: Little-endian and Big-endian

Comparing Little-Endian and Big-Endian Ordering
Host and Network Byte Order API:

  • For the correct interpretation of values across the system or a network, the network program must convert their internal representation(Host Byte Order) to Network Byte order(Big-Endian).
  • A host system may be little-endian but when sending data to a network, it must convert data into big-endian.
  • Likewise, a little-endian machine must first convert network data into little-endian before processing.
  • This can be done with a few systems call API defined in netinet/in.h are(for 32 bit long and 16-bit short)
    • unsigned long int htonl ( unsigned long int hostlong);
    • unsigned long int ntohl (unsigned long int netlong);
    • unsigned short int htons (unsigned short int hostshort);
    • unsigned short int ntohs (unsigned short int netshort);
  • Two important parameters in socket programming is Port number(16 bits) and IP address(32 bits), these two must be ordered correctly.
  • For port which is 16 bits, htons() and ntohs() can be used and for IP address of 32 bits htonl() and ntohl() can be used.

Diag-2: htonl-to-ntohl

Illustrating the use of htonl and ntohl function
In the AF_INET domain, the address is specified using a structure called sockaddr_in, defined in netinet/in.h, which contains at least three members.

struct sockaddr_in 
    short int family;                         /*AF_INET */
    unsigned short int sin_port;              /* Port Number */
    struct in_addr sin_addr;                  /* Internet Address */

struct in_addr
   unsigned long int s_addr;

Program to convert the Port and IP Address into Network Order(Big-Endian)

#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>

int main()
    struct sockaddr_in addr;
    unsigned short int port = 1234;
    unsigned long int ip = 2110443574;
    addr.sin_port= (htons (port));
    addr.sin_addr.s_addr = htonl(ip);
    /* Host Order(Little-Endian) */
    printf("\n Port in Host Order(Little-Endian) Hex: %x Decimal: 
           %d\n",port, port);
    printf("\n IP in Host Order(Little-Endian) Hex: %lx Decimal: 
           %lu\n",ip, ip);
    /* Network Order(Big-Endian) */
    printf("\n Port in Network Order(Big-Endian) Hex: %x Decimal: %d 
    printf("\n IP address in Network Order(Big-Endian) Hex: %x Decimal: 
           %d \n",addr.sin_addr.s_addr,addr.sin_addr.s_addr);
    return 0;


Port in Host Order(Little-Endian) Hex: 4d2 Decimal: 1234                                                                      
IP in Host Order(Little-Endian) Hex: 7dcad036 Decimal: 2110443574                                                             
Port in Network Order(Big-Endian) Hex: d204 Decimal: 53764                                                                    
IP address in Network Order(Big-Endian) Hex: 36d0ca7d Decimal:919652989                                                                

Program to check the Endiness of the system:

#include <stdio.h>
int main ()
  unsigned int x = 0x0102;
  char *c = (char*) &x;
  printf ("\n Value at c is: 0x%x\n", *c);
  if (*c == 0x02)
     printf ("Underlying architecture is little endian. \n")
     printf ("Underlying architecture is big endian. \n");
  return 0;

Relevant Posts:

Categories: Operating system (OS)

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