htons function in socket programing

Question

I am new to socket programming and I am  trying to understand the operation of htons    I ve read a few tutorials on the Internet like this and this one for instance  But I couldn t understand what htons   does exactly  I tried the following code    include  lt stdio h gt   include  lt sys types h gt    include  lt sys socket h gt   include  lt netinet in h gt   int main  int argc  char  argv           int sockfd  newsockfd  portno  clilen      char buffer 256       struct sockaddr in serv addr  cli addr      int  n          First call to socket   function        sockfd   socket AF INET  SOCK STREAM  0       if  sockfd  lt  0                 perror  ERROR opening socket            exit 1                Initialize socket structure        bzero  char     amp serv addr  sizeof serv addr        portno   5001      serv addr sin family   AF INET      serv addr sin addr s addr   INADDR ANY      serv addr sin port   htons portno           Now bind the host address using bind   call        if  bind sockfd   struct sockaddr     amp serv addr                            sizeof serv addr    lt  0                 perror  ERROR on binding             exit 1                 Now start listening for the clients  here process will       go in sleep mode and will wait for the incoming connection            listen sockfd 5       clilen   sizeof cli addr           Accept actual connection from the client        newsockfd   accept sockfd   struct sockaddr    amp cli addr                                    amp clilen       if  newsockfd  lt  0                 perror  ERROR on accept            exit 1                If connection is established then start communicating        bzero buffer 256       n   read  newsockfd buffer 255        if  n  lt  0                perror  ERROR reading from socket            exit 1             printf  Here is the message   s n  buffer           Write a response to the client        n   write newsockfd  I got your message  18       if  n  lt  0                perror  ERROR writing to socket            exit 1             return 0       The value of sin port was shown as 35091 while debugging and I don t understand how portno changed from 5001 to 35091  Could someone explain the reason for that change in value please

User · Answer

the htons   function converts values between host and network byte orders  There is a difference between big-endian and little-endian and network byte order depending on your machine and network protocol in use

User · Answer

It has to do with the order in which bytes are stored in memory  The decimal number 5001 is  0x1389 in hexadecimal  so the bytes involved are 0x13 and 0x89  Many devices store numbers in little-endian format  meaning that the least significant byte comes first  So in this particular example it means that in memory the number 5001 will be stored as  0x89 0x13   The htons   function makes sure that numbers are stored in memory in network byte order  which is with the most significant byte first  It will therefore swap the bytes making up the number so that in memory the bytes will be stored in the order  0x13 0x89   On a little-endian machine  the number with the swapped bytes is 0x8913 in hexadecimal  which is 35091 in decimal notation  Note that if you were working on a big-endian machine  the htons   function would not need to do any swapping since the number would already be stored in the right way in memory   The underlying reason for all this swapping has to do with the network protocols in use  which require the transmitted packets to use network byte order

User · Answer

It is done to maintain the arrangement of bytes which is sent in the network Endianness   Depending upon architecture of your device data can be arranged in the memory either in the big endian format or little endian format  In networking  we call the representation of byte order as network byte order and in our host  it is called host byte order  All network byte order is in big endian format If your host s memory computer architecture is in little endian format htons   function become necessity but in case of big endian format memory architecture it is not necessary You can find endianness of your computer programmatically too in the following way -      int x   1     if    char    amp x         cout lt  lt  Little Endian  lt  lt endl      else        cout lt  lt  Big Endian  lt  lt endl         and then decide whether to use htons   or not But in order to avoid the above line we always write htons   although it does no changes for Big Endian based memory architecture

User · Answer

htons is host-to-network short  This means it works on 16-bit short integers  i e  2 bytes   This function swaps the endianness of a short   Your number starts out at   0001 0011 1000 1001   5001  When the endianness is changed  it swaps the two bytes   1000 1001 0001 0011   35091

[c] htons() function in socket programing

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