Sunday, September 6, 2015

BASIC C Socket Programming In Unix For Newbies


1. Introduction
2. Different types of Internet Sockets
3. Structures
4. Conversions
5. IP Addresses
6. Important Functions

   6.1. socket()
   6.2. bind()
   6.3. connect()
   6.4. listen()
   6.5. accept()
   6.6. send()
   6.7. recv()
   6.8. sendto()
   6.9. recvfrom()
   6.10. close()
   6.11. shutdown()
   6.12. gethostname()

7. Some words about DNS

8. A Stream Server Example

9. A Stream Client Example

10. Last Words

11. Copyright


Are you trying to learn c socket programming? Or do you think that it's hard stuff?
Well, then you must read this basic tutorial to get basic ideas and concepts and to start
to work with sockets. Don't expect to be a "socket programming master" after reading this
tutorial. You'll only be that if you practice and read a lot.


In the first place I must explain what a socket is. In a very simple way, a socket is a way
to talk to other computer. To be more precise, it's a way to talk to other computers using
standard Unix file descriptors. In Unix, every I/O actions are done by writing or reading
to a file descriptor. A file descriptor is just an integer associated with an open file and it
can be a network connection, a terminal, or something else.

About the different types of internet sockets, there are many types but I'll just describe two
of them - Stream Sockets (SOCK_STREAM) and Datagram Sockets (SOCK_DGRAM).

"And what are the differences between this two types?" you may ask. Here's the answer:

        Stream Sockets - they're error free; if you send through the stream socket three
                                     items "A,B,C", they will arrive in the same order - "A,B,C" ;
                                     they use TCP ("Transmission Control Protocol") - this protocol
                                     assures the items' order.
      Datagram Sockets - they use UDP ("User Datagram Protocol"); they're connectionless
                                     because you don't need to have an open connection as in Stream
                                     Sockets - you build a packet with the destination information and
                                     send it out.

A lot more could be explained here about this two kind of sockets, but I think this is enough
to get the basic concept of socket. Understanding what a socket is and this two types of
internet sockets is a good start, but you need to learn how to "work" with them. You'll learn
it in the next sections.



The purpose of this section is not to teach you structures but to tell you how are they
used in C socket programming. If you don't know what a structure is, my advice is to read
a C Tutorial and learn it. For the moment, let's just say that a structure is a data type
that is an aggregate, that is, it contains other data types, which are grouped together into
a single user-defined type.

Structures are used in socket programming to hold information about the address.

The first structure is struct sockaddr that holds socket information.

            struct sockaddr{
                        unsigned short  sa_family;    /* address family */
                        char           sa_data[14];  /* 14 bytes of protocol address */

But, there's another structure (struct sockaddr_in) that help you to reference to the socket's

            struct sockaddr_in {
                        short int            sin_family;  /* Address family */
                        unsigned short int   sin_port;    /* Port */
                        struct in_addr             sin_addr;              /* Internet Address */
                        unsigned char            sin_zero[8]; /* Same size as struct sockaddr */

Note: sin_zero is set to all zeros with memset() or bzero() (See examples bellow).

The next structure is not very used but it is defined as an union.

As you can see in both examples bellow (Stream Client and Server Client) , when I declare for
example "client" to be of type sockaddr_in then I do client.sin_addr = (...)

Here's the structure anyway:

            struct in_addr {
                        unsigned long s_addr;

Finally, I think it's better talk about struct hostent. In the Stream Client Example, you can
see that I use this structure. This structure is used to get remote host information.

Here it is:

             struct hostent
              char *h_name;                 /* Official name of host.  */
              char **h_aliases;             /* Alias list.  */
              int h_addrtype;               /* Host address type.  */
              int h_length;                 /* Length of address.  */
              char **h_addr_list;           /* List of addresses from name server.  */
            #define h_addr  h_addr_list[0]  /* Address, for backward compatibility.  */

This structure is defined in header file netdb.h.

In the beginning, this structures will confuse you a lot, but after you start to write some
lines, and after seeing the examples, it will be easier for you understanding them. To see
how you can use them check the examples (section 8 and 9).


There are two types of byte ordering: most significant byte and least significant byte.
This former is called "Network Byte Order" and some machines store their numbers internally
in Network Byte Order.

There are two types you can convert: short and long.
Imagine you want to convert a long from Host Byte Order to Network Byte Order. What would you
do? There's a function called htonl() that would convert it =) The following functions are
used to convert :

            htons() -> "Host to Network Short"
            htonl()         -> "Host to Network Long"
            ntohs()         -> "Network to Host Short"
            ntohl() -> "Network to Host Long"

You must be thinking why do you need this. Well, when you finish reading this document, it will
all seems easier =) All you need is to read and a lot of practice =)

An important thing, is that sin_addr and sin_port (from struct sockaddr_in) must be in Network
Byte Order (you'll see in the examples the functions described here to convert and you'll start
to understand it).


In C, there are some functions that will help you manipulating IP addresses. We'll talk about
inet_addr() and inet_ntoa() functions.

            inet_addr() converts an IP address into an unsigned long. An example:

            dest.sin_addr.s_addr = inet_addr("");


            /*Remember that this is if you've a struct dest of type sockaddr_in*/

            inet_ntoa() converts string IP addresses to long. An example:


            char *IP;


            printf("Address is: %s\n",ip);


Remember that inet_addr() returns the address in Network Byte Order - so you don't need to
call htonl().


In this section I'll put the function' syntax, the header files you must include to call it,
and little comments. Besides the functions mentioned in this document, there are more, but
I decided to put only these ones here. Maybe I'll put them in a future version of this
document =) To see examples of these functions, you can check the stream client and stream
server source code (Sections 8 and 9)

  6.1. socket()

            int socket(int domain,int type,int protocol);

             Let's see the arguments:

                        domain   -> you can set "AF_INET" (set AF_INET to use ARPA internet protocols)
                                        or "AF_UNIX" if you want to create sockets for inside comunication.
                                        Those two are the most used, but don't think that there are just
                                        those. There are more I just don't mention them.
                        type     -> here you put the kind of socket you want (Stream or Datagram)
                                          If you want Stream Socket the type must be SOCK_STREAM
                                          If you want Datagram Socket the type must be SOCK_DGRAM
                        protocol -> you can just set protocol to 0

            socket() gives you a socket descriptor that you can use in later system calls or
            it gives you -1 on error (this is usefull for error checking routines).

  6.2. bind()


            int bind(int fd, struct sockaddr *my_addr,int addrlen);

            Let's see the arguments:

                        fd  -> is the socket file descriptor returned by socket() call
                        my_addr -> is a pointer to struct sockaddr
                        addrlen -> set it to sizeof(struct sockaddr)

            bind() is used when you care about your local port (usually when you use listen() )
            and its function is to associate a socket with a port (on your machine). It returns
            -1 on error.

            You can put your IP address and your port automatically:

                        server.sin_port = 0;                          /* bind() will choose a random port*/
                        server.sin_addr.s_addr = INADDR_ANY;  /* puts server's IP automatically */
            An important aspect about ports and bind() is that all ports bellow 1024 are reserved.
            You can set a  port above 1024 and bellow 65535 (unless the ones being used by other

  6.3. connect()


            int connect(int fd, struct sockaddr *serv_addr, int addrlen);

            Let's see the arguments:

                        fd    -> is the socket file descriptor returned by socket() call
                        serv_addr -> is a pointer to struct sockaddr that contains destination IP
                                         address and port
                        addrlen   -> set it to sizeof(struct sockaddr)

            connect() is used to connect to an IP address on a defined port. It returns -1 on

  6.4. listen()


            int listen(int fd,int backlog);

            Let's see the arguments:

                        fd  -> is the socket file descriptor returned by socket() call
                        backlog -> is the number of allowed connections

            listen() is used if you're waiting for incoming connections, this is, if you want
            someone to connect to your machine. Before calling listen(), you must call bind()
            or you'll be listening on a random port =) After calling listen() you must call
            accept() in order to accept incoming connection. Resuming, the sequence of system calls

                        1. socket()
                        2. bind()
                        3. listen()
                        4. accept() /* In the next section I'll explain how to use accept() */

            As all functions above described, listen() returns -1 on error.

  6.5. accept()


            int accept(int fd, void *addr, int *addrlen);

            Let's see the arguments:

                        fd  -> is the socket file descriptor returned by listen() call
                        addr    -> is a pointer to struct sockaddr_in where you can determine which host
                                       is calling you from which port
                        addrlen -> set it to sizeof(struct sockaddr_in) before its address is passed
                                       to accept()
            When someone is trying to connect to your computer, you must use accept() to get the
            connection. It's very simple to understand: you just get a connection if you accept =)

            Next, I'll give you a little example of accept() use because it's a little different
            from other functions.


              sin_size=sizeof(struct sockaddr_in);
              if ((fd2 = accept(fd,(struct sockaddr *)&client,&sin_size))==-1){ /* calls accept() */
                printf("accept() error\n");


            From now on, fd2 will be used for add send() and recv() calls.

  6.6. send()
            int send(int fd,const void *msg,int len,int flags);

            Let's see the arguments:

                        fd -> is the socket descriptor where you want to send data to
                        msg    -> is a pointer to the data you want to send
                        len    -> is the length of the data you want to send (in bytes)
                        flags  -> set it to 0

            This function is used to send data over stream sockets or CONNECTED datagram sockets.
            If you want to send data over UNCONNECTED datagram sockets you must use sendto().
            send() returns the number of bytes sent out and it will return -1 on error.

  6.7. recv()
            int recv(int fd, void *buf, int len, unsigned int flags);

            Let's see the arguments:

                        fd  -> is the socket descriptor to read from
                        buf     -> is the buffer to read the information into
                        len      -> is the maximum length of the buffer
                        flags   -> set it to 0

            As I said above about send(), this function is used to send data over stream sockets or
            CONNECTED datagram sockets. If you want to send data over UNCONNECTED datagram sockets
            you must use recvfrom().

            recv() returns the number of bytes read into the buffer and it'll return -1 on error.

  6.8. sendto()

            int sendto(int fd,const void *msg, int len, unsigned int flags,
                           const struct sockaddr *to, int tolen);

            Let's see the arguments:

                        fd  -> the same as send()
                        msg     -> the same as send()
                        len      -> the same as send()
                        flags   -> the same as send()
                        to        -> is a pointer to struct sockaddr
                        tolen  -> set it to sizeof(struct sockaddr)

            As you can see, sendto() is just like send(). It has only two more arguments : "to"
            and "tolen" =)

            sendto() is used for UNCONNECTED datagram sockets and it returns the number of bytes
            sent out and it will return -1 on error.

  6.9. recvfrom()

            int recvfrom(int fd,void *buf, int len, unsigned int flags
                             struct sockaddr *from, int *fromlen);

            Let's see the arguments:

                        fd        -> the same as recv()
                        buf     -> the same as recv()
                        len      -> the same as recv()
                        flags   -> the same as recv()
                        from  -> is a pointer to struct sockaddr
                        fromlen        -> is a pointer to a local int that should be initialised
                                       to sizeof(struct sockaddr)

            recvfrom() returns the number of bytes received and it'll return -1 on error.

  6.10. close()

             close() is used to close the connection on your socket descriptor. If you call close(),
            it won't be no more writes or reads and if someone tries to read/write will receive an

  6.11. shutdown()

            int shutdown(int fd, int how);

            Let's see the arguments:

                        fd        -> is the socket file descriptor you want to shutdown
                        how   -> you put one of those numbers:
                                                            0 -> receives disallowed
                                                            1 -> sends disallowed
                                                            2 -> sends and receives disallowed

            When how is set to 2, it's the same thing as close().

            shutdown() returns 0 on success and -1 on error.

  6.12. gethostname()


            int gethostname(char *hostname, size_t size);

            Let's see the arguments:

                        hostname  -> is a pointer to an array that contains hostname
                        size      -> length of the hostname array (in bytes)

            gethostname() is used to get the name of the local machine.


I created this section, because I think you should know what DNS is. DNS stands for "Domain
Name Service" and basically, it's used to get IP addresses. For example, I need to know the
IP address of and through DNS I'll get .

This is important, because functions we saw above (like bind() and connect()) work with IP

To see you how you can get IP address on c, I made a little example:

/* <---- ----="" code="" here="" source="" starts=""> */

#include    /* This is the header file needed for gethostbyname() */

int main(int argc, char *argv[])
  struct hostent *he;

  if (argc!=2){
     printf("Usage: %s \n",argv[0]);

  if ((he=gethostbyname(argv[1]))==NULL){
     printf("gethostbyname() error\n");

  printf("Hostname : %s\n",he->h_name);  /* prints the hostname */
  printf("IP Address: %s\n",inet_ntoa(*((struct in_addr *)he->h_addr))); /* prints IP address */
/* <---- ----="" code="" ends="" here="" source=""> */


In this section, I'll show you a nice example of a stream server. The source code is all
commented so that you ain't no possible doubts =)

Let's start:

/* <---- ----="" code="" here="" source="" starts=""> */

#include           /* These are the usual header files */

#define PORT 3550   /* Port that will be opened */
#define BACKLOG 2   /* Number of allowed connections */


  int fd, fd2; /* file descriptors */

  struct sockaddr_in server; /* server's address information */
  struct sockaddr_in client; /* client's address information */

  int sin_size;

  if ((fd=socket(AF_INET, SOCK_STREAM, 0)) == -1 ){  /* calls socket() */
    printf("socket() error\n");

  server.sin_family = AF_INET;        
  server.sin_port = htons(PORT);   /* Remember htons() from "Conversions" section? =) */
  server.sin_addr.s_addr = INADDR_ANY;  /* INADDR_ANY puts your IP address automatically */  
  bzero(&(server.sin_zero),8); /* zero the rest of the structure */

  if(bind(fd,(struct sockaddr*)&server,sizeof(struct sockaddr))==-1){ /* calls bind() */
      printf("bind() error\n");

  if(listen(fd,BACKLOG) == -1){  /* calls listen() */
      printf("listen() error\n");

  sin_size=sizeof(struct sockaddr_in);
  if ((fd2 = accept(fd,(struct sockaddr *)&client,&sin_size))==-1){ /* calls accept() */
    printf("accept() error\n");
  printf("You got a connection from %s\n",inet_ntoa(client.sin_addr) ); /* prints client's IP */
  send(fd2,"Welcome to my server.\n",22,0); /* send to the client welcome message */
  close(fd2); /*  close fd2 */

/* <---- ----="" code="" ends="" here="" source=""> */


/* <---- ----="" code="" here="" source="" starts=""> */

#include         /* netbd.h is needed for struct hostent =) */

#define PORT 3550          /* Open Port on Remote Host */
#define MAXDATASIZE 100   /* Max number of bytes of data */

int main(int argc, char *argv[])
  int fd, numbytes;              /* files descriptors */
  char buf[MAXDATASIZE];  /* buf will store received text */
  struct hostent *he;         /* structure that will get information about remote host */
  struct sockaddr_in server;  /* server's address information */

  if (argc !=2) {             /* this is used because our program will need one argument (IP) */
    printf("Usage: %s \n",argv[0]);

  if ((he=gethostbyname(argv[1]))==NULL){    /* calls gethostbyname() */
    printf("gethostbyname() error\n");

  if ((fd=socket(AF_INET, SOCK_STREAM, 0))==-1){  /* calls socket() */
    printf("socket() error\n");

  server.sin_family = AF_INET;
  server.sin_port = htons(PORT); /* htons() is needed again */
  server.sin_addr = *((struct in_addr *)he->h_addr);  /*he->h_addr passes "*he"'s info to "h_addr" */

  if(connect(fd, (struct sockaddr *)&server,sizeof(struct sockaddr))==-1){ /* calls connect() */
    printf("connect() error\n");

  if ((numbytes=recv(fd,buf,MAXDATASIZE,0)) == -1){  /* calls recv() */
    printf("recv() error\n");


      printf("Server Message: %s\n",buf); /* it prints server's welcome message =) */

      close(fd);   /* close fd =) */

/* <---- ----="" code="" ends="" here="" source=""> */



As I'm just a simple human, it's almost certain that there are some errors on this document.
When I say errors I mean English errors (because my language is not the English) but also
technical errors. Please email me if you detect any error =)

But you must understand that this is the first version of this document, so , it's natural not
to be very complete (as matter of fact I think it is ) and it's also very natural to have
stupid errors. However, I can be sure that source code presented in this document works fine.

If you need help concerning this subject you can email me at

SPECIAL THANKS TO: Ghost_Rider (my good old mate), Raven (for letting me write this tutorial)
                           and all my friends =)

All copyrights are reserved. You can distribute this tutorial freely, as long you don't change
any name or URL. You can't change a line or two, or add another lines, and then claim that this
tutorial is yours. If you want to change something, please email me at .

BASIC C Socket Programming In Unix For Newbies

Written by: BracaMan
E-mail    :
ICQ       : 41476410
URL      :

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