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Network Story 1

Network Story 2

Network Story 3

Network Story 4

Network Story 5

Network Story 6

Socket Example 1

Socket Example 2

Socket Example 3

Socket Example 4

Socket Example 5

Socket Example 6

Socket Example 7

Advanced TCP/IP 1

Advanced TCP/IP 2

Advanced TCP/IP 3

Advanced TCP/IP 4

Advanced TCP/IP 5

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Note:  Program examples if any, compiled using gcc on Linux Fedora Core 3 machine with several update, as normal user.  The Fedora machine used for the testing having the "No Stack Execute" disabled and the SELinux set to default configuration.



       listen() - listen for connections on a socket
       #include <sys/socket.h>
       int listen(int sockfd, int backlog);
  • sockfd is the usual socket file descriptor from the socket() system call.

  • backlog is the number of connections allowed on the incoming queue.

  • As an example, for the server, if you want to wait for incoming connections and handle them in some way, the steps are: first you listen(), then you accept().

  • The incoming connections are going to wait in this queue until you accept() (explained later) them and this is the limit on how many can queue up.

  • Again, as per usual, listen() returns -1 and sets errno on error.

  • We need to call bind() before we call listen() or the kernel will have us listening on a random port.

  • So if you’re going to be listening for incoming connections, the sequence of system calls you’ll make is something like this:




/* accept() goes here */


       accept() - accept a connection on a socket
       #include <sys/types.h>
       #include <sys/socket.h>
       int accept(int sockfd, struct sockaddr *addr, int *addrlen);
  • sockfd is the listen()ing socket descriptor.

  • addr will usually be a pointer to a local struct sockaddr_in.  This is where the information about the incoming connection will go (and with it you can determine which host is calling you from which port).

  • addrlen is a local integer variable that should be set to sizeof(struct sockaddr_in) before its address is passed to accept().

  • accept() will not put more than that many bytes into addr.  If it puts fewer in, it’ll change the value of addrlen to reflect that.

  • accept() returns -1 and sets errno if an error occurs.

  • Basically, after listen(), a server calls accept() to wait for the next client to connect.  accept() will create a new socket to be used for I/O with the new client.  The server then will continue to do further accepts with the original sockfd.

  • When someone try to connect() to your machine on a port that you are listen()ing on, their connection will be queued up waiting to be accepted.  You call accept() and you tell it to get the pending connection.

  • It’ll return to you a new socket file descriptor to use for this single connection.

  • Then, you will have two socket file descriptors where the original one is still listening on your port and the newly created one is finally ready to send() and recv().

  • The following is a program example that demonstrates the use of the previous functions.









[bodo@bakawali testsocket]$ cat test3.c

#include <unistd.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>


/* the port users will be connecting to */

#define MYPORT 3440

/* how many pending connections queue will hold */

#define BACKLOG 10


int main()


/* listen on sock_fd, new connection on new_fd */

int sockfd, new_fd;

/* my address information, address where I run this program */

struct sockaddr_in my_addr;

/* remote address information */

struct sockaddr_in their_addr;

int sin_size;


sockfd = socket(AF_INET, SOCK_STREAM, 0);

if(sockfd == -1)


  perror("socket() error lol!");




  printf("socket() is OK...\n");


/* host byte order */

my_addr.sin_family = AF_INET;

/* short, network byte order */

my_addr.sin_port = htons(MYPORT);

/* auto-fill with my IP */

my_addr.sin_addr.s_addr = INADDR_ANY;


/* zero the rest of the struct */

memset(&(my_addr.sin_zero), 0, 8);


if(bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)) == -1)


  perror("bind() error lol!");




  printf("bind() is OK...\n");


if(listen(sockfd, BACKLOG) == -1)


  perror("listen() error lol!");




  printf("listen() is OK...\n");


/* ...other codes to read the received data... */


sin_size = sizeof(struct sockaddr_in);

new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);


if(new_fd == -1)

  perror("accept() error lol!");


  printf("accept() is OK...\n");


/*.....other codes.......*/




return 0;


[bodo@bakawali testsocket]$ gcc test3.c -o test3

[bodo@bakawali testsocket]$ ./test3

socket() is OK...

bind() is OK...

listen() is OK...

  • Note that we will use the socket descriptor new_fd for all send() and recv() calls.

  • If you’re only getting one single connection ever, you can close() the listening sockfd in order to prevent more incoming connections on the same port, if you so desire.


int send(int sockfd, const void *msg, int len, int flags);

  • sockfd is the socket descriptor you want to send data to (whether it’s the one returned by socket() or the new one you got with accept()).

char *msg = "I was here!";

int len, bytes_sent;



len = strlen(msg);

bytes_sent = send(sockfd, msg, len, 0);



int recv(int sockfd, void *buf, int len, unsigned int flags);


       write() - write to a file descriptor
       #include <unistd.h>
       ssize_t write(int fd, const void *buf, size_t count);



       read() - read from a file descriptor
       #include <unistd.h>
       ssize_t read(int fd, void *buf, size_t count);

close() and shutdown()

       close() - close a file descriptor
       #include <unistd.h>
       int close(int sockfd);


int shutdown(int sockfd, int how);

  1. 0 – Further receives are disallowed.

  2. 1 – Further sends are disallowed.

  3. 2 – Further sends and receives are disallowed (like close()).

Continue on next Module…More in-depth discussion about TCP/IP suite is given in Advanced TCP/IP Programming.








Further reading and digging:


  1. Check the best selling C/C++, Networking, Linux and Open Source books at Amazon.com.

  2. Protocol sequence diagram examples.

  3. Another site for protocols information.

  4. RFCs.

  5. GCC, GDB and other related tools.







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