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Networking and Socket programming tutorial in C

, 21 Aug 2014 CPOL
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Networking and Socket programming tutorial in C.

This article is for programmers with the following requirements:

network5

Before you start learning socket programming make sure you already have a certain basic knowledge to network such as understand what is IP address, TCP, UDP.

Before we start our tutorial, keep in mind that the following tutorial only works for Linux OS environment. If you are using Windows, I have to apologize to you because Windows has its own socket programming and it is different from Linux even though the connection concept is the same. Well, first copy and paste the following code and run it on server and client, respectively.

Both code can be run on the same computer.

It is always easy to understand after getting the code work.

Socket-server.c

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
 
int main(void)
{
  int listenfd = 0,connfd = 0;
  
  struct sockaddr_in serv_addr;
 
  char sendBuff[1025];  
  int numrv;  
 
  listenfd = socket(AF_INET, SOCK_STREAM, 0);
  printf("socket retrieve success\n");
  
  memset(&serv_addr, '0', sizeof(serv_addr));
  memset(sendBuff, '0', sizeof(sendBuff));
      
  serv_addr.sin_family = AF_INET;    
  serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); 
  serv_addr.sin_port = htons(5000);    
 
  bind(listenfd, (struct sockaddr*)&serv_addr,sizeof(serv_addr));
  
  if(listen(listenfd, 10) == -1){
      printf("Failed to listen\n");
      return -1;
  }
     
  
  while(1)
    {
      
      connfd = accept(listenfd, (struct sockaddr*)NULL ,NULL); // accept awaiting request
  
      strcpy(sendBuff, "Message from server");
      write(connfd, sendBuff, strlen(sendBuff));
 
      close(connfd);    
      sleep(1);
    }
 
 
  return 0;
}

Socket-client.c

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
 
int main(void)
{
  int sockfd = 0,n = 0;
  char recvBuff[1024];
  struct sockaddr_in serv_addr;
 
  memset(recvBuff, '0' ,sizeof(recvBuff));
  if((sockfd = socket(AF_INET, SOCK_STREAM, 0))< 0)
    {
      printf("\n Error : Could not create socket \n");
      return 1;
    }
 
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_port = htons(5000);
  serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
 
  if(connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr))<0)
    {
      printf("\n Error : Connect Failed \n");
      return 1;
    }
 
  while((n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0)
    {
      recvBuff[n] = 0;
      if(fputs(recvBuff, stdout) == EOF)
    {
      printf("\n Error : Fputs error");
    }
      printf("\n");
    }
 
  if( n < 0)
    {
      printf("\n Read Error \n");
    }
 
 
  return 0;
}

After debugging both source files, run Socket-server.out, then run Socket-client. Attention here, never mess up with the order of executing Socket-server.out and Socket-client. Socket-server must be executed first then execute Socket-client.out and never try to break Socket-server forever loop. It means, you need to open two terminals to run each of the outputs.

When you execute Socket-cli, I guess you will get the following result:

network2 network1

If you see the message above, congratulations, you have success with your first step to networking programming. Otherwise, do some checking on your development environment or try to run some simple code for instance hello world.

Why both server and client on the same computer?

The answer is the server and client both are software but not hardware. It means what is happening on the top is there are two different software executed. To be more precise, the server and client are two different processes with different jobs. If you are experienced with constructing a server you might find out that a server can be built on a home computer by installing a server OS. It is because server is a kind of software.

Understand sockets

Imagine a socket as a seaport that allows a ship to unload and gather shipping, whereas socket is the place where a computer gathers and puts data into the internet.

network3

Configure Socket

Things that need to be initialized are listed as follows:

  1. Using TCP or UDP
  2. Additional protocol
  3. Permit the incoming IP address
  4. Assign the port used

At the beginning, a socket function needs to be declared to get the socket descriptor.

int socket(int domain, int type, int protocol)
Domain AF_UNIX - connect inside same machine AF_INET – connect with different machine
Type SOCK_STREAM – TCP connection SOCK_DGRAM – UDP connection 
Protocol Define here when there is any additional protocol. Otherwise, define it as 0

Next, decide which struct needs to be used based on what domain is used above.

AF_UNIX AF_INET
struct sockaddr_un
  {
    sa_family_t sun_family ;
    char sun_path[];
  };
struct sockaddr_in
  {
    short int  sin_family ;
    int        sin_port;
    struct in_addr sin_addr;
  };
Use struct sockaddr_un if you are using AF_UNIX on your domain. It is required to include <sys/un.h> Use struct sockaddr_in if you are using AF_INT on your domain.

On this article, I will explain sockadd_in that showed on the code above.

serv_addr.sin_family = AF_INET;
Define the domain used
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
Permit any incoming IP address by declare INADDR_ANY
serv_addr.sin_port = htons(5000);
Declare port 5000 to be used.

Based on example above, server is using port 5000. You can check it by following command

sudo netstat -ntlp

Then, you will see following list

network6

Inside red bracket, you will found 0.0.0.0:5000 and Socket-server, it means port 5000 is used and listen to any valid incoming address.

On client side, serv_addr.sin_port = htons(127.0.0.1) is declared in order to listen internal network.

The flow chart below shows the interaction between client and server. The flow chart might looks complicated but make sure you don’t lost your patient due to the following flow chart. Because every process on the flow chart is needed and it acts a very important roles on network connection.

network4

After all setup on struct sockaddr_in is done, declare bind function. As flow chart, bind function must be declared on both server and client.

bind function

server_socket & client_socket Put socket description retrieved on the top
address Put struct sockaddr_in into it as domain is AF_INET. If your domain is AF_UNIX, tried put struct sockaddr_un here.
address_len Put the length of the address

Server and client will start interact with each other after the bind function and it is the most important session. From what flow chart shows, listen, accept, connect, three function play a very important roles.

Imagine that server looks like an ATM, and only one person can be used the ATM. So, what happen if there is 2 or more people come at one time? The answer is simple, lining up and wait the front people finished using with ATM. It is exactly same as what happening in server.

Listen function acts as waiting room, asking the traffic wait on the waiting room. Accept function acts as person who asking the traffic waiting inside the waiting room to be ready for the meeting between server. Last, connect function acts as the person who want to carry out some work with server.

listen function

server_socket Put socket description retrieved on the top
backlog Define the maximum of awaiting request

accept function

server_socket Put socket description retrieved on the top
client_address Put null here if there is no any special request to specify address.
address_len Put null here if second parameter is null
return Return information of client socket description. Use it for interact between client and server.

connect function

client_socket Put socket description retrieved on the top
address Put the struct sockaddr defined on the top
address_len Put the length of the address

Finally, after the request accepted. What should server and client do is send and read data. It is the most simple part in whole of this article. read function used to read the buffer data and write function used to send the data. That’s all.

read function

socket_description Put server or client socket description depend on reading data from server or client
read buffer Content of the data retrieved
read buffer length Length of the output string

write function

socket_description Put server or client socket description depend on sending data to server or client
write buffer Data to be send
write buffer length Length of the output string

Personal Comment

This article was publish on 2013/5/1 and I was still new to networking programming on this period. Maybe there is some point that I am not make clear enough, I have tried all of my best to present all my knowledge to this article. Hope you can get the good basic beginning over here. Thank you.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Edison Heng

Japan Japan
Hi! Thank you everyone who reading my article. My major is electronic and programming. Right now I am doing foreign study at Japan. I will like sharing to everyone with my works and if you do interesting with my works, please leave you comment on my blog. Any comments is welcoming.

Comments and Discussions

 
Questioncomment PinmemberMember 110527902-Sep-14 1:21 
GeneralMy vote of 5 PinmemberMember 110368871-Sep-14 21:54 
Bugmemset() error PinmemberChad3F27-Aug-14 21:20 
GeneralRe: memset() error PinmemberEdison Heng27-Aug-14 22:17 
Questionnot working PinmemberMember 1090220518-Aug-14 5:18 
AnswerRe: not working PinmemberEdison Heng18-Aug-14 15:45 
Generalcomment PinmemberMember 1080047613-May-14 21:59 
GeneralRe: comment PinmemberMember 1090220518-Aug-14 5:17 
QuestionVery nice representation. PinmemberBaldevS18-Feb-14 20:21 
QuestionGreat Article! Excellent Explanations, code, diagrams.... PinmemberMember 1060114718-Feb-14 9:16 
AnswerRe: Great Article! Excellent Explanations, code, diagrams.... PinmemberEdison Heng18-Feb-14 10:37 
QuestionWonderful Article Pinmembersayooj cyriac13-Nov-13 9:15 
QuestionSuperb tutorial PinmemberMember 1024039729-Aug-13 3:55 
QuestionIts crisp and clear PinmemberMember 1022113320-Aug-13 7:18 
GeneralMy vote of 5 Pinmemberhimrpei26-Jul-13 2:45 

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