Linux tcp клиент сервер

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A simple in-kernel tcp client and server implemented as LKMs

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abysamross/simple-linux-kernel-tcp-client-server

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README.md

A simple in-kernel tcp client and server implemented as LKMs (linux kernel version 4.1.3)

This is an attempt to build a tcp server, entirely in kernel space, that supports mulitple tcp clients. The tcp client is also entirely in kernel space.

The client and server are built as loadable kernel modules.

To try this out:

1. clone this repo to server machine.
2. open network_server.c
   • change the port number of your server.
   • sorry, this will soon be changed to module parameter.
3. make
4. sudo insmod network_server.ko
5. keep observing dmesg out.
6. clone this repo to client machine.
7. before inserting the client, open network_client.c
   • change the ip and port number to that of your server.
   • again, this too will soon be changed to module parameters.
8. make
9. sudo insmod network_client.ko

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A simple in-kernel tcp client and server implemented as LKMs

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How to Code a Server and Client in C with Sockets on Linux – Code Examples

In a previous example we learnt about the basics of socket programming in C. In this example we shall build a basic ECHO client and server. The server/client shown here use TCP sockets or SOCK_STREAM.

Tcp sockets are connection oriented, means that they have a concept of independent connection on a certain port which one application can use at a time.

The concept of connection makes TCP a «reliable» stream such that if errors occur, they can be detected and compensated for by resending the failed packets.

Server

Lets build a very simple web server. The steps to make a webserver are as follows :

1. Create socket
2. Bind to address and port
3. Put in listening mode
4. Accept connections and process there after.

/* C socket server example */ #include #include //strlen #include #include //inet_addr #include //write int main(int argc , char *argv[]) < int socket_desc , client_sock , c , read_size; struct sockaddr_in server , client; char client_message[2000]; //Create socket socket_desc = socket(AF_INET , SOCK_STREAM , 0); if (socket_desc == -1) < printf("Could not create socket"); >puts("Socket created"); //Prepare the sockaddr_in structure server.sin_family = AF_INET; server.sin_addr.s_addr = INADDR_ANY; server.sin_port = htons( 8888 ); //Bind if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0) < //print the error message perror("bind failed. Error"); return 1; >puts("bind done"); //Listen listen(socket_desc , 3); //Accept and incoming connection puts("Waiting for incoming connections. "); c = sizeof(struct sockaddr_in); //accept connection from an incoming client client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c); if (client_sock < 0) < perror("accept failed"); return 1; >puts("Connection accepted"); //Receive a message from client while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 ) < //Send the message back to client write(client_sock , client_message , strlen(client_message)); >if(read_size == 0) < puts("Client disconnected"); fflush(stdout); >else if(read_size == -1) < perror("recv failed"); >return 0; >

The above code example will start a server on localhost (127.0.0.1) port 8888
Once it receives a connection, it will read some input from the client and reply back with the same message.
To test the server run the server and then connect from another terminal using the telnet command like this

Client

Now instead of using the telnet program as a client, why not write our own client program. Quite simple again

/* C ECHO client example using sockets */ #include //printf #include //strlen #include //socket #include //inet_addr #include int main(int argc , char *argv[]) < int sock; struct sockaddr_in server; char message[1000] , server_reply[2000]; //Create socket sock = socket(AF_INET , SOCK_STREAM , 0); if (sock == -1) < printf("Could not create socket"); >puts("Socket created"); server.sin_addr.s_addr = inet_addr("127.0.0.1"); server.sin_family = AF_INET; server.sin_port = htons( 8888 ); //Connect to remote server if (connect(sock , (struct sockaddr *)&server , sizeof(server)) < 0) < perror("connect failed. Error"); return 1; >puts("Connected\n"); //keep communicating with server while(1) < printf("Enter message : "); scanf("%s" , message); //Send some data if( send(sock , message , strlen(message) , 0) < 0) < puts("Send failed"); return 1; >//Receive a reply from the server if( recv(sock , server_reply , 2000 , 0) < 0) < puts("recv failed"); break; >puts("Server reply :"); puts(server_reply); > close(sock); return 0; >

The above program will connect to localhost port 8888 and then ask for commands to send. Here is an example, how the output would look

$ gcc client.c && ./a.out Socket created Connected Enter message : hi Server reply : hi Enter message : how are you

Server to handle multiple connections

The server in the above example has a drawback. It can handle communication with only 1 client. Thats not very useful.

One way to work around this is by using threads. A thread can be assigned for each connected client which will handle communication with the client.

/* C socket server example, handles multiple clients using threads */ #include #include //strlen #include //strlen #include #include //inet_addr #include //write #include //for threading , link with lpthread //the thread function void *connection_handler(void *); int main(int argc , char *argv[]) < int socket_desc , client_sock , c , *new_sock; struct sockaddr_in server , client; //Create socket socket_desc = socket(AF_INET , SOCK_STREAM , 0); if (socket_desc == -1) < printf("Could not create socket"); >puts("Socket created"); //Prepare the sockaddr_in structure server.sin_family = AF_INET; server.sin_addr.s_addr = INADDR_ANY; server.sin_port = htons( 8888 ); //Bind if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0) < //print the error message perror("bind failed. Error"); return 1; >puts("bind done"); //Listen listen(socket_desc , 3); //Accept and incoming connection puts("Waiting for incoming connections. "); c = sizeof(struct sockaddr_in); //Accept and incoming connection puts("Waiting for incoming connections. "); c = sizeof(struct sockaddr_in); while( (client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c)) ) < puts("Connection accepted"); pthread_t sniffer_thread; new_sock = malloc(1); *new_sock = client_sock; if( pthread_create( &sniffer_thread , NULL , connection_handler , (void*) new_sock) < 0) < perror("could not create thread"); return 1; >//Now join the thread , so that we dont terminate before the thread //pthread_join( sniffer_thread , NULL); puts("Handler assigned"); > if (client_sock < 0) < perror("accept failed"); return 1; >return 0; > /* * This will handle connection for each client * */ void *connection_handler(void *socket_desc) < //Get the socket descriptor int sock = *(int*)socket_desc; int read_size; char *message , client_message[2000]; //Send some messages to the client message = "Greetings! I am your connection handler\n"; write(sock , message , strlen(message)); message = "Now type something and i shall repeat what you type \n"; write(sock , message , strlen(message)); //Receive a message from client while( (read_size = recv(sock , client_message , 2000 , 0)) >0 ) < //Send the message back to client write(sock , client_message , strlen(client_message)); >if(read_size == 0) < puts("Client disconnected"); fflush(stdout); >else if(read_size == -1) < perror("recv failed"); >//Free the socket pointer free(socket_desc); return 0; >

Run the above server and connect from multiple clients and it will handle all of them. There are other ways to handle multiple clients, like select, poll etc.

We shall talk about them in some other article. Till then practise the above code examples and enjoy.

A Tech Enthusiast, Blogger, Linux Fan and a Software Developer. Writes about Computer hardware, Linux and Open Source software and coding in Python, Php and Javascript. He can be reached at [email protected] .

64 Comments

1. Zamer Chaudhary April 16, 2022 at 2:02 pm Hey sir i need you help related to Develop a client/server application using Linux TCP sockets and the C programming language.
   I will share the more information on mail. Please respond me.
   [email protected]

1. Alex June 8, 2017 at 4:04 pm Yes, it fixes the bug with memory leakage but disables the multiple clients functionality, so this example doesn’t really work 🙁 I cann’t solve this problem yet.

1. David November 5, 2021 at 1:01 am I’m wondering that, too. As soon as I change from the localhost address, I start getting message refused on the client.

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A thin and simple C++ TCP client server

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elhayra/tcp_server_client

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README.md

A thin and simple C++ TCP client and server library with examples.

Currently, both linux and mac are supported

Simple tcp server-client examples. They are optimized for simplicity and ease of use/read but not for performance. However, I believe tuning this code to suite your needs should be easy in most cases. You can find code examples of both server and client under the ‘examples’ directory. Both the library and the examples are well documented.

The server is thread-safe, and can handle multiple clients at the same time, and remove dead clients resources automatically.

build the examples and static library file:

$ cd tcp_server_client $ ./build

run the server and client examples: Navigate into the build folder and run in the terminal:

This project is set to use CMake to build both the client example and the server example. In addition, CMake builds a static library file to hold the common code to both server and client.

In order to build the project you can either use the build.sh script:

$ cd tcp_server_client $ ./build

$ cd tcp_server_client $ mkdir build $ cmake .. $ make

The build process generate three files: libtcp_client_server.a , tcp_client and tcp_server . The last two are the executables of the examples which can be executed in the terminal.

Building Only Server or Client

By default, the CMake configuration builds both server and client. However, you can use flags to build only one of the apps as follows:

Disabling the server build

To build only the client, disable the server build by replace the cmake .. call by:

And then run the make command as usual.

Disabling the client build

To build only the server, disable the client build by replace the cmake .. call by:

And then run the make command as usual.

Navigate into the build folder and run in the terminal:

You should see a menu output on the screen, we’ll get back to that. In a different terminal, run the client:

You should see a similar menu for the client too. In addition, as mentioned, the client will try to connect to the server right away, so you should also see an output messages on both client and server terminals indicating that the connection succeeded. Now, feel free to play with each of the client/server menus. You can exchange messages b/w client and server, print active clients etc. You can also spawn more clients in other terminals to see how the server handles multiple clients.

After playing with the examples, go into the examples source code and have a look at the main() function to learn how the server and client interacts. The examples are heavily documented. In addition, you can also look at the public functions in tcp_client.h and tcp_server.h to learn what APIs are available.

Both server and client are using the observer design pattern to register and handle events. When registering to an event with a callback, you should make sure that:

• The callback is fast (not doing any heavy lifting tasks) because those callbacks are called from the context of the server or client.
• No server / client function calls are made in those callbacks to avoid possible deadlock.

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A thin and simple C++ TCP client server

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