Memo to myself about how to implement in C a TCP client-server application.

Server side:

#define _POSIX_C_SOURCE 200112L #include <stdlib.h> #include <stdio.h> #include <sys/socket.h> #include <arpa/inet.h> #include <unistd.h> #include <netdb.h> #include <string.h> #include <poll.h> // Helper function to print a message and exit void failed(char const* const msg) { printf("server> error: %s\n", msg); perror("server> perror"); exit(1); } int main(int argc, char** argv) { // Get the IP of the machine running the server char hostname[256]; gethostname(hostname, sizeof(hostname)); struct hostent* host = gethostbyname(hostname); char* hostIP = inet_ntoa(*((struct in_addr*)host->h_addr_list[0])); printf("server> host IP %s\n", hostIP); // Create the socket short serverSocket = socket(AF_INET, SOCK_STREAM, 0); if(serverSocket < 0) failed("socket()"); printf("server> socket created\n"); // Set the SO_REUSEADDR flag to avoid the following bind() command to // fail due to a hanging port from a previous execution of the server // (it may take a few minutes for the port to become available again) int reuseAddr = 1; int ret = setsockopt( serverSocket, SOL_SOCKET, SO_REUSEADDR, (char*)&reuseAddr, sizeof(reuseAddr)); if(ret < 0) failed("setsockopt()"); printf("server> set SO_REUSEADDR\n"); // Bind the socket to an address made of the host ip and a port automatically // determined (port must be in [1024,32768]). `cat /etc/services` and // `lsof -n | grep LISTEN` give a listing of used ports. `fuser -k xxxx/tcp` // kills the process using the port xxxx of the local machine. int serverPort = 1024; do { ++serverPort; struct sockaddr_in serverAddr = {0}; serverAddr.sin_family = AF_INET; serverAddr.sin_addr.s_addr = inet_addr(hostIP); serverAddr.sin_port = htons(serverPort); ret = bind(serverSocket, (struct sockaddr*)&serverAddr, sizeof(serverAddr)); } while(ret < 0 && serverPort < 32769); if(ret < 0) failed("bind()"); printf("server> socket bound to port %d\n", serverPort); // Save the server IP and port to a file for the client if(argc != 2) exit(1); FILE* fp = fopen(argv[1], "w"); if(fp == NULL) failed("fopen()"); fprintf(fp, "%s %d\n", hostIP, serverPort); fclose(fp); // Listen for incoming connections without queueing int sizeQueue = 1; ret = listen(serverSocket, sizeQueue); if(ret < 0) failed("listen"); printf("server> listening to connection\n"); // Wait for an incoming connection from a client printf("server> waiting for incoming connection 1mn max\n"); struct pollfd fds = {.fd = serverSocket, .events = POLLIN, .revents = 0}; int timeoutMs = 60 * 1000; ret = poll(&fds, 1, timeoutMs); if(ret == 0) { printf("server> timeout, giveup waiting for client\n"); exit(1); } else if(ret < 0) failed("poll()"); // Accept the incoming connection from a client printf("server> accepting the incoming connection\n"); struct sockaddr_in client; size_t clientStructSize = sizeof(client); int incomingConnection = accept( serverSocket, (struct sockaddr*)&client, (socklen_t*)&clientStructSize); if (incomingConnection < 0) failed("accept()"); printf("server> incoming connection accepted\n"); // Loop to process that connection until the client send the end // command int flagLoop = 1; while(flagLoop) { // Wait for data from the client printf("server> waiting for data from client 2s max\n"); struct pollfd fdsData = {.fd = incomingConnection, .events = POLLIN, .revents = 0}; timeoutMs = 2 * 1000; ret = poll(&fdsData, 1, timeoutMs); if(ret == 0) { printf("server> timeout, giveup waiting data from client\n"); exit(1); } else if(ret < 0) failed("poll()"); // Receive data from the incoming connection #define INCOMING_DATA_MAX_SIZE 1000 char incomingData[INCOMING_DATA_MAX_SIZE] = {0}; ssize_t incomingDataSize = read(incomingConnection, incomingData, INCOMING_DATA_MAX_SIZE); if(incomingDataSize < 0) failed("read()"); printf("server> incoming data received (%ld bytes)\nserver> %s\n", incomingDataSize, incomingData); // Update the flag if the client requests to close the connection if(strcmp(incomingData, "end") == 0) flagLoop = 0; // Send back data to the client #define SENT_DATA_MAX_SIZE 1000 char sentData[SENT_DATA_MAX_SIZE] = {0}; snprintf( sentData, SENT_DATA_MAX_SIZE, "echo from server: %s", incomingData); ssize_t sentDataSize = write(incomingConnection, sentData, strlen(sentData)); if(sentDataSize < 0) failed("write()"); printf("server> sent back data\n"); } // Close the connection with the client ret = close(incomingConnection); if(ret < 0) failed("close(incomingConnection)"); printf("server> closed incoming connection\n"); // Close the server socket ret = close(serverSocket); if(ret < 0) failed("close(serverSocket)"); printf("server> closed the socket\n"); // Return the success code return 0; }

Client side:

#define _POSIX_C_SOURCE 200112L #include <stdlib.h> #include <stdio.h> #include <sys/socket.h> #include <sys/time.h> #include <arpa/inet.h> #include <unistd.h> #include <netdb.h> #include <string.h> // Helper function to print a message and exit void failed(char const* const msg) { printf("client> error: %s\n", msg); perror("client> perror"); exit(1); } int main(int argc, char** argv) { // Load the server IP and port from the file in argument if(argc != 2) exit(1); int serverPort = 0; char hostIP[20] = {0}; FILE* fp = fopen(argv[1], "r"); if(fp == NULL) failed("fopen()"); fscanf(fp, "%20s %d\n", hostIP, &serverPort); fclose(fp); printf("client> host IP %s:%d\n", hostIP, serverPort); // Create the socket #define DEFAULT_PROTOCOL 0 short clientSocket = socket(AF_INET, SOCK_STREAM, DEFAULT_PROTOCOL); if(clientSocket < 0) failed("socket()"); printf("client> socket created\n"); // Connect to the server struct sockaddr_in serverAddr = {0}; serverAddr.sin_family = AF_INET; serverAddr.sin_addr.s_addr = inet_addr(hostIP); serverAddr.sin_port = htons(serverPort); int ret = connect(clientSocket, (struct sockaddr*)&serverAddr, sizeof(serverAddr)); if(ret < 0) failed("connect()"); printf("client> connected to server\n"); // Loop to simulate several data exchange between the server and client int nbLoop = 3; for(int iLoop = 0; iLoop < nbLoop; ++iLoop) { // Send data to the server #define SENT_DATA_MAX_SIZE 1000 char sentData[SENT_DATA_MAX_SIZE] = {0}; if(iLoop < nbLoop - 1) { snprintf(sentData, SENT_DATA_MAX_SIZE, "data sent from client %d", iLoop); } else { snprintf(sentData, SENT_DATA_MAX_SIZE, "end"); } ret = send(clientSocket, sentData, strlen(sentData), 0); if(ret < 0) failed("send()"); printf("client> data sent\n"); // Receive data back from the server #define INCOMING_DATA_MAX_SIZE 1000 char incomingData[INCOMING_DATA_MAX_SIZE] = {0}; ssize_t incomingDataSize = read(clientSocket, incomingData, INCOMING_DATA_MAX_SIZE); if(incomingDataSize < 0) failed("read()"); printf("client> incoming data received (%ld bytes)\nclient> %s\n", incomingDataSize, incomingData); } // Close the connection with the server ret = close(clientSocket); if(ret < 0) failed("close()"); printf("client> closed the socket\n"); // Return the success code return 0; }

Compile with gcc -o server server.c -std=c99 and gcc -o client client.c -std=c9, and run with ./server serverIp.txt &; sleep 1; ./client serverIp.txt.