TCP Echo implementation using C

In the following program, we will demonstrate, how a connection can be established between client and server using Transport Layer Protocol - TCP. This code is self-explanatory. Hence, first you will learn to develop server-side application. Then, you will learn to develop client side application.

Source code of Echo Server

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> void error(const char *msg) { perror(msg); exit(1); } int main(int argc, char *argv[]) { int sock_file_descriptor; // socket file descriptor int client_sock_file_descriptor; // client socket file descriptor int server_sock_port_no; // port number socklen_t client_socket_length; // client socket length char buff[256]; // buffer struct sockaddr_in server_address; // address of this server struct sockaddr_in client_address; // address of a client int max_queue_len_4_pending_conn = 5; // maximum length for the queue of pending connections int data_length; if (argc < 2) error("Error!\n This program requires port number as command line argument."); sock_file_descriptor = socket(AF_INET, SOCK_STREAM, 0); if (sock_file_descriptor < 0) error("Unable to open socket....."); /* set all the bits of server_address by 0 (zero) */ bzero((char *) &server_address, sizeof(server_address)); /* convert the string to integer and set the value to vaiable server_sock_port_no */ server_sock_port_no = atoi(argv[1]); /* prepare server_address variable */ server_address.sin_family = AF_INET; server_address.sin_addr.s_addr = INADDR_ANY; server_address.sin_port = htons(server_sock_port_no); /* bind function returns negative number if there is any error */ if (bind(sock_file_descriptor, (struct sockaddr *) &server_address, sizeof(server_address)) < 0) error("Error in Binding"); /* this function returns 0 on success and -1 on failure */ listen(sock_file_descriptor, max_queue_len_4_pending_conn); client_socket_length = sizeof(client_address); /* accept function is used to accept a connection request on the server socket socket. * The return value of accept is the file descriptor for the new socket. * After accept, the original socket socket remains open and unconnected, * and continues listening until you close it. */ client_sock_file_descriptor = accept(sock_file_descriptor, (struct sockaddr *) &client_address, &client_socket_length); if (client_sock_file_descriptor < 0) error("Unable to accept"); /* reset the buffer */ bzero(buff, 256); /* read the data sent by client */ data_length = read(client_sock_file_descriptor, buff, sizeof(buff)); if (data_length < 0) error("Error in reading data"); printf("Here is the message: %s\n", buff); /* write the data for the client */ data_length = write(client_sock_file_descriptor, buff, sizeof(buff)); if (data_length < 0) error("Error in writing data"); close(client_sock_file_descriptor); close(sock_file_descriptor); return 0; }

Source code of Echo Client

#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <netdb.h> void error(const char *msg) { perror(msg); exit(0); } int main(int argc, char *argv[]) { int sock_file_descriptor; int server_sock_port_no; int data_length; struct sockaddr_in server_address; struct hostent *server; char buff[256]; if (argc < 3) { fprintf(stderr, "Usage: %s hostname port\n", argv[0]); exit(0); } server_sock_port_no = atoi(argv[2]); sock_file_descriptor = socket(AF_INET, SOCK_STREAM, 0); if (sock_file_descriptor < 0) error("Error opening socket"); server = gethostbyname(argv[1]); if (server == NULL) error("Error, no such host\n"); bzero((char *) &server_address, sizeof(server_address)); server_address.sin_family = AF_INET; bcopy((char *) server->h_addr, (char *)&server_address.sin_addr.s_addr, server->h_length); server_address.sin_port = htons(server_sock_port_no); if (connect(sock_file_descriptor, (struct sockaddr *) &server_address, sizeof(server_address)) < 0) error("Error in connecting..."); printf("Please enter the message: "); /* reset the buffer */ bzero(buff, 256); /* read the data from standard input */ fgets(buff, 255, stdin); /* send the content of buffer to server */ data_length = write(sock_file_descriptor, buff, strlen(buff)); if (data_length < 0) error("Error in writing"); /* reset the buffer */ bzero(buff, 256); /* read the content received from server */ data_length = read(sock_file_descriptor, buff, 255); if (data_length < 0) error("Error in reading"); printf("%s\n", buff); close(sock_file_descriptor); return 0; }

Happy Exploring!