Sockets Programming in C using TCP/IP

#include <sys/types.h>
#include <sys/socket.h>

int socket(int domain, int type, int protocol);

#include <unistd.h>

int close(int fd);

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>

int bind(int sockfd, const struct sockaddr *addr,
         socklen_t addrlen);

#include <sys/types.h>
#include <sys/socket.h>

int listen(int sockfd, int backlog);

#include <sys/types.h>
#include <sys/socket.h>

int connect(int sockfd, const struct sockaddr *addr,
            socklen_t addrlen);

#include <sys/types.h>
#include <sys/socket.h>

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

/* tcp_server.c */
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>

#include "die_with_error.h"

#define MAXPENDING 1024
#define RCVBUFSIZE 4096

int main(void) {
    /* Create a TCP socket */
    /* Create socket for incoming connections */
    int serv_sock;
    if ((serv_sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
        die_with_error("socket() failed");
    }

    /* Assign a port to socket */
    int serv_port = 5100;
    struct sockaddr_in serv_addr;
    serv_addr.sin_family = AF_INET;                 /* Internet address family */
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);  /* Any incoming interface */
    serv_addr.sin_port = htons(serv_port);           /* Local port */
    if (bind(serv_sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        die_with_error("bind() failed");
    }

    /* Set socket to listen */
    /* Mark the socket so it will listen for incoming connections */
    if (listen(serv_sock, MAXPENDING) < 0) {
        die_with_error("listen() failed");
    }

    /* Repeatedly: */
    /* Run forever */
    for (;;) {
        /* Accept new connectionb */
        /* Server is now blocked waiting for connection from a client */
        int client_sock;
        struct sockaddr client_addr;
        int addr_len;
        addr_len = sizeof(client_addr);
        if ((client_sock = accept(serv_sock, (struct sockaddr *)&client_addr, &addr_len)) < 0) {
            die_with_error("accept() failed");
        }

        struct sockaddr_in *c_addr = (struct sockaddr_in *)&client_addr;
        char c_ip_addr[INET6_ADDRSTRLEN];
        inet_ntop(c_addr -> sin_family, &(c_addr -> sin_addr), c_ip_addr, addr_len);
        int c_port = ntohs(c_addr -> sin_port);
        printf("%s:%d =>\n", c_ip_addr, c_port);

        /* Receive mesage from client */
        int recv_msg_size;
        char echo_buf[RCVBUFSIZE];
        if ((recv_msg_size = recv(client_sock, echo_buf, RCVBUFSIZE, 0)) < 0) {
            die_with_error("first recv() failed");
        }

        /* Send received string and receive again until end of transmission */
        while (recv_msg_size > 0) { /* zero indicates end of transmission */
            if (send(client_sock, echo_buf, recv_msg_size, 0) != recv_msg_size) {
                die_with_error("repeat send() failed");
            }
            printf(echo_buf);
            memset(echo_buf, '\0', RCVBUFSIZE);

            if ((recv_msg_size = recv(client_sock, echo_buf, RCVBUFSIZE, 0)) < 0) {
                die_with_error("recv() failed");
            }
        }

        /* Close the connection */
        close(client_sock);
        printf("%s:%d <=\n", c_ip_addr, c_port);
    }
}

/* tcp_client.c */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#include "die_with_error.h"

#define RCVBUFSIZE 4096


int main(void) {
    /* Create a TCP socket */
    /* Create a reliable, stream socket using TCP */
    int client_sock;
    if ((client_sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
        die_with_error("socket() failed");
    }

    /* Establish connection */
    char *serv_ip = "127.0.0.1";
    int serv_port = 5100;
    struct sockaddr_in serv_addr;
    serv_addr.sin_family = AF_INET;                     /* Internet address family */
    serv_addr.sin_addr.s_addr = inet_addr(serv_ip);     /* Server IP address*/
    serv_addr.sin_port = htons(serv_port);              /* Server port */
    if (connect(client_sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        die_with_error("connect() failed");
    }

    /* Communicate */
    int read_len;
    char *read_buf = (char*)malloc(RCVBUFSIZE * sizeof(char));
    int recv_msg_size;
    char echo_buf[RCVBUFSIZE];
    for(;;) {
        read_buf = fgets(read_buf, sizeof(read_buf), stdin);
        if(read_buf == NULL) {
            exit(EXIT_SUCCESS);
        }

        read_len = strlen(read_buf); /* Determine input length *//* Send the string to the server */
        if (send(client_sock, read_buf, read_len, 0) != read_len) {
            die_with_error("send() sent a different number of bytes than expected");
        }

        /* Receive mesage from server */
        if ((recv_msg_size = recv(client_sock, echo_buf, RCVBUFSIZE, 0)) < 0) {
            die_with_error("recv() failed");
        }

        fputs(echo_buf, stdout);
        memset(read_buf, '\0', RCVBUFSIZE);
        memset(echo_buf, '\0', RCVBUFSIZE);
    }

    /* Close the connection */
    close(client_sock);
}

$ gcc die_with_error.h die_with_error.c tcp_server.c -o tcp_server
$ ./tcp_server
127.0.0.1:45934 =>
Hello world!
127.0.0.1:45934 <=
^C

$ gcc die_with_error.h die_with_error.c tcp_client.c -o tcp_client
$ echo 'Hello world!' | ./tcp_client
Hello world!