sample-tcp-server

// src https://www.cs.cmu.edu/afs/cs/academic/class/15213-f99/www/class26/tcpserver.c
// modified to show the impact of bind(2) v listen(2) on tcp connection as visible by netstat -atp


/*
 * tcpserver.c - A simple TCP echo server
 * usage: tcpserver <port>
 */

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define BUFSIZE 1024

#if 0
/*
 * Structs exported from in.h
 */

/* Internet address */
struct in_addr {
                                unsigned int s_addr;
};

/* Internet style socket address */
struct sockaddr_in  {
                                unsigned short int sin_family; /* Address family */
                                unsigned short int sin_port;   /* Port number */
                                struct in_addr sin_addr;         /* IP address */
                                unsigned char sin_zero[...];   /* Pad to size of 'struct sockaddr' */
};

/*
 * Struct exported from netdb.h
 */

/* Domain name service (DNS) host entry */
struct hostent {
                                char    *h_name;        /* official name of host */
                                char    **h_aliases;    /* alias list */
                                int     h_addrtype;     /* host address type */
                                int     h_length;       /* length of address */
                                char    **h_addr_list;  /* list of addresses */
}
#endif

/*
 * error - wrapper for perror
 */
void error(char *msg) {
                                perror(msg);
                                exit(1);
}

int main(int argc, char **argv) {
                                int parentfd; /* parent socket */
                                int childfd; /* child socket */
                                int portno; /* port to listen on */
                                int clientlen; /* byte size of client's address */
                                struct sockaddr_in serveraddr; /* server's addr */
                                struct sockaddr_in clientaddr; /* client addr */
                                struct hostent *hostp; /* client host info */
                                char buf[BUFSIZE]; /* message buffer */
                                char *hostaddrp; /* dotted decimal host addr string */
                                int optval; /* flag value for setsockopt */
                                int n; /* message byte size */

                                /*
                                 * check command line arguments
                                 */
                                if (argc != 2) {
                                                                fprintf(stderr, "usage: %s <port>\n", argv[0]);
                                                                exit(1);
                                }
                                portno = atoi(argv[1]);

                                /*
                                 * socket: create the parent socket
                                 */
                                parentfd = socket(AF_INET, SOCK_STREAM, 0);
                                if (parentfd < 0)
                                                                error("ERROR opening socket");

                                /* setsockopt: Handy debugging trick that lets
                                 * us rerun the server immediately after we kill it;
                                 * otherwise we have to wait about 20 secs.
                                 * Eliminates "ERROR on binding: Address already in use" error.
                                 */
                                /*
                                         optval = 1;
                                         setsockopt(parentfd, SOL_SOCKET, SO_REUSEADDR,
                                         (const void *)&optval , sizeof(int));
                                         */
                                /*
                                 * build the server's Internet address
                                 */
                                bzero((char *) &serveraddr, sizeof(serveraddr));

                                /* this is an Internet address */
                                serveraddr.sin_family = AF_INET;

                                /* let the system figure out our IP address */
                                serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);

                                /* this is the port we will listen on */
                                serveraddr.sin_port = htons((unsigned short)portno);

                                /*
                                 * bind: associate the parent socket with a port
                                 */
                                if (bind(parentfd, (struct sockaddr *) &serveraddr,
                                                                                                                                sizeof(serveraddr)) < 0)
                                                                error("ERROR on binding");

                                // wait here
                                int dummy  = getchar( );


                                /*
                                 * listen: make this socket ready to accept connection requests
                                 */
                                if (listen(parentfd, 5) < 0) /* allow 5 requests to queue up */
                                                                error("ERROR on listen");





                                /*
                                 * main loop: wait for a connection request, echo input line,
                                 * then close connection.
                                 */
                                clientlen = sizeof(clientaddr);
                                while (1) {

                                                                /*
                                                                 * accept: wait for a connection request
                                                                 */
                                                                childfd = accept(parentfd, (struct sockaddr *) &clientaddr, &clientlen);
                                                                if (childfd < 0)
                                                                                                error("ERROR on accept");

                                                                /*
                                                                 * gethostbyaddr: determine who sent the message
                                                                 */
                                                                hostp = gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr,
                                                                                                                                sizeof(clientaddr.sin_addr.s_addr), AF_INET);
                                                                if (hostp == NULL)
                                                                                                error("ERROR on gethostbyaddr");
                                                                hostaddrp = inet_ntoa(clientaddr.sin_addr);
                                                                if (hostaddrp == NULL)
                                                                                                error("ERROR on inet_ntoa\n");
                                                                printf("server established connection with %s (%s)\n",
                                                                                                                                hostp->h_name, hostaddrp);

                                                                /*
                                                                 * read: read input string from the client
                                                                 */
                                                                bzero(buf, BUFSIZE);
                                                                n = read(childfd, buf, BUFSIZE);
                                                                if (n < 0)
                                                                                                error("ERROR reading from socket");
                                                                printf("server received %d bytes: %s", n, buf);

                                                                /*
                                                                 * write: echo the input string back to the client
                                                                 */
                                                                n = write(childfd, buf, strlen(buf));
                                                                if (n < 0)
                                                                                                error("ERROR writing to socket");

                                                                close(childfd);
                                }
}