Question

I am working through examples in UNIX Network Programming and I’ve adapted “daytimeclientserv.c” into this code here. The server sends the date/time string to the client as expected except for the very first request it receives upon starting up. When I first run the server program (on another computer in the LAN) it creates the listening socket, binds it and then waits for connections. Upon receiving the first request it prints the date/time string to its own stdout (terminal) instead of to the socket and the client program hangs forever waiting. However, all subsequent requests are sent to the clients correctly. Using gdb, I noticed that connfd is always set to zero. It is set to zero on the first request and also on all future ones.

I also have a few other questions related to this:

• if the server listens on one socket (listenfd) and then reconnects on another (connfd) with connect(), how does the client deal with the change of socket? It was my understanding that a socket is uniquely identified by four parts: servIPaddr, servPort, clientIPaddr, clientPort

• how can i run the server (on linux) without being root

• how can i cleanly close the listening socket, so that i can use it again. I get a bind error if I quit the server program with SIGINT (Ctrl-C). So far I’ve been using gdb, and using a “call close(listenfd)” to manually call the function. But is there a way to do this if I am not using gdb (ie. debugging the client application only).

Any help is greatly appreciated.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>

#define BUFFER 80
int main(int argc, char **argv) {
  int listenfd, connfd;
  char buf[BUFFER];
  struct sockaddr_in servaddr;
  time_t ticks;
  struct sockaddr *ptr;
  char *ret;

  if ( (listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
    perror("socket error");
    return 1;
  }

  memset(&servaddr, 0, sizeof(servaddr));
  memset(buf, 0, BUFFER);
  servaddr.sin_family = AF_INET;
  servaddr.sin_port = htons(13);
  servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

  ptr = (struct sockaddr*) &servaddr;
  if ( bind(listenfd, ptr ,sizeof(servaddr)) < 0) {
    perror("bind error");
    return 2;
  }

  if ( listen(listenfd, 128) < 0) {
    perror("listen error");
    return 3;
  }

  ptr = NULL;
  while ( 1 ) {
    if ( connfd = accept(listenfd, ptr, NULL) < 0) {
      perror("accept error");
      return 4;
    } else {
      ticks = time(NULL);
      ret = ctime(&ticks);
      sprintf(buf, "%.24s\n", ret);
      if ( write(connfd, buf, strlen(buf)) < 0) {
        perror("write error");
        close(connfd);
    }
  }

  return 0;
}

Answer 1

Here was my hunch: On a terminal (tty), stdout and stdin are the same physical device. Therefore writing to filedescriptor 0 (stdin) might actually work and result in terminal output.

You need parentheses around this

   if ( connfd = accept(listenfd, ptr, NULL) < 0) {

Like so

  if ( (connfd = accept(listenfd, ptr, NULL)) < 0) {

Or connfd will be assigned ‘0’

Update Just tested this, and this is indeed the culprit. Next time, compile with gcc -Wall and the compiler would have told you this (and several other issues of good form/style). That way, you won’t have to rely on having the hunch to find the error.

Fixed version:

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>

#define BUFFER 80
int main(int argc, char **argv) {
    int listenfd, connfd;
    char buf[BUFFER];
    struct sockaddr_in servaddr;
    time_t ticks;
    struct sockaddr *ptr;
    char *ret;

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    if ( listenfd < 0 ) {
        perror("socket error");
        return 1;
    }

    memset(&servaddr, 0, sizeof(servaddr));
    memset(buf, 0, BUFFER);
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(13);
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

    ptr = (struct sockaddr*) &servaddr;
    if ( bind(listenfd, ptr ,sizeof(servaddr) ) < 0) {
        perror("bind error");
        return 2;
    }

    if ( listen(listenfd, 128) < 0 ) {
        perror("listen error");
        return 3;
    }

    ptr = NULL;
    while ( 1 ) {
        connfd = accept(listenfd, ptr, NULL);
        if ( connfd < 0 ) {
            perror("accept error");
            return 4;
        } else {
            ticks = time(NULL);
            ret = ctime(&ticks);
            sprintf(buf, "%.24s\n", ret);
            if ( write(connfd, buf, strlen(buf)) < 0) {
                perror("write error");
                close(connfd);
            }
        }
    }

    return 0;
}