A Simple Daytime Server

#include "unp.h"
#include <time.h>

int main(int argc, char **argv)
{
    int listenfd, connfd;
    struct sockaddr_in    servaddr;
    char buff[MAXLINE];
    time_t ticks;

    listenfd = Socket(AF_INET, SOCK_STREAM, 0);

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(13);    /* daytime server */

    Bind(listenfd, (SA *) &servaddr, sizeof(servaddr));

    Listen(listenfd, LISTENQ);

    for ( ; ; )
    {
        connfd = Accept(listenfd, (SA *) NULL, NULL);

        ticks = time(NULL);
        snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks));
        Write(connfd, buff, strlen(buff));

        Close(connfd);
    }
}

Our server handles only one client at a time. If multiple client connections arrive at about the same time, the kernel queues them, up to some limit, and returns them to accept one at a time. This daytime server, which requires calling two library functions, time and ctime, is quite fast. But if the server took more time to service each client (say a few seconds or a minute), we would need some way to overlap the service of one client with another client.

The server that we show in here is called an iterative server because it iterates through each client, one at a time. There are numerous techniques for writing a concurrent server, one that handles multiple clients at the same time.

int Socket(int family, int type, int protocol)
{
    int n;

    if ((n = socket(family, type, protocol)) < 0)
        err_sys("socket error");

    return(n);
}

void Bind(int fd, const struct sockaddr *sa, socklen_t salen)
{
    if (bind(fd, sa, salen) < 0)
        err_sys("bind error");
}

void Listen(int fd, int backlog)
{
    char *ptr;

    /*4can override 2nd argument with environment variable */
    if ((ptr = getenv("LISTENQ")) != NULL)
        backlog = atoi(ptr);

    if (listen(fd, backlog) < 0)
        err_sys("listen error");
}

int Accept(int fd, struct sockaddr *sa, socklen_t *salenptr)
{
    int n;

again:
    if ((n = accept(fd, sa, salenptr)) < 0)
    {
#ifdef    EPROTO
        if (errno == EPROTO || errno == ECONNABORTED)
#else
        if (errno == ECONNABORTED)
#endif
            goto again;
        else
            err_sys("accept error");
    }

    return (n);
}

void Write(int fd, void *ptr, size_t nbytes)
{
    if (write(fd, ptr, nbytes) != nbytes)
        err_sys("write error");
}

void Close(int fd)
{
    if (close(fd) == -1)
        err_sys("close error");
}