#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>

#include <unistd.h>
#include <string.h>
#include <stdio.h>

#define    QLEN           5    /* maximum connection queue length    */#define    BUFSIZE        4096

externint    errno;
int        errexit(constchar *format, ...);
int        passiveTCP(constchar *service, int qlen);
int        echo(int fd);

/*------------------------------------------------------------------------ * main - Concurrent TCP server for ECHO service *------------------------------------------------------------------------ */int
main(int argc, char *argv[])
{
    char    *service = "echo";    /* service name or port number    */struct sockaddr_in fsin;    /* the from address of a client    */int    msock;            /* master server socket        */
    fd_set    rfds;            /* read file descriptor set    */
    fd_set    afds;            /* active file descriptor set    */int    alen;            /* from-address length        */int    fd, nfds;
    
    switch (argc) {
    case    1:
        break;
    case    2:
        service = argv[1];
        break;
    default:
        errexit("usage: TCPmechod [port]\n");
    }

    msock = passiveTCP(service, QLEN);

    nfds = getdtablesize();
    FD_ZERO(&afds);
    FD_SET(msock, &afds);

    while (1) {
        memcpy(&rfds, &afds, sizeof(rfds));

        if (select(nfds, &rfds, (fd_set *)0, (fd_set *)0,
                (struct timeval *)0) < 0)
            errexit("select: %s\n", strerror(errno));
        if (FD_ISSET(msock, &rfds)) {
            int    ssock;

            alen = sizeof(fsin);
            ssock = accept(msock, (struct sockaddr *)&fsin,
                &alen);
            if (ssock < 0)
                errexit("accept: %s\n",
                    strerror(errno));
            FD_SET(ssock, &afds);
        }
        for (fd=0; fd<nfds; ++fd)
            if (fd != msock && FD_ISSET(fd, &rfds))
                if (echo(fd) == 0) {
                    (void) close(fd);
                    FD_CLR(fd, &afds);
                }
    }
}

/*------------------------------------------------------------------------ * echo - echo one buffer of data, returning byte count *------------------------------------------------------------------------ */int
echo(int fd)
{
    char    buf[BUFSIZ];
    int    cc;

    cc = read(fd, buf, sizeof buf);
    if (cc < 0)
        errexit("echo read: %s\n", strerror(errno));
    if (cc && write(fd, buf, cc) < 0)
        errexit("echo write: %s\n", strerror(errno));
    return cc;
}