sockets/is_echo_sv.c

/* is_echo_sv.c

   An implementation of the TCP "echo" service.

   NOTE: this program must be run under a root login, in order to allow the
   "echo" port (7) to be bound. Alternatively, for test purposes, you can
   replace the SERVICE name below with a suitable unreserved port number
   (e.g., "51000"), and make a corresponding change in the client.

   See also is_echo_cl.c.
*/
#include <signal.h>
#include <syslog.h>
#include <sys/wait.h>
#include "become_daemon.h"
#include "inet_sockets.h"       /* Declarations of inet*() socket functions */
#include "tlpi_hdr.h"

#define SERVICE "echo"          /* Name of TCP service */
#define BUF_SIZE 4096

static void             /* SIGCHLD handler to reap dead child processes */
grimReaper(int sig)
{
    int savedErrno;             /* Save 'errno' in case changed here */

    savedErrno = errno;
    while (waitpid(-1, NULL, WNOHANG) > 0)
        continue;
    errno = savedErrno;
}

/* Handle a client request: copy socket input back to socket */

static void
handleRequest(int cfd)
{
    char buf[BUF_SIZE];
    ssize_t numRead;

    while ((numRead = read(cfd, buf, BUF_SIZE)) > 0) {
        if (write(cfd, buf, numRead) != numRead) {
            syslog(LOG_ERR, "write() failed: %s", strerror(errno));
            exit(EXIT_FAILURE);
        }
    }

    if (numRead == -1) {
        syslog(LOG_ERR, "Error from read(): %s", strerror(errno));
        exit(EXIT_FAILURE);
    }
}

int
main(int argc, char *argv[])
{
    int lfd, cfd;               /* Listening and connected sockets */
    struct sigaction sa;

    if (becomeDaemon(0) == -1)
        errExit("becomeDaemon");

    /* Establish SIGCHLD handler to reap terminated child processes */

    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = grimReaper;
    if (sigaction(SIGCHLD, &sa, NULL) == -1) {
        syslog(LOG_ERR, "Error from sigaction(): %s", strerror(errno));
        exit(EXIT_FAILURE);
    }

    lfd = inetListen(SERVICE, 10, NULL);
    if (lfd == -1) {
        syslog(LOG_ERR, "Could not create server socket (%s)", strerror(errno));
        exit(EXIT_FAILURE);
    }

    for (;;) {
        cfd = accept(lfd, NULL, NULL);  /* Wait for connection */
        if (cfd == -1) {
            syslog(LOG_ERR, "Failure in accept(): %s", strerror(errno));
            exit(EXIT_FAILURE);
        }

        /* Handle each client request in a new child process */

        switch (fork()) {
        case -1:
            syslog(LOG_ERR, "Can't create child (%s)", strerror(errno));
            close(cfd);                 /* Give up on this client */
            break;                      /* May be temporary; try next client */

        case 0:                         /* Child */
            close(lfd);                 /* Unneeded copy of listening socket */
            handleRequest(cfd);
            _exit(EXIT_SUCCESS);

        default:                        /* Parent */
            close(cfd);                 /* Unneeded copy of connected socket */
            break;                      /* Loop to accept next connection */
        }
    }
}