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 */
         }
     }
 }