This is sockets/is_echo_v2_sv.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 60-2 (page 1252).

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download sockets/is_echo_v2_sv.c

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Function list (Bold in this list means a function is not static)

/* is_echo_v2_sv.c

   An implementation of the TCP "echo" service that can either be invoked from
   the command line, or via inetd(8) if we specify the "-i" command-line option.

   If run from the command line, 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.

   If run from inetd(8), place a line similar to the following in
   /etc/inetd.conf (you will need to modify <some-path> as required):

        echo stream tcp nowait root /some-path/is_echo_v2_sv is_echo_v2_sv -i

   Note that this program is very similar to is_echo_sv.c: all we've
   done is add a few extra lines of code to handle the "-i" option.

   See also is_echo_sv.c.
#include <syslog.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include "become_daemon.h"
#include "inet_sockets.h"       /* Declares our 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)
    errno = savedErrno;
static void             /* Handle client: copy socket input back to socket */
handleRequest(int cfd)
    ssize_t numRead;
    char buf[BUF_SIZE];

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

    if (numRead == -1) {
        syslog(LOG_ERR, "Error from read(): %s", strerror(errno));
main(int argc, char *argv[])
    /* The "-i" option means we were invoked from inetd(8), so that
       all we need to do is handle the connection on STDIN_FILENO */

    if (argc > 1 && strcmp(argv[1], "-i") == 0) {

    if (becomeDaemon(0) == -1)

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

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

    for (;;) {
        int cfd = accept(lfd, NULL, NULL);      /* Wait for connection */
        if (cfd == -1) {
            syslog(LOG_ERR, "Failure in accept(): %s",
            continue;           /* Try next */

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

        case 0:                 /* Child */
            close(lfd);         /* Don't need copy of listening socket */

        default:                /* Parent */
            close(cfd);         /* Don't need copy of connected socket */
            break;              /* Loop to accept next connection */


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