This is sockets/is_echo_sv.c (Listing 60-4, page 1252), an example from the book, The Linux Programming Interface.

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

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/* 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)
    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));

    if (numRead == -1) {
        syslog(LOG_ERR, "Error from read(): %s", strerror(errno));
main(int argc, char *argv[])
    int lfd, cfd;               /* Listening and connected sockets */
    struct sigaction sa;

    if (becomeDaemon(0) == -1)

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

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

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

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

        /* 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 */

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


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