This is sockets/is_echo_cl.c (Listing 61-2, page 1258), 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_cl.c

   A simple client to communicate with the standard "echo" server.

   In many Linux distributions, the "echo" server is not started by default.
   Normally it is implemented internally by inetd(8), and to enable we must
   do the following *as root*:

   1.   Edit the file /etc/inetd.conf, and uncomment the two lines
        for the the "echo" service by removing the '#' character at the
        beginning of the line. The lines typically look like this:

                # echo    stream  tcp     nowait  root    internal
                # echo    dgram   udp     wait    root    internal

        and we must change them to this:

                echo    stream  tcp     nowait  root    internal
                echo    dgram   udp     wait    root    internal

   2.   Inform inetd(8) of the change using the following command:

                bash# killall -HUP inetd

   If your system uses xinetd(8) instead of inetd(8), then read the
   xinetd.conf(5) manual page. (You may also find that your system has a GUI
   admin tool that allows you to easily enable/disable the "echo" service.)

   See also is_echo_sv.c.
#include "inet_sockets.h"
#include "tlpi_hdr.h"

#define BUF_SIZE 100
main(int argc, char *argv[])
    int sfd;
    ssize_t numRead;
    char buf[BUF_SIZE];

    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s host\n", argv[0]);

    sfd = inetConnect(argv[1], "echo", SOCK_STREAM);
    if (sfd == -1)

    switch (fork()) {
    case -1:

    case 0:             /* Child: read server's response, echo on stdout */
        for (;;) {
            numRead = read(sfd, buf, BUF_SIZE);
            if (numRead <= 0)                   /* Exit on EOF or error */
            printf("%.*s", (int) numRead, buf);

    default:            /* Parent: write contents of stdin to socket */
        for (;;) {
            numRead = read(STDIN_FILENO, buf, BUF_SIZE);
            if (numRead <= 0)                   /* Exit loop on EOF or error */
            if (write(sfd, buf, numRead) != numRead)
                fatal("write() failed");

        /* Close writing channel, so server sees EOF */

        if (shutdown(sfd, SHUT_WR) == -1)


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