This is pipes/fifo_seqnum_client.c (Listing 44-8, page 914), 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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/* fifo_seqnum_client.c

   A simple client that uses a well-known FIFO to request a (trivial)
   "sequence number service". This client creates its own FIFO (using a
   convention agreed upon by client and server) which is used to receive a reply
   from the server. The client then sends a request to the server consisting of
   its PID and the length of the sequence it wishes to be allocated. The client
   then reads the server's response and displays it on stdout.

   See fifo_seqnum.h for the format of request and response messages.

   The server is in fifo_seqnum_server.c.
#include "fifo_seqnum.h"

static char clientFifo[CLIENT_FIFO_NAME_LEN];
static void             /* Invoked on exit to delete client FIFO */
main(int argc, char *argv[])
    int serverFd, clientFd;
    struct request req;
    struct response resp;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [seq-len]\n", argv[0]);

    /* Create our FIFO (before sending request, to avoid a race) */

    umask(0);                   /* So we get the permissions we want */
            (long) getpid());
    if (mkfifo(clientFifo, S_IRUSR | S_IWUSR | S_IWGRP) == -1
                && errno != EEXIST)
        errExit("mkfifo %s", clientFifo);

    if (atexit(removeFifo) != 0)

    /* Construct request message, open server FIFO, and send message */

    req.pid = getpid();
    req.seqLen = (argc > 1) ? getInt(argv[1], GN_GT_0, "seq-len") : 1;

    serverFd = open(SERVER_FIFO, O_WRONLY);
    if (serverFd == -1)
        errExit("open %s", SERVER_FIFO);

    if (write(serverFd, &req, sizeof(struct request)) !=
            sizeof(struct request))
        fatal("Can't write to server");

    /* Open our FIFO, read and display response */

    clientFd = open(clientFifo, O_RDONLY);
    if (clientFd == -1)
        errExit("open %s", clientFifo);

    if (read(clientFd, &resp, sizeof(struct response))
            != sizeof(struct response))
        fatal("Can't read response from server");

    printf("%d\n", resp.seqNum);


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