This is pipes/fifo_seqnum_server.c (Listing 44-7, page 912), 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_server.c

   An example of a server using a FIFO to handle client requests.
   The "service" provided is the allocation of unique sequential
   numbers. Each client submits a request consisting of its PID, and
   the length of the sequence it is to be allocated by the server.
   The PID is used by both the server and the client to construct
   the name of the FIFO used by the client for receiving responses.

   The server reads each client request, and uses the client's FIFO
   to send back the starting value of the sequence allocated to that
   client. The server then increments its counter of used numbers
   by the length specified in the client request.

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

   The client is in fifo_seqnum_client.c.
#include <signal.h>
#include "fifo_seqnum.h"
main(int argc, char *argv[])
    int serverFd, dummyFd, clientFd;
    char clientFifo[CLIENT_FIFO_NAME_LEN];
    struct request req;
    struct response resp;
    int seqNum = 0;                     /* This is our "service" */

    /* Create well-known FIFO, and open it for reading */

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

    /* Open an extra write descriptor, so that we never see EOF */

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

    /* Let's find out about broken client pipe via failed write() */

    if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)    errExit("signal");

    for (;;) {                          /* Read requests and send responses */
        if (read(serverFd, &req, sizeof(struct request))
                != sizeof(struct request)) {
            fprintf(stderr, "Error reading request; discarding\n");
            continue;                   /* Either partial read or error */

        /* Open client FIFO (previously created by client) */

        snprintf(clientFifo, CLIENT_FIFO_NAME_LEN, CLIENT_FIFO_TEMPLATE,
                (long) req.pid);
        clientFd = open(clientFifo, O_WRONLY);
        if (clientFd == -1) {           /* Open failed, give up on client */
            errMsg("open %s", clientFifo);

        /* Send response and close FIFO */

        resp.seqNum = seqNum;
        if (write(clientFd, &resp, sizeof(struct response))
                != sizeof(struct response))
            fprintf(stderr, "Error writing to FIFO %s\n", clientFifo);
        if (close(clientFd) == -1)

        seqNum += req.seqLen;           /* Update our sequence number */


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