This is svmsg/svmsg_file_client.c (Listing 46-9, page 960), 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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/* svmsg_file_client.c

   Send a message to the server svmsg_file_server.c requesting the
   contents of the file named on the command line, and then receive the
   file contents via a series of messages sent back by the server. Display
   the total number of bytes and messages received. The server and client
   communicate using System V message queues.
#include "svmsg_file.h"

static int clientId;
static void
    if (msgctl(clientId, IPC_RMID, NULL) == -1)
main(int argc, char *argv[])
    struct requestMsg req;
    struct responseMsg resp;
    int serverId, numMsgs;
    ssize_t msgLen, totBytes;

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

    if (strlen(argv[1]) > sizeof(req.pathname) - 1)
        cmdLineErr("pathname too long (max: %ld bytes)\n",
                (long) sizeof(req.pathname) - 1);
    /*FIXME: above: should use %zu here, and remove (long) cast */

    /* Get server's queue identifier; create queue for response */

    serverId = msgget(SERVER_KEY, S_IWUSR);
    if (serverId == -1)
        errExit("msgget - server message queue");

    clientId = msgget(IPC_PRIVATE, S_IRUSR | S_IWUSR | S_IWGRP);
    if (clientId == -1)
        errExit("msgget - client message queue");

    if (atexit(removeQueue) != 0)

    /* Send message asking for file named in argv[1] */

    req.mtype = 1;                      /* Any type will do */
    req.clientId = clientId;
    strncpy(req.pathname, argv[1], sizeof(req.pathname) - 1);
    req.pathname[sizeof(req.pathname) - 1] = '\0';
                                        /* Ensure string is terminated */

    if (msgsnd(serverId, &req, REQ_MSG_SIZE, 0) == -1)

    /* Get first response, which may be failure notification */

    msgLen = msgrcv(clientId, &resp, RESP_MSG_SIZE, 0, 0);
    if (msgLen == -1)

    if (resp.mtype == RESP_MT_FAILURE) {
        printf("%s\n",;      /* Display msg from server */

    /* File was opened successfully by server; process messages
       (including the one already received) containing file data */

    totBytes = msgLen;                  /* Count first message */
    for (numMsgs = 1; resp.mtype == RESP_MT_DATA; numMsgs++) {
        msgLen = msgrcv(clientId, &resp, RESP_MSG_SIZE, 0, 0);
        if (msgLen == -1)

        totBytes += msgLen;

    printf("Received %ld bytes (%d messages)\n", (long) totBytes, numMsgs);
    /*FIXME: above: should use %zd here, and remove (long) cast (or perhaps
       better, make totBytes size_t, and use %zu)*/



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