pmsg/pmsg_receive.c

This is pmsg/pmsg_receive.c (Listing 52-5, page 1076), 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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Function list (Bold in this list means a function is not static)

/* pmsg_receive.c

   Usage as shown in usageError().

   Receive a message from a POSIX message queue, and write it on
   standard output.

   See also pmsg_send.c.

   Linux supports POSIX message queues since kernel 2.6.6.
*/
#include <mqueue.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"
static void
usageError(const char *progName)
{
    fprintf(stderr, "Usage: %s [-n] mq-name\n", progName);
    fprintf(stderr, "    -n           Use O_NONBLOCK flag\n");
    exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
    int flags, opt;
    mqd_t mqd;
    unsigned int prio;
    void *buffer;
    struct mq_attr attr;
    ssize_t numRead;

    flags = O_RDONLY;
    while ((opt = getopt(argc, argv, "n")) != -1) {
        switch (opt) {
        case 'n':   flags |= O_NONBLOCK;        break;
        default:    usageError(argv[0]);
        }
    }

    if (optind >= argc)
        usageError(argv[0]);

    mqd = mq_open(argv[optind], flags);
    if (mqd == (mqd_t) -1)
        errExit("mq_open");

    /* We need to know the 'mq_msgsize' attribute of the queue in
       order to determine the size of the buffer for mq_receive() */

    if (mq_getattr(mqd, &attr) == -1)
        errExit("mq_getattr");

    buffer = malloc(attr.mq_msgsize);
    if (buffer == NULL)
        errExit("malloc");

    numRead = mq_receive(mqd, buffer, attr.mq_msgsize, &prio);
    if (numRead == -1)
        errExit("mq_receive");

    printf("Read %ld bytes; priority = %u\n", (long) numRead, prio);
    /*FIXME: above: should use %zd here, and remove (long) cast */
    if (write(STDOUT_FILENO, buffer, numRead) == -1)
        errExit("write");
    write(STDOUT_FILENO, "\n", 1);

    exit(EXIT_SUCCESS);
}

 

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