The Linux Programming Interface

1074 Chapter 52

Listing 52-4: Writing a message to a POSIX message queue

––––––––––––––––––––––––––––––––––––––––––––––––––––––––– pmsg/pmsg_send.c

#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] name msg [prio]\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;

flags = O_WRONLY;

while ((opt = getopt(argc, argv, "n")) != -1) {

switch (opt) {

case 'n': flags |= O_NONBLOCK; break;

default: usageError(argv[0]);

}

}

if (optind + 1 >= argc)

usageError(argv[0]);

mqd = mq_open(argv[optind], flags);

if (mqd == (mqd_t) -1)

errExit("mq_open");

prio = (argc > optind + 2)? atoi(argv[optind + 2]) : 0;

if (mq_send(mqd, argv[optind + 1], strlen(argv[optind + 1]), prio) == -1)

errExit("mq_send");

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––––– pmsg/pmsg_send.c

52.5.2 Receiving Messages

The mq_receive() function removes the oldest message with the highest priority

from the message queue referred to by mqdes and returns that message in the

buffer pointed to by msg_ptr.