CODEWITHSUNDEEP
clinuxmessage-queuefreebsd
Aleksei Averchenko
Aleksei Averchenko

Reputation: 1776

FreeBSD doesn't preserve POSIX message queue descriptions over forks

I have a test program:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <mqueue.h>
#include <errno.h>
#include <fcntl.h>

int main() {
  struct mq_attr attrs;
  attrs.mq_maxmsg = 10;
  attrs.mq_msgsize = sizeof(int);

  const char name[] = "/test-queue";

  mqd_t q = mq_open(name, O_CREAT | O_RDWR, 0600, &attrs);
  if (q == (mqd_t)-1) {
    perror("mq_open");
    exit(EXIT_FAILURE);
  }

  mq_unlink(name); // it doesn't matter if I do this at the end or not

  if (fork()) {
    int msg = 666;
    if (mq_send(q, (const char *)&msg, sizeof(msg), 1)) {
      perror("mq_send");
      exit(EXIT_FAILURE);
    }
  } else {
    int msg;
    unsigned priority;
    if (mq_receive(q, (char *)&msg, sizeof(msg), &priority) == -1) {
      perror("mq_receive");
      exit(EXIT_FAILURE);
    }
    printf("%d\n", msg);
  }

  mq_close(q);

  return 0;
}

I compile this program using gcc -std=c99 -Wall -o mqtest mqtest.c -lrt on two platforms:

On Linux, everything works. On FreeBSD, I get mq_receive: Bad file descriptor. Moving the mq_unlink call to the end of main() doesn't help. Is there a way to fix this, or do I have to postpone marking the queue for deletion and reopen it after the fork?

Upvotes: 2

Views: 719

Answers (1)

Roland Smith
Roland Smith

Reputation: 43533

FreeBSD does preserve message queue descriptors. See mq_open(2):

FreeBSD implements message queue based on file descriptor. The descriptor is inherited by child after fork(2). The descriptor is closed in a new image after exec(3). The select(2) and kevent(2) system calls are supported for message queue descriptor.

Edit:

The structure that mqd_t points to does contain a descriptor. But if you test that file descriptor just after the fork() using fcntl(), it also returns EBADF.

This is a bug in FreeBSD. But wether the bug is in the docs or in the implementation I cannot say.

Upvotes: 0

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