Sys V IPC msgsnd(), msgrcv() after fork()

Question

I have a program running on Linux that fork()s after a TCP connection was accept()ed. Before the fork, it connects to a message queue via msgget() and happily sends and receives messages. At some point in the program, both the parent and the child will be waiting at the same time on a msgrcv() using the same msgtype. A separate process then sends a message via msgsnd() using this same msgtype.

However, only one of the forked processes returns from msgrcv(), and it also seems to depend on the path, the parent and the child took. It is very repeatable. In one case, only the parent receives the message, in another case only the child receives the message, leaving the other one waiting infinitely.

Does anyone have a hint on what could go wrong and why not both the parent and the child always receive the message?

I wrote two little test programs, recv.c and send.c, see below. It turns out that the parent and the child only receive every other message. It seems to be strictly "every other", not even by chance which of the two receives a message. This would very well explain what's happening to my software.

Is this how message queues are supposed to work? Can I not send a message to multiple recipients?

/* recv.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/wait.h>

int main(void)
{
  int msgid = msgget(247, 0666 | IPC_CREAT);
  pid_t cldpid = fork();
  struct msgform 
  {
    long mtype;
    char mbuf[16];
  } msg;
  msg.mtype = 1;
  if (cldpid == 0) 
  {
    while(true)
    {
      printf("Child waiting\n");
      msgrcv(msgid, &msg, sizeof(msg), 1, 0);
      printf("Child done\n");
    }
  }
  while(true)
  {
    printf("Parent waiting\n");
    msgrcv(msgid, &msg, sizeof(msg), 1, 0);
    printf("Parent done\n");
  }
  return 0;
}

and

/* send.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/wait.h>

int main(void)
{
  int msgid = msgget(247, 0666 | IPC_CREAT);
  struct msgform {
    long mtype;
    char mbuf[16];
  } msg;
  msg.mtype = 1;
  msgsnd(msgid, &msg, sizeof(msg), IPC_NOWAIT);
  return 0;
}

Thanks