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POSystem V Msg Queues on Linux don't work as expected
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copied!<p>I have the following application which replicates an issue I'm having in a larger application with system v message queues. basically, the main function generates a key, then creates a message queue with msgget(). Then 3 forks are spawned, each with a different id. each of them runs msgrcv with a different posative number (so they are waiting for different messages).</p> <p>Main then sleeps for a few seconds a sends a message to id = 3. However it <em>isn't</em> the third thread that wakes up but a different one instead. This code is completely isolated so you can try it out yourself. What's wrong with this code?</p> <pre><code>#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> #include <sys/wait.h> struct dummy_struct { long mtype; char message[255]; }; int msg_queue_id; void recv_thread(int id); int main(int argc, char **argv) { int i; key_t key; struct dummy_struct dummy = { 3, "hello" }; //create a unique key if (key = ftok("/mnt/mydocuments/code/sys_v_fork_test/main.c", 'a') == -1) { printf("ftok didn't work\n"); exit(1); } //create the unix sys 5 message queue if ((msg_queue_id = msgget(key, 0644 | IPC_CREAT)) == -1) { printf("msgget failed\n"); exit(1); } else printf("my message queue id: %i\n", msg_queue_id); //fork off multiple recievers for (i = 1; i < 4; i++) // <- NOTE: 1 -> 4 { if (fork() == 0) recv_thread(i); } printf("sleeping\n"); sleep(5); //wait a little then send a message printf("sending message\n"); if (msgsnd(msg_queue_id, &dummy, sizeof(struct dummy_struct), 0) == -1) { printf("msgsnd failed\n"); } printf("main thread exiting"); _exit(0); } void recv_thread(int id) { struct dummy_struct dummy; printf("recv_thread with id: %i\n", id); if (msgrcv(msg_queue_id, &dummy, sizeof(struct dummy_struct), id, 0) == -1) printf("error in msgrcv\n"); else printf("thread %i got %s back\n", id, dummy.message); } </code></pre> <p>If I wait for 2 that means messages whose struct contains a mtype set to <em>exactly</em> 2. 3 for 3 and so one. My point of reference was this guide: <a href="http://www.ecst.csuchico.edu/~beej/guide/ipc/mq.html" rel="nofollow noreferrer">http://www.ecst.csuchico.edu/~beej/guide/ipc/mq.html</a>. Can anyone help please? (you may need to modify the ftok line of code to point to a valid file on your own machine to test successfully). I'm running Fedora 10 on an EeePC 1000H</p>

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