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).
Main then sleeps for a few seconds a sends a message to id = 3. However it isn’t 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?
#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); } If I wait for 2 that means messages whose struct contains a mtype set to exactly 2. 3 for 3 and so one. My point of reference was this guide: http://www.ecst.csuchico.edu/~beej/guide/ipc/mq.html. 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
Ahh think i’ve fixed it. It’s because I was using an int rather than a long for the first member set in the structure for the ‘mtype’. passing in { 1l, ‘hello’ } instead and changing the definition of i to long seems to have fixed it