How to use Libev io with mqueue - c

Question

How do i use ev_io with mqueues? I'm trying to do the following with no luck.
#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "ev.h"
#define MAX_Q_SIZE 255
#define MY_QUEUE "/test_queue"
typedef struct __test_ctxt_t
{
ev_timer timeout_watcher[32];
ev_io stdin_watcher;
struct ev_loop *loop;
mqd_t mq;
int data;
}test_ctxt_t;
static test_ctxt_t *g_ctxt = NULL;
static void mq_callback(EV_P_ struct ev_io *w, int revents)
{
test_ctxt_t *ctxt = (test_ctxt_t *)w;
struct mq_attr attr;
char msg[256];
int rcvd_msg_size;
rcvd_msg_size = mq_receive(ctxt->mq, msg, MAX_Q_SIZE, NULL);
if (rcvd_msg_size >= 0)
{
msg[rcvd_msg_size] = '\0';
printf("Received: %s\n", msg);
if (strcmp(msg, "stop") == 0)
{
printf("Exiting....\n");
ev_unloop (EV_A_ EVUNLOOP_ONE);
}
}
}
static void timeout_cb1 (EV_P_ struct ev_timer *w, int revents)
{
puts ("timeout timeout_cb1");
//ev_unloop (EV_A_ EVUNLOOP_ONE);
}
static void timeout_cb2 (EV_P_ struct ev_timer *w, int revents)
{
puts ("timeout timeout_cb2");
//ev_unloop (EV_A_ EVUNLOOP_ONE);
}
static void timeout_cb3 (EV_P_ struct ev_timer *w, int revents)
{
puts ("timeout timeout_cb3");
//ev_unloop (EV_A_ EVUNLOOP_ONE);
}
int main (void)
{
struct mq_attr attr;
g_ctxt = (test_ctxt_t *)calloc(1, sizeof(test_ctxt_t));
g_ctxt->loop = ev_default_loop (0);
/* initialize the queue attributes */
attr.mq_flags = 0;
attr.mq_maxmsg = 10;
attr.mq_msgsize = 255;
g_ctxt->mq = mq_open(MY_QUEUE, O_CREAT | O_RDONLY, 0644, &attr);
if (g_ctxt->mq == -1)
{
printf("Unable to open Queue");
return -1;
}
ev_io_init(&g_ctxt->stdin_watcher, mq_callback, g_ctxt->mq, EV_READ);
ev_io_start(g_ctxt->loop, &g_ctxt->stdin_watcher);
ev_timer_init (&g_ctxt->timeout_watcher[0], timeout_cb1, 10, 0.);
ev_timer_start (g_ctxt->loop, &g_ctxt->timeout_watcher[0]);
ev_loop (g_ctxt->loop, 0);
return 0;
}
I'm able to get the timer call back but io callback is never called on sending a messsage to the queue. Is it possible to use POSIX mqueue with libev ?

Answer 1

There are basically three ways, how to do that:
1) On Linux platform, the message queue handler is a valid file descriptor.
Polling message queue descriptors
On Linux, a message queue descriptor is actually a file descriptor,
and can be monitored using select(2), poll(2), or epoll(7). This is
not portable.
From mq_overview man page
You can utilize ev_io watcher to listen for incoming messages.
This is LINUX specific and non-portable code.
2) You can use function mq_notify to configure a signal that will be send whenever then message queue receives a new message. This signal can be handled by a ev_signal watcher from libev.
This seems to be more portable code but due to a signal interaction, the performance will be a bit lower.
See mq_notify man page.
3) Switch to datagram-oriented sockets, UNIX or network. These offer the roughly equal ability to send a message (aka datagram). You will get connectionless, best effort delivery, unreliable, message delivery service.
This approach is very portable.
Macintosh note:
There is no POSIX message queue implementation in macOS so the portability of such a code is limited in regards to this Apple platform. There is a possibility to mimic the same function on NSOperationQueue or Grand Central Dispatch but it will certainly result in a extensive amount of the work.