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Date: Wed, 30 Apr 2008 20:31:39 +0200
From: Philippe Gerum <rpm@xenomai.org>
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Sender: Philippe Gerum <philippe.gerum@domain.hid>
Subject: Re: [Xenomai-help] Segfault Problems with Message Queues
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To: "Karch, Joshua" <jkarch@domain.hid>
Cc: xenomai@xenomai.org

Karch, Joshua wrote:
> Hi All,
> 
> I tried implementing inter-task message queues and following the
> msg_queue.c example
> (http://www.xenomai.org/documentation/trunk/html/api/msg__queue_8c-example.html)
> and when I try to run the implementation after fixing some of the code
> (declaring fail() to printf an error, declaring int err, and changing
> &task_body to &consumer), I get the following error
> msg_queue[3897]: segfault at 00000000 eip b7cf8933 esp bf8dce04 error
> 6.  This comes out of the rt_queue_alloc function.
> 
> Has the implementation of message passing between userspace tasks
> changed?  Is there a more recent example?  My end goal is to send non
> blocking messages from one producer task at 50Hz to two reading tasks
> tasks with similar (though not exactly same) loop rates-- i.e. None of
> the tasks are synchronized, though they all run at about 50Hz.  Should I
> use message queues to do this?
> 
> Thank you,
> 
> Joshua Karch
> 
> Attached is the code I used..
> 
> #include <sys/mman.h>
> #include <stdio.h>
> #include <string.h>
> #include <native/task.h>
> #include <native/queue.h>
> 
> #define TASK_PRIO  99 /* Highest RT priority */
> #define TASK_MODE  0  /* No flags */
> #define TASK_STKSZ 0  /* Stack size (use default one) */
> 
> RT_QUEUE q_desc;
> 
> RT_TASK task_desc;
> 

We need to know whether an error actually occurred. Nothing guarantees us that
all stdio buffers would be flushed upon SEGV. Does the executable still behave
the same way with that patch?

> void fail()
> {
>         printf("error\n");

+	exit(99);
> }
> 
> 
> void consumer (void *cookie)
> 
> {
>     ssize_t len;
>     void *msg;
>     int err;
> 
>     /* Bind to a queue which has been created elsewhere, either in
>        kernel or user-space. The call will block us until such queue
>        is created with the expected name. The queue should have been
>        created with the Q_SHARED mode set, which is implicit when
>        creation takes place in user-space. */
> 
>     err = rt_queue_bind(&q_desc,"SomeQueueName",TM_INFINITE);
> 
>     if (err)
>         fail();
> 
>     /* Collect each message sent to the queue by the queuer() routine,
>        until the queue is eventually removed from the system by a call
>        to rt_queue_delete(). */
> 
>     while ((len = rt_queue_receive(&q_desc,&msg,TM_INFINITE)) > 0)
>         {
>         printf("received message> len=%d bytes, ptr=%p, s=%s\n",
>                len,msg,(const char *)msg);
>         rt_queue_free(&q_desc,msg);
>         }
> 
>     /* We need to unbind explicitly from the queue in order to
>        properly release the underlying memory mapping. Exiting the
>        process unbinds all mappings automatically. */
> 
>     rt_queue_unbind(&q_desc);
> 
>     if (len != -EIDRM)
>         /* We received some unexpected error notification. */
>         fail();
> 
>     /* ... */
> }
> 
> 
> int main (int argc, char *argv[])
> 
> {
>     static char *messages[] = { "hello", "world", NULL };
>     int n, len;
>     void *msg;
>     int err;
> 
>     mlockall(MCL_CURRENT|MCL_FUTURE);
> 
>     err = rt_task_create(&task_desc,
>                          "MyTaskName",
>                          TASK_STKSZ,
>                          TASK_PRIO,
>                          TASK_MODE);
>     if (!err)
>         rt_task_start(&task_desc,&consumer,NULL);
> 
>     /* ... */
> 
>     for (n = 0; messages[n] != NULL; n++)
>         {
>         len = strlen(messages[n]) + 1;
>         /* Get a message block of the right size. */
>         msg = rt_queue_alloc(&q_desc,len);
> 
>         if (!msg)
>             /* No memory available. */
>             fail();
> 
>         strcpy((char *)msg,messages[n]);
>         rt_queue_send(&q_desc,msg,len,Q_NORMAL);
>         }
> 
>     rt_task_delete(&task_desc);
> }
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Xenomai-help mailing list
> Xenomai-help@domain.hid
> https://mail.gna.org/listinfo/xenomai-help


-- 
Philippe.