core/msg: make msg_avail() return 0 on no queue

For the caller there should be no difference if there is no message
in the queue and if there can't be a message in the queue.

The current API works as one would expect if there is a message queue,
but once called from a thread that does not have a message queue
configured, code that does

    while (msg_avail())

will end up in an infinite loop.

Remove this foot-gun from the API by making the return value of
msg_avail() independend of the availability of a message queue.

core/include/msg.h

@@ -362,9 +362,9 @@ int msg_reply_int(msg_t *m, msg_t *reply);
  * @brief Check how many messages are available in the message queue
  *
  * @return Number of messages available in our queue on success
- * @return -1, if no caller's message queue is initialized
+ * @return 0, if no caller's message queue is initialized
  */
-int msg_avail(void);
+unsigned msg_avail(void);
 
 /**
  * @brief Initialize the current thread's message queue.

core/msg.c

@@ -433,20 +433,20 @@ static int _msg_receive(msg_t *m, int block)
     DEBUG("This should have never been reached!\n");
 }
 
-int msg_avail(void)
+unsigned msg_avail(void)
 {
     DEBUG("msg_available: %" PRIkernel_pid ": msg_available.\n",
           thread_getpid());
 
     thread_t *me = thread_get_active();
 
-    int queue_index = -1;
+    unsigned queue_count = 0;
 
     if (thread_has_msg_queue(me)) {
-        queue_index = cib_avail(&(me->msg_queue));
+        queue_count = cib_avail(&(me->msg_queue));
     }
 
-    return queue_index;
+    return queue_count;
 }
 
 void msg_init_queue(msg_t *array, int num)
@@ -462,11 +462,11 @@ void msg_queue_print(void)
     unsigned state = irq_disable();
     thread_t *thread = thread_get_active();
 
-    int msg_counter = msg_avail();
+    unsigned msg_counter = msg_avail();
 
-    if (msg_counter <= -1) {
+    if (msg_counter < 1) {
         /* no msg queue */
-        printf("No message queue\n");
+        printf("No messages or no message queue\n");
         return;
     }
     cib_t *msg_queue = &thread->msg_queue;
@@ -474,9 +474,9 @@ void msg_queue_print(void)
     int first_msg = cib_peek(msg_queue);
 
     printf("Message queue of thread %" PRIkernel_pid "\n", thread->pid);
-    printf("    size: %u (avail: %d)\n", msg_queue->mask + 1, msg_counter);
+    printf("    size: %u (avail: %u)\n", msg_queue->mask + 1, msg_counter);
 
-    for (int i = 0; i < msg_counter; i++) {
+    for (unsigned i = 0; i < msg_counter; i++) {
         msg_t *m = &msg_array[(first_msg + i) & msg_queue->mask];
         printf("    * %u: sender: %" PRIkernel_pid ", type: 0x%04" PRIu16
                ", content: %" PRIu32 " (%p)\n", i, m->sender_pid, m->type,

tests/msg_avail/main.c

@@ -39,7 +39,7 @@ int main(void)
     puts("[START]");
 
     /* add message to own queue */
-    for (int idx = 0; idx < MSG_QUEUE_LENGTH; ++idx) {
+    for (unsigned idx = 0; idx < MSG_QUEUE_LENGTH; ++idx) {
         msges[idx].type = idx;
         msg_send_to_self(msges + idx);
         LOG_INFO("+ add msg: %d\n", idx);
@@ -49,11 +49,11 @@ int main(void)
         }
     }
     /* receive available messages in queue */
-    for (int idx = msg_avail(); idx > 0; --idx) {
+    for (unsigned idx = msg_avail(); idx > 0; --idx) {
         msg_t msg;
         msg_receive(&msg);
         LOG_INFO("- got msg: %d\n", (MSG_QUEUE_LENGTH - idx));
-        if ((int)msg.type != (MSG_QUEUE_LENGTH - idx)
+        if (msg.type != (MSG_QUEUE_LENGTH - idx)
             || msg_avail() != idx - 1) {
             puts("[FAILED]");
             return 1;

tests/msg_queue_print/tests/01-run.py

@@ -14,9 +14,8 @@ from testrunner import run
 
 
 def testfunc(child):
-    child.expect("No message queue")
+    child.expect("No messages or no message queue")
-    child.expect(r"Message queue of thread \d+\r\n")
+    child.expect("No messages or no message queue")
-    child.expect_exact('size: 8 (avail: 0)')
     child.expect(r"Message queue of thread \d+\r\n")
     child.expect_exact('size: 8 (avail: 8)')
     if os.environ.get('BOARD') == 'native':