kernel/tasks: implement function to get process control block by ID

hermit/include/hermit/tasks.h

@@ -190,6 +190,18 @@ int wakeup_task(tid_t);
  */
 int block_current_task(void);
 
+/** @brief Get a process control block
+ *
+ * @param id	ID of the task to retrieve
+ * @param task	Location to store pointer to task
+ * @return
+ *  - 0 on success
+ *  - -ENOMEM (-12) if @p task is NULL
+ *  - -ENOENT ( -2) if @p id not in task table
+ *  - -EINVAL (-22) if there's no valid task with @p id
+ */
+int get_task(tid_t id, task_t** task);
+
 /** @brief Block current task until timer expires
  *
  * @param deadline Clock tick, when the timer expires

hermit/kernel/tasks.c

@@ -877,6 +877,26 @@ get_task_out:
 	return NULL;
 }
 
+
+int get_task(tid_t id, task_t** task)
+{
+	if (BUILTIN_EXPECT(task == NULL, 0)) {
+		return -ENOMEM;
+	}
+
+	if (BUILTIN_EXPECT(id >= MAX_TASKS, 0)) {
+		return -ENOENT;
+	}
+
+	if (BUILTIN_EXPECT(task_table[id].status == TASK_INVALID, 0)) {
+		return -EINVAL;
+	}
+
+	*task = &task_table[id];
+
+	return 0;
+}
+
 void reschedule(void)
 {
 	size_t** stack;