Fix: lab3 on-board problems

kernel/lib/ringbuffer.c

@@ -24,11 +24,11 @@ int ringbuffer_push(ringbuffer_t *buf, uint8_t val)
     return ERR_OUT_OF_BOUND;
 
   *buf->write++ = val;
-  ++buf->size;
 
   if (buf->write == buf->data + buf->cap)
     buf->write = buf->data;
 
+  ++buf->size;
   return NO_ERROR;
 }
 
@@ -38,10 +38,18 @@ uint8_t ringbuffer_bump(ringbuffer_t *buf)
     exit(ERR_INVALID_OP);
 
   uint8_t ret = *buf->read++;
-  --buf->size;
 
   if (buf->read == buf->data + buf->cap)
     buf->read = buf->data;
 
+  --buf->size;
   return ret;
 }
+
+uint8_t ringbuffer_peek(ringbuffer_t *buf)
+{
+  if (buf->size == 0)
+    exit(ERR_INVALID_OP);
+
+  return *buf->read;
+}