--- a/mb_ds_util.h	Mon Mar 22 14:57:48 2021 +0100
+++ b/mb_ds_util.h	Mon Jun 07 11:21:26 2021 +0200
@@ -97,6 +97,7 @@
         int data_start;     /* offset within *data were valid data starts */
         int data_end;       /* offset within *data were valid data ends   */
         int max_data_start; /* optimization parameter! When should it be normalised? */
+        int marked_for_purge; /* Number of bytes to be deleted in next call to lb_data_purge() */
         } lb_buf_t;
 
 /* NOTE: lb = Linear Buffer */
@@ -111,6 +112,7 @@
   buf->data_size  = size;
   buf->data_start = 0;
   buf->data_end   = 0;
+  buf->marked_for_purge = 0;
   buf->max_data_start = max_data_start;
   buf->data = (u8 *)malloc(size);
   return buf->data;
@@ -122,9 +124,12 @@
 }
 
 static inline u8 *lb_normalize(lb_buf_t *buf) {
-  return (u8 *)memmove(buf->data,
-                       buf->data + buf->data_start,
-                       buf->data_end - buf->data_start);
+  u8 *ptr = (u8 *)memmove(buf->data,
+                          buf->data + buf->data_start,
+                          buf->data_end - buf->data_start);
+  buf->data_end -= buf->data_start;
+  buf->data_start = 0;
+  return ptr;
 }
 
 static inline u8 *lb_data(lb_buf_t *buf) {
@@ -140,12 +145,17 @@
     buf->data_end = buf->data_size - 1;
 }
 
+static inline void lb_data_mark_for_purge(lb_buf_t *buf, int count) {
+  buf->marked_for_purge += count;
+}
+
 static inline u8 *lb_data_purge(lb_buf_t *buf, int count) {
-  buf->data_start += count;
-  if (buf->data_start > buf->data_end)
-    buf->data_start = buf->data_end;
+  buf->data_start += count + buf->marked_for_purge;
+  buf->marked_for_purge = 0;
 
-  if ((buf->data_end == buf->data_size) || (buf->data_start >= buf->max_data_start))
+  if      (buf->data_start >= buf->data_end)
+    buf->data_start = buf->data_end = 0; // no bytes in buffer, might just as well normalize it
+  else if ((buf->data_end == buf->data_size) || (buf->data_start >= buf->max_data_start))
     return lb_normalize(buf);
 
   return buf->data + buf->data_start;
@@ -153,6 +163,7 @@
 
 static inline void lb_data_purge_all(lb_buf_t *buf) {
   buf->data_start = buf->data_end = 0;
+  buf->marked_for_purge = 0;
 }
 
 static inline u8 *lb_free(lb_buf_t *buf) {

--- a/mb_rtu.c	Mon Mar 22 14:57:48 2021 +0100
+++ b/mb_rtu.c	Mon Jun 07 11:21:26 2021 +0200
@@ -1422,8 +1422,11 @@
 #endif
     /* set the data pointer */
   *recv_data_ptr = lb_data(&(buf->data_buf));
-    /* remove the frame bytes off the buffer */
-  lb_data_purge(&(buf->data_buf), frame_length);
+    /* Mark the frame bytes to be deleted off the buffer by the next call to lb_data_purge() */
+	/* Notice that we cannot delete the frame bytes right away, as they will still be accessed by whoever
+	 * called read_frame(). We can only delete this data on the next call to read_frame() 
+	 */
+  lb_data_mark_for_purge(&(buf->data_buf), frame_length);
     /* reset the search_history flag */
   buf->frame_search_history = 0;
     /* if the buffer becomes empty, then reset boundary flag */
@@ -1532,6 +1535,16 @@
   *recv_data_ptr = NULL;
 
   /*===================================*
+   * Delete any previously received data that has already been returned as a valid frame in *
+   *===================================*/  
+   /* Delete any previously received data that has already been returned as a valid frame in 
+    * the previous invocation of read_frame().
+	* Notice that the data that will be deleted hass ben marked for deletion by calling
+	* lb_data_mark_for_purge() in return_frame()
+	*/
+  lb_data_purge(&(recv_buf->data_buf), 0);
+
+  /*===================================*
    * Check for frame in left over data *
    *===================================*/
   /* If we have any data left over from previous call to read_frame()

--- a/mb_rtu_private.h	Mon Mar 22 14:57:48 2021 +0100
+++ b/mb_rtu_private.h	Mon Jun 07 11:21:26 2021 +0200
@@ -49,7 +49,7 @@
   * due to the algorithm used to work around aborted frames.
   */
 #define RECV_BUFFER_SIZE_SMALL  (MAX_RTU_FRAME_LENGTH + 10)
-#define RECV_BUFFER_SIZE_LARGE  (2 * MAX_RTU_FRAME_LENGTH)
+#define RECV_BUFFER_SIZE_LARGE  (2 * (MAX_RTU_FRAME_LENGTH))
 
 
 /* Frame lengths... */

--- a/mb_util.h	Mon Mar 22 14:57:48 2021 +0100
+++ b/mb_util.h	Mon Jun 07 11:21:26 2021 +0200
@@ -86,7 +86,7 @@
 #define TCP_HEADER_LENGTH       6
 
  /* Global Frame sizes */
-#define MAX_RTU_FRAME_LENGTH MAX_L2_FRAME_LENGTH + RTU_FRAME_CRC_LENGTH
+#define MAX_RTU_FRAME_LENGTH (MAX_L2_FRAME_LENGTH + RTU_FRAME_CRC_LENGTH)
 #define MAX_ASC_FRAME_LENGTH ((MAX_L2_FRAME_LENGTH * L2_TO_ASC_CODING) +             \
                               ASC_FRAME_HEADER_LENGTH + ASC_FRAME_TAIL_LENGTH +      \
                               ASC_FRAME_LRC_LENGTH)