diff --git a/src/lcdbuf.h b/src/lcdbuf.h
index a0342ca..a24b689 100644
--- a/src/lcdbuf.h
+++ b/src/lcdbuf.h
@@ -94,12 +94,12 @@ void LcdBuffer_Write(struct LcdBuffer *self, lcdbuf_pos_t row, lcdbuf_pos_t col,
 /* Callbacks - need to be implemented by the application! */
 
 /** Clear the entire screen (CGRAM can be left unchanged, but they will be written anew if needed) */
-void LcdBuffer_IO_Clear();
+extern void LcdBuffer_IO_Clear();
 
 /** Write character data at position */
-void LcdBuffer_IO_WriteAt(lcdbuf_pos_t row, lcdbuf_pos_t col, const uint8_t *buf, lcdbuf_count_t len);
+extern void LcdBuffer_IO_WriteAt(lcdbuf_pos_t row, lcdbuf_pos_t col, const uint8_t *buf, lcdbuf_count_t len);
 
 /** Write CGRAM data. Data is always 8 bytes long. */
-void LcdBuffer_IO_WriteCGRAM(uint8_t position, const uint8_t* data);
+extern void LcdBuffer_IO_WriteCGRAM(uint8_t position, const uint8_t* data);
 
 #endif //HD44780UTF_LCDBUF_H