1 files changed, 5 insertions, 0 deletions

diff --git a/src/buffer.h b/src/buffer.h
index 8c64a24e804..3c91bdfe570 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -306,6 +306,11 @@ do								\
   }                                                             \
 while (0)
 
+/* Maximum number of bytes in a buffer.
+   A buffer cannot contain more bytes than a 1-origin fixnum can represent,
+   nor can it be so large that C pointer arithmetic stops working.  */
+#define BUF_BYTES_MAX min (MOST_POSITIVE_FIXNUM - 1, min (SIZE_MAX, PTRDIFF_MAX))
+
 /* Return the address of byte position N in current buffer.  */
 
 #define BYTE_POS_ADDR(n) \