py: Fix handling of negative numbers in struct.pack of q/Q.

1 files changed, 4 insertions, 0 deletions

diff --git a/py/binary.c b/py/binary.c
index 8b5c05ab3..ee00c4e34 100644
--- a/py/binary.c
+++ b/py/binary.c
@@ -291,6 +291,10 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte **
             #endif
             {
                 val = mp_obj_get_int(val_in);
+                // sign extend if needed
+                if (BYTES_PER_WORD < 8 && size > sizeof(val) && is_signed(val_type) && (mp_int_t)val < 0) {
+                    memset(p + sizeof(val), 0xff, size - sizeof(val));
+                }
             }
     }