From a94f6aa25ffa074fa88752a1953f1badd63c2afe Mon Sep 17 00:00:00 2001
From: jyjsunny <jyjiangsunny@gmail.com>
Date: Thu, 11 Dec 2025 07:37:09 +0000
Subject: [PATCH] add_unit_id_check

---
 src/modbus.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/modbus.c b/src/modbus.c
index e3737bb2f..2c1209e2e 100644
--- a/src/modbus.c
+++ b/src/modbus.c
@@ -806,6 +806,12 @@ int modbus_reply(modbus_t *ctx,
     sft.function = function;
     sft.t_id = ctx->backend->get_response_tid(req);
 
+    if (slave != 0 && slave != 255) {
+        errno = EINVAL;
+        printf("The Unit Identifier must be 0x00 or 0xFF\n");
+        return -1;
+    }
+
     /* Data are flushed on illegal number of values errors. */
     switch (function) {
     case MODBUS_FC_READ_COILS: