From 7bb760e83bffeae02cab97406ca4a6a600c2ee63 Mon Sep 17 00:00:00 2001
From: Ana Rodriguez Lopez <rodrilopez.ana@gmail.com>
Date: Thu, 9 Jan 2025 10:43:37 +0100
Subject: [PATCH] New API functions to access constructed response frame

---
 src/modbus.c | 32 ++++++++++++++++++++++++++++++--
 src/modbus.h |  4 ++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/src/modbus.c b/src/modbus.c
index e3737bb2..f851d2ec 100644
--- a/src/modbus.c
+++ b/src/modbus.c
@@ -783,12 +783,32 @@ int modbus_reply(modbus_t *ctx,
                  const uint8_t *req,
                  int req_length,
                  modbus_mapping_t *mb_mapping)
+{
+    uint8_t rsp[MAX_MESSAGE_LENGTH];
+    int rsp_length;
+
+    rsp_length = modbus_reply_construct(ctx, req, req_length, mb_mapping, rsp);
+    if (rsp_length > 0) {
+        return send_msg(ctx, rsp, rsp_length);
+    }
+    return rsp_length;
+}
+
+/* Analyses the request and constructs a response.
+
+   If an error occurs, this function constructs the response
+   accordingly.
+*/
+int modbus_reply_construct(modbus_t *ctx,
+                           const uint8_t *req,
+                           int req_length,
+                           modbus_mapping_t *mb_mapping,
+                           uint8_t *rsp)
 {
     unsigned int offset;
     int slave;
     int function;
     uint16_t address;
-    uint8_t rsp[MAX_MESSAGE_LENGTH];
     int rsp_length = 0;
     sft_t sft;
 
@@ -1189,7 +1209,15 @@ int modbus_reply(modbus_t *ctx,
         !(ctx->quirks & MODBUS_QUIRK_REPLY_TO_BROADCAST)) {
         return 0;
     }
-    return send_msg(ctx, rsp, rsp_length);
+    return rsp_length;
+}
+
+int modbus_reply_send(modbus_t *ctx, uint8_t *rsp, int rsp_length)
+{
+    if (rsp_length > 0) {
+        return send_msg(ctx, rsp, rsp_length);
+    }
+    return rsp_length;
 }
 
 int modbus_reply_exception(modbus_t *ctx, const uint8_t *req, unsigned int exception_code)
diff --git a/src/modbus.h b/src/modbus.h
index fa7ec4a2..e594a82e 100644
--- a/src/modbus.h
+++ b/src/modbus.h
@@ -275,6 +275,10 @@ MODBUS_API int modbus_reply(modbus_t *ctx,
                             int req_length,
                             modbus_mapping_t *mb_mapping);
 MODBUS_API int
+modbus_reply_construct(modbus_t *ctx, const uint8_t *req,
+                       int req_length, modbus_mapping_t *mb_mapping, uint8_t *rsp);
+MODBUS_API int modbus_reply_send(modbus_t *ctx, uint8_t *rsp, int rsp_length);
+MODBUS_API int
 modbus_reply_exception(modbus_t *ctx, const uint8_t *req, unsigned int exception_code);
 MODBUS_API int modbus_enable_quirks(modbus_t *ctx, unsigned int quirks_mask);
 MODBUS_API int modbus_disable_quirks(modbus_t *ctx, unsigned int quirks_mask);