how to read from callback function

Using Arduino Programming Questions
1

Hi everyone.
I'm new to IOT and I'm having a very simple question.
In my code I have subscribe "appClient" topic and I publish my messages under "Esp8266Client" topic.
I have written a callback code which publish whatever it gets in its payload.
why received messages and published messages are different from each other?

This is my callback function:

void callback(char* topic, byte* payload, unsigned int length)
{
  String Sdata;

   for (int i = 1; i <= length; i++)
   {
    Sdata += "-" + String(payload[i], DEC);
   }
  
  String topic_R = "ESP8266Client";
  client.publish(topic_R.c_str() , Sdata.c_str());

}

here is the result show in broker:

appClient 1
ESP8266Client -105

appClient 12
ESP8266Client -50-108

appClient 123
ESP8266Client -50-51-105

appClient 1234
ESP8266Client -50-51-52-101

appClient 1111
ESP8266Client -49-49-49-101

2

Why are you using Strings ?

Try this

void callback(char* topic, byte* payload, unsigned int length)
{
  char buffer[30;
  memcpy(buffer, payload);  //copy the payload to the buffer
  buffer[length] = '\0';  //terminate the string (NOT a String !
  char * outTopic = "steamer/ESP8266Client";
  client.publish(outTopic , buffer);
}
3

thank you very much UKHeliBob

it worked with a little correction.

4

Please post the working version for future readers of the thread
I see that I missed the closing ] in the declaration of buffer

5

for (int i = 1; i <= length; i++) If length is the number of bytes in payload, then i will miss the first byte in payload AND access past the end of payload.

6

thank you all.
this is the working version of this code:

void callback(char* topic, byte* payload, unsigned int length)
{
  char buffer1[30];
  memcpy(buffer1, payload, length);  //copy the payload to the buffer
  buffer1[length] = '\0';  //terminate the string (NOT a String !)
  char * outTopic = "ESP8266Client";
  client.publish(outTopic , buffer1);
}