Parsing the payload of the AT commands from the full response string

Question

I want to parse the actual payload from the output of AT commands.

For instance: in the example below, I'd want to read only "2021/11/16,11:12:14-32,0"

AT+QLTS=1                          // command
+QLTS: "2021/11/16,11:12:14-32,0"  // response

OK

In the following case, I'd need to only read 12345678.

AT+CIMI     // command
12345678   // example response

So the point is: not all commands have the same format for the output. We can assume the response is stored in a string array.

I have GetAtCmdRsp() already implemented which stores the response in a char array.

void GetPayload()
{
  char rsp[100] = {0};
  GetAtCmdRsp("AT+QLTS=1", rsp);
  // rsp now contains +QLTS: "2021/11/16,11:12:14-32,0"
  // now, I need to parse "2021/11/16,11:12:14-32,0" out of the response
  
  memset(rsp, 0, sizeof(rsp));

  GetAtCmdRsp("AT+CIMI", rsp);
  // rsp now contains 12345678   
  // no need to do additional parsing since the output already contains the value I need
}

I was thinking of doing char *start = strstr(rsp, ":") + 1; to get the start of the payload but some responses may only contain the payload as it's the case with AT+CIMI

Perhaps could regex be a good idea to determine the pattern +<COMMAND>: in a string?

Answer 1

In order to parse AT command responses a good starting point is understanding all the possible formats they can have.
So, rather than implementing a command specific routine, I would discriminate commands by "type of response":

1. Commands with no payload in their answers, for example

    AT
    OK
   

2. Commands with no header in their answers, such as

    AT+CIMI
    12345678
   
    OK
   

3. Commands with a single header in their answers

    AT+QLTS=1
    +QLTS: "2021/11/16,11:12:14-32,0"
   
    OK
   

4. Command with multi-line responses.
   Every line could of "single header" type, like in +CGDCONT:

   AT+CDGCONT?
   +CGDCONT: 1,"IP","epc.tmobile.com","0.0.0.0",0,0
   +CGDCONT: 2,"IP","isp.cingular","0.0.0.0",0,0
   +CGDCONT: 3,"IP","","0.0.0.0",0,0
   
   OK
   

   Or we could even have mixed types, like in +CGML:

    AT+CMGL="ALL"
   
    +CMGL: 1,"REC READ","+XXXXXXXXXX","","21/11/25,10:20:00+00"
    Good morning! How are you?
   
    +CMGL: 2,"REC READ","+XXXXXXXXXX","","21/11/25,10:33:33+00"
    I'll come a little late. See you. Bruce Wayne
   
    OK
   

   (please note how it could have also "empty" lines, that is \r\n).

At the moment I cannot think about any other scenario.
In this way you'll be able to define an enum like

typedef enum
{
    AT_RESPONSE_TYPE_NO_RESPONSE,
    AT_RESPONSE_TYPE_NO_HEADER,
    AT_RESPONSE_TYPE_SINGLE_HEADER,
    AT_RESPONSE_TYPE_MULTILINE,
    AT_RESPONSE_TYPE_MAX
}

and pass it to your GetAtCmdRsp( ) function in order to parser the response accordingly. If implement the differentiation in that function, or after it (or in an external function is your choice.

---

A solution without explicit categorization

Once you have clear all the scenarios that might ever occur, you can think about a general algorithm working for all of them:

1. Get the full response resp after the command echo and before the closing OK or ERROR. Make sure that the trailing \r\n\r\nOK is removed (or \r\nERROR. Or \r\nNO CARRIER. Or whatever the terminating message of the response might be).
   Make also sure to remove the command echo

2. If strlen( resp ) == 0 we belong to the NO_RESPONSE category, and the job is done

3. If the response contains \r\ns in it, we have a MULTILINE answer. So, tokenize it and place every line into an array element resp_arr[i]. Make sure to remove trailing \r\n

4. For every line in the response (for every resp_arr[i] element), search for <CMD> : pattern (not only :, that might be contained in the payload as well!). Something like that:

    size_t len = strlen( resp_cur_line );
    char *payload;
   
    if( strstr( "+YOURCMD: ", resp_cur_line) == NULL )
    {
        // We are in "NO_HEADER" case
        payload = resp_cur_line;
    }
    else
    {
        // We are in "HEADER" case
        payload = resp_cur_line + strlen( "+YOURCMD: " );
    }
   

   Now payload pointer points to the actual payload.

   Please note how, in case of MULTILINE answer, after splitting the lines into array elements every loop will handle correctly also the mixed scenarios like the one in +CMGL, as you'll be able to distinguish the lines containing the header from those containing data (and from the empty lines, of course). For a deeper analysis about +CMGL response parsing have a look to this answer.