mosquitto MQTT parsing issue

Question

I am using libmosquitto and the parson library in a C application running on my IoT device for doing stuff with MQTT and parsing JSON.

When I publish a message to my device using the mosquitto_pub command on a Linux terminal as follows :

mosquitto_pub -t "mytopic/test" -u "admin" -P "admin" -h 192.168.5.100 -m "{"value1": 1, "value2":  2, "value3":  3}"

I am succesfully receiving the message on the device as I am subscribed to it, however, I cannot parse the values at all using json_object_dotget_value

JSON_Value*  root_value   = json_parse_string(payload);
JSON_Object* root_object  = json_value_get_object(root_value);

JSON_Value* value1 =  json_object_dotget_value(root_object, "value1");
JSON_Value* value2 =  json_object_dotget_value(root_object, "value2");
JSON_Value* value3 =  json_object_dotget_value(root_object, "value3");

The values returned are NULL.

I know that i'm leaving out lots of code here. However, the problem is not on the receiving end, because when another application in Python publishes this message, the parsing works fine.

There is something wrong with my mosquitto_pub command, but I can't see it.

I'd appreciate any help. Thanks.

What does `payload` contain? That would be the first thing to start with. You say you receive it successfully, so it contains the sent message? — Sami Kuhmonen, Jun 28 '21 at 17:09
You may have to escape the `"` in the argument to your sender program (or use _single_ quotes): `-m "{"value1": 1, "value2": 2, "value3": 3}"` --> `-m '{"value1": 1, "value2": 2, "value3": 3}'` — Craig Estey, Jun 28 '21 at 17:24
@CraigEstey - That is the correct answer. Post it as an answer and get all the credit and glory! — JD Allen, Jun 29 '21 at 01:11

score 3 · Accepted Answer · answered Jun 29 '21 at 02:36

Per your comment, your program is fine.

In the invocation, you may have to escape the " in the argument to your sender program (or use single quotes)

With -m "{"value1": 1, "value2": 2, "value3": 3}", the args [as the program sees them] are:

-m
{value1: 1, value2:  2, value3:  3}

Notice that the " got stripped around (e.g.) value1 and json needs them.

So, change to -m '{"value1": 1, "value2": 2, "value3": 3}' and the args become:

-m
{"value1": 1, "value2":  2, "value3":  3}

mosquitto MQTT parsing issue

1 Answers1