How to handle POST request with JSON payload on ESP32 server?

Question

I am trying to receive json data from http server on ESP32, but I get core panic every time I try to access data inside my json. POST request is done using simple html form:

<form method="post" enctype='text/plain'>
    <fieldset>
        <legend>Fermentation:</legend>
        <input name="type" type="radio" value="Ale" /> Ale <br />
        <input checked="checked" name="type" type="radio" value="Lager" /> Lager <br />
        <label for="primary-duration">Duration of primary</label><br />
        <input name="primary-duration" type="text" value="5" /> <br />
        <input type="submit" value="Submit"><br />

    </fieldset>
</form>

char *buf contains data from POST like this: type=Lager primary-duration=5\0 After reading the data into buf I am parsing it using cJSON

cJSON *root = cJSON_Parse(buf);

and extracting "type" object

const cJSON *typeJSON = cJSON_GetObjectItemCaseSensitive(root, "type");

after getting my cJSON object it is properly recognized as a string by _IsString(), but I get "LoadProhibited" panic when trying to access it.

if (cJSON_IsString(typeJSON) && (typeJSON->valuestring != NULL))
{
    printf("%s", typeJSON->valuestring);
}else if(cJSON_IsString(typeJSON))
{
    ESP_LOGE(SERVER_HANDLER_TAG, "String error: Not a string");
}else
{
    ESP_LOGE(SERVER_HANDLER_TAG, "String error: empty string"); //I am always here, trying to print string (typeJSON->valuestring) results in kernel panic
}

I would be vary thankful for any advice.

Luca Polito · Accepted Answer · 2021-09-20 12:56:28Z

Your <form> has attribute enctype='text/plain'; that means that the POST body will not contain data encoded in JSON.

Indeed, string "type=Lager primary-duration=5" is not valid JSON.

Unfortunately, enctype='application/json' is not available, so you have to serialize the form's fields manually, and then make a POST request with JSON data.

For instance:

<form action="javascript:void(0);">
  <input type="text" id="mytext1" />
  <button onclick="submitData()">Submit</button>
</form>

<script>
  async function submitData() {
    const mytext1 = document.getElementById("mytext1").value;

    const body = { mytext1 };

    const response = await fetch("target_page", {
      method: "POST",
      headers: {
        "Content-Type": "application/json"
      },
      body: JSON.stringify(body)
    });

    // if your server responds with text
    const serverResponse = await response.text();

    ...
  }
</script>

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