`cJSON_CreateNumber` is truncating numbers outside the bounds of `INT_MIN` - `INT_MAX`

[raphael-bmec-co]

Issue

Calls to the following may result in truncated values:
void JSONVar::operator=(unsigned int i)
void JSONVar::operator=(long l)
void JSONVar::operator=(unsigned long ul)

All these methods wrap a call to CJSON_PUBLIC(cJSON *) cJSON_CreateNumber(double num) shown below which sets valuedouble correctly but truncates valueint for values outside the bounds of INT_MIN - INT_MAX.

CJSON_PUBLIC(cJSON *) cJSON_CreateNumber(double num)
{
    cJSON *item = cJSON_New_Item(&global_hooks);
    if(item)
    {
        item->type = cJSON_Number;
        item->valuedouble = num;

        /* use saturation in case of overflow */
        if (num >= INT_MAX)
        {
            item->valueint = INT_MAX;
        }
        else if (num <= (double)INT_MIN)
        {
            item->valueint = INT_MIN;
        }
        else
        {
            item->valueint = (int)num;
        }
    }

    return item;
}

The corresponding getters all access the valueint.

JSONVar::operator unsigned int () const
{
    return cJSON_IsNumber (_json) ? _json->valueint : 0;
}

JSONVar::operator long () const
{
    return cJSON_IsNumber (_json) ? _json->valueint : 0;
}

JSONVar::operator unsigned long () const
{
    return cJSON_IsNumber (_json) ? _json->valueint : 0;
}

Steps to reproduce

Sample code:

#include <Arduino_JSON.h>

void setup() {

  Serial.begin(115200);

  JSONVar jsonVar;

  jsonVar["unsignedInt"] = UINT_MAX;
  jsonVar["longMax"] = LONG_MAX;
  jsonVar["longMin"] = LONG_MIN;
  jsonVar["unsignedLong"] = ULONG_MAX;

  Serial.println("Expected: " + String((double) jsonVar["unsignedInt"]) + " got: " + String((unsigned int) jsonVar["unsignedInt"]));
  Serial.println("Expected: " + String((double) jsonVar["longMax"]) + " got: " + String((long) jsonVar["longMax"]));
  Serial.println("Expected: " + String((double) jsonVar["longMin"]) + " got: " + String((long) jsonVar["longMin"]));
  Serial.println("Expected: " + String((double) jsonVar["unsignedLong"]) + " got: " + String((unsigned long) jsonVar["unsignedLong"]));

}

void loop() {
}

Output:

Expected: 4294967295.00 got: 2147483647
Expected: 2147483647.00 got: 2147483647
Expected: -2147483648.00 got: -k
Expected: 4294967295.00 got: 2147483647

Potential fixes

1. Fix the implementation of cJSON_CreateNumber (unlikely see Clean up the cJSON struct DaveGamble/cJSON#63) OR
2. Use valuedouble and static_cast to the correct type.

JSONVar::operator unsigned int () const
{
    return cJSON_IsNumber (_json) ? static_cast<unsigned int>_json->valuedouble: 0;
}

JSONVar::operator long () const
{
    return cJSON_IsNumber (_json) ? static_cast<long>_json->valuedouble: 0;
}

JSONVar::operator unsigned long () const
{
    return cJSON_IsNumber (_json) ? static_cast<unsigned long>_json->valuedouble: 0;
}

Workaround:
When reading values of the types listed above use cast to double to get the valuedouble and then cast to the desired type.

e.g. static_cast((double) jsonVar["unsignedLong"])