PROGMEM support?

[brunnels]

I'm pretty low on memory in one of my projects so I'm trying to scrape a few bytes together by moving strings into PROGMEM. Would it be possible to add support for this or should it work and I'm doing it wrong?

// normal way
root["relays"].as<uint8_t>();
// using PROGMEM macro that doesn't work but does compile
root[(const char*)F("relays")].as<uint8_t>();

[bblanchon]

It doesn't work with the current version of ArduinoJson.
Reading from Flash memory requires a bit of extra work: you need to call pgm_read_byte() etc.

I'll try to add that feature in 5.x but I don't make any promise yet because I fear that it will increase the code size significantly.

[puppetmaster886]

I think maybe is easier to support PSTR() rather than F().

[JimGaleForce]

in my sketch, I have a struct which has a type and a pointer... if the type is 'P' (or whatever), it reads the pointer thru the pgm_read_byte(), otherwise, normal string/int/bool/etc. this adds little overhead.

[JarredSteenvoorden]

In the meantime until this is implemented I have been using the following macro to reduce memory usage.

#define PSTR_READ_INTO(buffer_name, string_literal)     char buffer_name[sizeof(string_literal)]; strcpy_P(buffer_name, (char*)PSTR(string_literal))

PSTR_READ_INTO(relays, "relays");
PSTR_READ_INTO(another_one, "another_one");

root[relays].as<uint8_t>();
root[another_one].as<uint8_t>();

[q2dg]

Sorry if I'm a little offtopic, but I just wanted to note that in ARM Arduinos there's no PROGMEM stuff. Instead, we can use some libraries to do the trick (https://github.com/cmaglie/FlashStorage is one example). Maybe this should be taken into account.

[bblanchon]

I just realize that this feature is partially supported since v5.0, thanks to an implicit conversion to String.

This is supported:

root[F("sensor")] = "gps";

This is not supported:

root[F("sensor")] = F("gps");

but can be made to work like this:

root[F("sensor")] = String(F("gps"));

However, I do not recommend to use this because it's terribly inefficient. Each flash string has to be first copied to the heap (this is done by the String constructor), then copied into the JsonBuffer.
I'll try to implement a direct copy to the JsonBuffer.

[bblanchon]

Support for PROGMEM has been added in ArduinoJson 5.7.1.
Please note that the content of the flash string is copied into the JsonBuffer when needed (just like it's done for String), so the JsonBuffer will be bigger.

@q2dg, I took your comment into account: I concentrated the logic for handling strings in a very small area so that support for other types of string can be added easily in the future.
Have a look at commit 1ce16ce to see how I added support for PROGMEM; it's a good start if you want to add another type of string.