AsyncWebServer can send text content directly from Flash ?

[MarusGradinaru]

Hi !
I am using the library on ESP32 with Arduino framework in Platform IO.
I want to know if the AsyncWebServer.send() commmand can send flash constants defined with F(), like this:

const __FlashStringHelper* msgError = F("Internal error encountered.");

void handleTestRes(AsyncWebServerRequest *request) {
request->send(500, MIME_PLAIN, msgError);
}

I test it with F() and it works... But I want to know if it was pure luck or it is really intented to work this way ?
An if the library sends the string directly from Flash, or it copy it to RAM first ?

[mathieucarbou]

Hi,
It sends from flash if you use the uint8_t overloads. Otherwise if you pass it as a const char* or String, it will be copied in a buffer.

[MarusGradinaru]

Great ! I test it and it works...
Can you tell me if send() suports a flash stored string too ? I mean, that can be use it directly, not convert it to String...
Or is considered to short to optimeze it ?

[mathieucarbou]

You are on ESP32: all strings are stored on flash. Only ESP8266 uses F() macros and such. On ESP32, you do not need to. Just use the method with the const utin8_t* params.

[MarusGradinaru]

@mathieucarbou but then, if all strings are sored in flash and there is no need for F() macro, and I define const char *msgError = "This is an error message"; and then I use request->send(500, MIME_PLAIN, msgError); that selects the const char* overload, will it still send it directly from flash ?
Edit: Ah, I see you already answer to this in the first comment...
Then, if only the const uint8_t send directly from flash, can I cast my msgError to that type ?

[mathieucarbou]

will it still send it directly from flash ?

no because the lib does not know if your const char* pointer is from a static location or dynamically allocated one, which could eventually become out of scope or be freed and then crash.

if you have const char* strings stored in flash, cast them to const utin8_t* to avoid buffering in a String