HotTips

Minimising Memory Usage

Literals use up memory, so its a good idea to move them to flash. Be aware though that it can be slower! Further reading, see the header FlashString.h in the sming source code

F macro Instructs the compiler to store the literal in flash (of which there is plenty) rather than RAM (which is limited)

Example: String system = F("system");

_F macro This function uses the stack, and is generally preferable to the F() macro, but you need to be mindful of scope. The content is lost as soon as the containing block goes out of scope. Used as a function parameter, that means the end of the function call.

Example: println(_F("Debug started"));

Example: commandOutput->print(_F("Welcome to the Tcp Command executor\r\n"));

Bad: char* s = _F("string")

An assignment such as this will not work because the temporary will be out of scope after the statement, hence s will point to garbage. In this instance PSTR_ARRAY(s, "string") can be used.

PSTR_ARRAY

PSTR_ARRAY(_name, _str) - creates and loads string into named stack buffer Ensures loaded string stays in scope, unlike _F() Example:

    String testfunc() {
	//char * test = "This is a string"; <<- BAD
	PSTR_ARRAY(test, "This is a string");
	m_printf(test);
	...
	return test;
    }

JSON keys

Example: root[F("offset")] = something;

Bad: root[_F("offset")] = something;

According to the ArduinoJson docs it should take an internal copy of char* strings, but it doesn't! Tip - Use the F() macro without leading underscore instead.

Sming/Sming/Libraries/ArduinoJson/include/ArduinoJson/JsonObjectKey.hpp

Webpages and Spiffs

FlashString turns out to be very useful for sending web pages, javascript, CSS and so on. Many examples for the ESP8266 exist where a Spiffs file system is used for this purpose, but in fact Spiffs is not ideal. If you want to release a new version of your software, and your web pages are in spiffs, you now have two things to release, so there is double the chance of something going wrong. Plus you have the challenge of preserving any user files while refreshing just a few.

One solution is to use a FlashString hooked up to a FlashMemoryStream instead. In the example below, the CSS file is sent compressed to save time and space. The browser asks for core.js and gets a compressed version.

IMPORT_FSTR(flash_corejsgz,	"web/build/core.js.gz")

void onSendCore_js(HttpRequest &request, HttpResponse &response)
{
	response.headers[HTTP_HEADER_CONTENT_ENCODING] = _F("gzip");
	auto stream = new FlashMemoryStream(flash_corejsgz);
	response.sendDataStream(stream, MimeType::MIME_JS);
}