This is a C++ library to simplify the definition and use of data structures stored in program (flash) memory in an embedded microcontroller.

It was developed for use with Sming and the ESP8266 but could be ported to other platforms relatively easily.

Perhaps the most common use for PROGMEM data is passing strings around, usually as a pointer to a ‘C’ NUL-terminated char array. There are a couple of problems with this:

  1. If we need to know how long the string is, we need to call strlen_P(), which is really expensive computationally.

  2. We can’t include NUL characters in the string.

Both of these are easily solved by passing the length along with the string, like this:

char myString[] PROGMEM = "hello, this is a string";
Serial.println(FPSTR(myString), sizeof(myString) - 1);

Of course, passing two parameters instead of one gets tiresome and is not very C++, is it?

This library implements C++ objects stored in flash memory, using macros to create the data structures. The object interfaces are implemented using class templates for performance and flexibility.

The classes are all in the FSTR namespace.

Sming Integration

Sming provides several aliases to provide compatibility with existing code:

Other pages


Used by

SoC support

  • esp32

  • esp32c2

  • esp32c3

  • esp32s2

  • esp32s3

  • esp8266

  • host

  • rp2040