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In this code for heating monitor, there appears to be macros called FLASH_ARRAY and FLASH_TABLE. I can't find any reference to them either in this code, Arduino, or on the Internet. What do they do?



  {DEVICE_UTE,             0, 55, 15,  7, MAIN_TYPE_TEXT_CELCIUS_WITH_NEGATIVE, 103},
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2 Answers 2

up vote 4 down vote accepted

After further digging, this appears to be from Mikal Hart's Flash library.

The reason this library was developed is:

Storing static program data in flash/PROGMEM is a tricky part of Arduino programming. To save precious RAM, a novice user already at odds with unfamiliar C++ syntax must digest such daunting concepts as prog_char, PSTR(), PROGMEM, pgm_read_word(), etc. Even seasoned users get tripped up by the indirection and typecasting that are required to retrieve valid PROGMEM data. Add to that a couple of apparent bugs in the implementation, and it’s clear that PROGMEM is a complicated mess.

I have written a new library, Flash, which abstracts away most of this complexity. It provides new String, Array, Table, and String Array types that make ROM-based data collections as easy to use as “normal” types. Each overrides the C++ [] operator, so to extract individual elements one uses familiar array access syntax:

Specifically, these are macros and a simple helper class to make it easier to create arrays and tables of values in program memory.

The two macros are declared as so:

// Example: FLASH_ARRAY(float, temperatures, 98.1, 98.5, 99.1, 102.1);
#define FLASH_ARRAY(type, name, values...) \
  static const type name##_flash[] PROGMEM = { values }; \
  _FLASH_ARRAY<type> name(name##_flash, sizeof(name##_flash) / sizeof(type));

// Example: FLASH_TABLE(uint8_t, fonts, 7, {ON, OFF, ON, ON, OFF, ON, OFF}, {OFF, ON, OFF, ON, OFF, ON, OFF});
#define FLASH_TABLE(type, name, cols, values...) \
  static const type name##_flash[][cols] PROGMEM = { values }; \
  _FLASH_TABLE<type> name((const PROGMEM type *)name##_flash, sizeof(name##_flash) / sizeof(name##_flash[0]), cols);

The helper functions include size and override the operator [] so that they can easily be accessed like so


I disagree with zmo - this is more than boilerplate. The area of PROGMEM causes a lot of questions from Arduino users, and it is very easy to do things wrong when creating arrays in PROGMEM.

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Could you add an explanation of the macros within the answer? This answer contains only a link and won't be of much use, if the link goes offline. –  AsheeshR Mar 5 at 1:24
Cybergibbons, this is nice, handy and useful, but this adds definitely a non trivial overhead: it's encapsulating an object within a full instance. When you only got 2k of RAM, every byte is precious. –  zmo Mar 5 at 16:39
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The flash array is from Mikal Hart's flash library as you show, and what it does is that they declare an array or a matrix (table) object in the PROGMEM memory:

#define FLASH_ARRAY(type, name, values...) \
static const type name##_flash[] PROGMEM = { values }; \
_FLASH_ARRAY<type> name(name##_flash, sizeof(name##_flash) / sizeof(type));

which is an instance of the _FLASH_ARRAY class defined in that same file, which is a facility to access PROGMEM/Flash memory.

Basically, it's a lot of boilerplate for what already does the PSTR() macro amongst other from pgmspace.h. I personally would avoid using that and prefer to use the original macros along with my own index and pointers.

BTW, except for some ardunio IDE magic, I don't think that can compile, as the Flash.h is not included.


To make another point against the FLASH_TABLE/FLASH_ARRAY boilerplate/overhead, since avc-gcc 4.8 has been introduced the __flash qualifier which is a replacement for the PROGMEM macro, simplifying the whole stuff:

// to use string literals without having to cast
#define FSTR(X) ((const __flash char[]) { X })

// create string pstr in the .progmem.data address space
const __flash char* pstr = FSTR ("foo"); 

and no more use of the pgm_read_*() functions, you can now access the content of the flash memory using *pstr, as shown is this thread. In case you want to keep a code compatible with older version of avr-gcc, here's a post that offers macros to deal with both systems.

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