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Guide to PROGMEM on ESP8266 and Arduino IDE | ||
=========================================== | ||
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Intro | ||
----- | ||
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PROGMEM is a Arduino AVR feature that has been ported to ESP8266 to | ||
ensure compatability with existing Arduino libraries, as well as, saving | ||
RAM. On the esp8266 declaring a string such as ``const char * xyz = | ||
"this is a string"`` will place this string in RAM, not flash. It is | ||
possible to place a String into flash, and then load it into RAM when | ||
it is needed. On an 8bit AVR this process is very simple. On the 32bit | ||
ESP8266 there are conditions that must be met to read back from flash. | ||
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On the ESP8266 PROGMEM is a macro: | ||
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.. code:: cpp | ||
#define PROGMEM ICACHE_RODATA_ATTR | ||
``ICACHE_RODATA_ATTR`` is defined by: | ||
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.. code:: cpp | ||
#define ICACHE_RODATA_ATTR __attribute__((section(".irom.text"))) | ||
Which places the variable in the .irom.text section ie flash. Placing strings in | ||
flash requires using any of the methods above. | ||
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| ### Declare a global string to be stored in flash. | ||
.. code:: cpp | ||
static const char xyz[] PROGMEM = "This is a string stored in flash"; | ||
Declare a flash string within code block. | ||
----------------------------------------- | ||
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For this you can use the PSTR macro. Which are all defined in | ||
`pgmspace.h <https://github.com/esp8266/Arduino/blob/master/cores/esp8266/pgmspace.h>`__ | ||
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.. code:: cpp | ||
#define PGM_P const char * | ||
#define PGM_VOID_P const void * | ||
#define PSTR(s) (__extension__({static const char __c[] PROGMEM = (s); &__c[0];})) | ||
In practice: | ||
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.. code:: cpp | ||
void myfunction(void) { | ||
PGM_P xyz = PSTR("Store this string in flash"); | ||
const char * abc = PSTR("Also Store this string in flash"); | ||
} | ||
The two examples above will store these strings in flash. To retrieve | ||
and manipulate flash strings they must be read from flash in 4byte words. | ||
In the Arduino IDE for esp8266 there are several functions that can help | ||
retrieve strings from flash that have been stored using PROGMEM. Both of | ||
the examples above return ``const char *``. However use of these pointers, | ||
without correct 32bit alignment you will cause a segmentation fault and | ||
the ESP8266 will crash. You must read from the flash 32 bit aligned. | ||
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Functions to read back from PROGMEM | ||
----------------------------------- | ||
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Which are all defined in | ||
`pgmspace.h <https://github.com/esp8266/Arduino/blob/master/cores/esp8266/pgmspace.h>`__ | ||
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.. code:: cpp | ||
int memcmp_P(const void* buf1, PGM_VOID_P buf2P, size_t size); | ||
void* memccpy_P(void* dest, PGM_VOID_P src, int c, size_t count); | ||
void* memmem_P(const void* buf, size_t bufSize, PGM_VOID_P findP, size_t findPSize); | ||
void* memcpy_P(void* dest, PGM_VOID_P src, size_t count); | ||
char* strncpy_P(char* dest, PGM_P src, size_t size); | ||
char* strcpy_P(dest, src) | ||
char* strncat_P(char* dest, PGM_P src, size_t size); | ||
char* strcat_P(dest, src) | ||
int strncmp_P(const char* str1, PGM_P str2P, size_t size); | ||
int strcmp_P(str1, str2P) | ||
int strncasecmp_P(const char* str1, PGM_P str2P, size_t size); | ||
int strcasecmp_P(str1, str2P) | ||
size_t strnlen_P(PGM_P s, size_t size); | ||
size_t strlen_P(strP) | ||
char* strstr_P(const char* haystack, PGM_P needle); | ||
int printf_P(PGM_P formatP, ...); | ||
int sprintf_P(char *str, PGM_P formatP, ...); | ||
int snprintf_P(char *str, size_t strSize, PGM_P formatP, ...); | ||
int vsnprintf_P(char *str, size_t strSize, PGM_P formatP, va_list ap); | ||
There are a lot of functions there but in reality they are ``_P`` | ||
versions of standard c functions that are adapted to read from the | ||
esp8266 32bit aligned flash. All of them take a ``PGM_P`` which is | ||
essentially a ``const char *``. Under the hood these functions all use, a | ||
process to ensure that 4 bytes are read, and the request byte is returned. | ||
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This works well when you have designed a function as above that is | ||
specialised for dealing with PROGMEM pointers but there is no type | ||
checking except against ``const char *``. This means that it is totally | ||
legitimate, as far as the compiler is concerned, for you to pass it any | ||
``const char *`` string, which is obviously not true and will lead to | ||
undefined behaviour. This makes it impossible to create any overloaded | ||
functions that can use flash strings when they are defined as ``PGM_P``. | ||
If you try you will get an ambiguous overload error as ``PGM_P`` == | ||
``const char *``. | ||
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Enter the \_\_FlashStringHelper... This is a wrapper class that allows flash | ||
strings to be used as a class, this means that type checking and function | ||
overloading can be used with flash strings. Most people will be familiar with | ||
the ``F()`` macro and possibly the FPSTR() macro. These are defined in `WString.h <https://github.com/esp8266/Arduino/blob/master/cores/esp8266/WString.h#L37>`__: | ||
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.. code:: cpp | ||
#define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer)) | ||
#define F(string_literal) (FPSTR(PSTR(string_literal))) | ||
So ``FSPTR()`` takes a PROGMEM pointer to a string and casts it to this | ||
``__FlashStringHelper`` class. Thus if you have defined a string as | ||
above ``xyz`` you can use ``FPSTR()`` to convert it to | ||
``__FlashStringHelper`` for passing into functions that take it. | ||
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.. code:: cpp | ||
static const char xyz[] PROGMEM = "This is a string stored in flash"; | ||
Serial.println(FPSTR(xyz)); | ||
The ``F()`` combines both of these methods to create an easy and quick | ||
way to store an inline string in flash, and return the type | ||
``__FlashStringHelper``. For example: | ||
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.. code:: cpp | ||
Serial.println(F("This is a string stored in flash")); | ||
Although these two functions provide a similar function, they serve | ||
different roles. ``FPSTR()`` allows you to define a global flash string | ||
and then use it in any function that takes ``__FlashStringHelper``. | ||
``F()`` allows you to define these flash strings in place, but you can't | ||
use them anywhere else. The consequence of this is sharing common | ||
strings is possible using ``FPSTR()`` but not ``F()``. | ||
``__FlashStringHelper`` is what the String class uses to overload its | ||
constructor: | ||
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.. code:: cpp | ||
String(const char *cstr = ""); // constructor from const char * | ||
String(const String &str); // copy constructor | ||
String(const __FlashStringHelper *str); // constructor for flash strings | ||
This allows you to write: | ||
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.. code:: cpp | ||
String mystring(F("This string is stored in flash")); | ||
How do I write a function to use \_\_FlashStringHelper? Simples: cast the pointer back to a PGM\_P and use the ``_P`` functions shown above. This an example implementation for String for the concat function. | ||
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.. code:: cpp | ||
unsigned char String::concat(const __FlashStringHelper * str) { | ||
if (!str) return 0; // return if the pointer is void | ||
int length = strlen_P((PGM_P)str); // cast it to PGM_P, which is basically const char *, and measure it using the _P version of strlen. | ||
if (length == 0) return 1; | ||
unsigned int newlen = len + length; | ||
if (!reserve(newlen)) return 0; // create a buffer of the correct length | ||
strcpy_P(buffer + len, (PGM_P)str); //copy the string in using strcpy_P | ||
len = newlen; | ||
return 1; | ||
} | ||
How do I declare a global flash string and use it? | ||
-------------------------------------------------- | ||
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.. code:: cpp | ||
static const char xyz[] PROGMEM = "This is a string stored in flash. Len = %u"; | ||
void setup() { | ||
Serial.begin(115200); Serial.println(); | ||
Serial.println( FPSTR(xyz) ); // just prints the string, must convert it to FlashStringHelper first using FPSTR(). | ||
Serial.printf_P( xyz, strlen_P(xyz)); // use printf with PROGMEM string | ||
} | ||
How do I use inline flash strings? | ||
---------------------------------- | ||
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.. code:: cpp | ||
void setup() { | ||
Serial.begin(115200); Serial.println(); | ||
Serial.println( F("This is an inline string")); // | ||
Serial.printf_P( PSTR("This is an inline string using printf %s"), "hello"); | ||
} | ||
How do I declare and use data in PROGMEM? | ||
----------------------------------------- | ||
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.. code:: cpp | ||
const size_t len_xyz = 30; | ||
const uint8_t xyz[] PROGMEM = { | ||
0x53, 0x61, 0x79, 0x20, 0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, | ||
0x74, 0x6f, 0x20, 0x4d, 0x79, 0x20, 0x4c, 0x69, 0x74, 0x74, | ||
0x6c, 0x65, 0x20, 0x46, 0x72, 0x69, 0x65, 0x6e, 0x64, 0x00}; | ||
void setup() { | ||
Serial.begin(115200); Serial.println(); | ||
uint8_t * buf = new uint8_t[len_xyz]; | ||
if (buf) { | ||
memcpy_P(buf, xyz, len_xyz); | ||
Serial.write(buf, len_xyz); // output the buffer. | ||
} | ||
} | ||
How do I declare some data in PROGMEM, and retrieve one byte from it. | ||
--------------------------------------------------------------------- | ||
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Declare the data as done previously, then use ``pgm_read_byte`` to get | ||
the value back. | ||
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.. code:: cpp | ||
const size_t len_xyz = 30; | ||
const uint8_t xyz[] PROGMEM = { | ||
0x53, 0x61, 0x79, 0x20, 0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, | ||
0x74, 0x6f, 0x20, 0x4d, 0x79, 0x20, 0x4c, 0x69, 0x74, 0x74, | ||
0x6c, 0x65, 0x20, 0x46, 0x72, 0x69, 0x65, 0x6e, 0x64, 0x00 | ||
}; | ||
void setup() { | ||
Serial.begin(115200); Serial.println(); | ||
for (int i = 0; i < len_xyz; i++) { | ||
uint8_t byteval = pgm_read_byte(xyz + i); | ||
Serial.write(byteval); // output the buffer. | ||
} | ||
} | ||
In summary | ||
---------- | ||
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It is easy to store strings in flash using ``PROGMEM`` and ``PSTR`` but | ||
you have to create functions that specifically use the pointers they | ||
generate as they are basically ``const char *``. On the other hand | ||
``FPSTR`` and ``F()`` give you a class that you can do implicit | ||
conversions from, very useful when overloading functions, and doing | ||
implicit type conversions. It is worth adding that if you wish to store | ||
an ``int``, ``float`` or pointer these can be stored and read back | ||
directly as they are 4 bytes in size and therefor will be always | ||
aligned! | ||
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Hope this helps. |
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