Flash Strings


Strings are basically just arrays of char, but have additional methods to allow them to be used more easily. These methods are consistent with Wiring String, so should be reasonably familiar.

  • length() returns the number of characters in the String, excluding the NUL terminator

  • size() returns the number of bytes of storage used

For example, “123” is actually stored as { '1', '2', '3', '\0' } so the length is 3 and the size is 4. However, “1234” is stored as { '1', '2', '3', '4', '\0' } so the length is 4 and the size is 8.

Using Strings


You can use FSTR::String or the Sming-provided FlashString alias to work with Strings.

Within a function:

DEFINE_FSTR_LOCAL(myFlashString, "This is my flash string");

Serial.printf("myFlashString has %u chars and occupies %u bytes\n", myFlashString.length(), myFlashString.size());

To use Strings across translation units, we do this in the header:


And in a source file:

DEFINE_FSTR(myFlashString, "I am a flash string\0I've got a Naughty NUL.");

You can generally use a Flash String anywhere you can use a regular Wiring String as it has an implicit ::String() operator. Note that WString is used within the library for disambiguation.

Inline Strings

Use the FS() macro to create Flash Strings inline:

Serial.println(FS("A Flash String"));


The macro makes use of FS_PTR() which creates the structure and returns a pointer to it. It behaves like a function call, although the compiler inlines the code.

Therefore FS() may only be used within functions. At file scope you’ll get this error:

statement-expressions are not allowed outside functions nor in template-argument lists

The example above doesn’t provide any improvement over F as there are no Flash String overloads available, so is equivalent to this:

String s = FS("A Flash String");

However, it’s rather different if you pass it to a function which recognises Flash Strings, like this:

FSTR::println(Serial, FS("A Flash String"));

This is equivalent to:

FS("A Flash String").printTo(Serial);

FSTR::String::printTo() uses no heap and imposes no restriction on the string length.

Nested Inline Strings

It would be really useful to be able to use inline Strings this within nested structures, and this can be done provided those structures are in RAM.


Inline Strings cannot be used when defining Vectors or Maps.

Here’s is a simplified structure we will attempt to initialize:

static const struct {
   FlashString* string;
} flashData PROGMEM = { FS_PTR("Inline Flash String") };

The static flashData structure gets initialised at runtime on first use, as per C++ rules. This attempts to copy our pointer into the flashData structure which clearly it cannot do as it’s in PROGMEM, so we get a LOAD/STORE error. We must remove PROGMEM.

Avoiding the heap

Instead of using a temporary Wiring String, you can use LOAD_FSTR() to load the content into a temporary stack buffer:

DEFINE_FSTR(globalTest, "This is a testing string");

void func()
   LOAD_FSTR(local, globalTest);
   printf("%s, %u characters, buffer is %u bytes\n", local, globalTest.length(), sizeof(local));

You can do this with inline Flash Strings using FSTR_ARRAY():

FSTR_ARRAY(buffer, "text");

Is roughly equivalent to:

char name[] = "text";

Except the buffer is word aligned, so sizeof(name) may differ.



Define an inline String and return a pointer to it.


The rather obscure asm statement is required to prevent the compiler from discarding the symbol at link time, which leads to an ‘undefined reference’ error


Define an inline FSTR::String and return it as a copy.


    Serial.println(FS("This is a Flash String"));


Declare a global FSTR::String& reference.


Define the FSTR::String object using DEFINE_STR()

  • name

DEFINE_FSTR(name, str)

Define a FSTR::String object with global reference.


DEFINE_FSTR(test, “This is a test\0Another test\0hello”)

The data includes the nul terminator but the length does not.


Like DEFINE_FSTR except reference is declared static constexpr.


Define a FSTR::String data structure.

  • name – Name of data structure

  • str – Quoted string content

LOAD_FSTR(name, fstr)

Load a FSTR::String object into a named local (stack) buffer.


DEFINE_FSTR(globalTest, "This is a testing string")
LOAD_FSTR(local, globalTest)
printf("%s, %u characters, buffer is %u bytes\n", local, globalTest.length(), sizeof(local));

FSTR_ARRAY(name, str)

Define a flash FSTR::String and load it into a named char[] buffer on the stack.


Equivalent to char name[] = "text" except the buffer is word aligned. Faster than using a temporary Wiring String and avoids using the heap.

  • name – Name of char[] buffer

  • str – Content of FSTR::String

IMPORT_FSTR(name, file)

Define a FSTR::String containing data from an external file.



  • name – Name for the FSTR::String object

  • file – Absolute path to the file containing the content


Like IMPORT_FSTR except reference is declared static constexpr.


declare a table of FlashStrings


Use a Vector or Map

Declares a simple table. Example:

DEFINE_FSTR(fstr1, "Test string #1");
DEFINE_FSTR(fstr2, "Test string #2");

FSTR_TABLE(table) = {

Table entries may be accessed directly as they are word-aligned. Examples:

debugf("fstr1 = '%s'", FSTR::String(*table[0]).c_str());
debugf("fstr2.length() = %u", table[1]->length());

  • name – name of the table

String Class

class FSTR::String : public FSTR::Object<String, char>

describes a counted string stored in flash memory

Public Functions

inline size_t size() const

Get the number of bytes used to store the String.


Always an integer multiple of 4 bytes

inline flash_string_t data() const

Get a WString-compatible pointer to the flash data.

bool equals(const char *cstr, size_t len = 0) const

Check for equality with a C-string.


loads string into a stack buffer for the comparison, no heap required

  • cstr

  • len – Length of cstr (optional)


bool – true if strings are identical

bool equals(const String &str) const

Check for equality with another String.




bool – true if strings are identical

inline StringPrinter printer() const

Supports printing of large String objects.

Avoids implicit String() cast when working with large FlashStrings:

IMPORT_FSTR(largeString, PROJECT_DIR "/files/large-text.txt");