Flash layout

Even though file system is stored on the same flash chip as the program, programming new sketch will not modify file system contents. This allows to use file system to store sketch data, configuration files, or content for Web server.

The following diagram illustrates flash layout used in Arduino environment:

^              ^       ^               ^     ^
Sketch    OTA update   File system   EEPROM  WiFi config (SDK)

File system size depends on the flash chip size. Depending on the board which is selected in IDE, you have the following options for flash size:

Board Flash chip size, bytes File system size, bytes
Generic module 512k 64k, 128k
Generic module 1M 64k, 128k, 256k, 512k
Generic module 2M 1M
Generic module 4M 3M
Adafruit HUZZAH 4M 1M, 3M
ESPresso Lite 1.0 4M 1M, 3M
ESPresso Lite 2.0 4M 1M, 3M
NodeMCU 0.9 4M 1M, 3M
NodeMCU 1.0 4M 1M, 3M
Olimex MOD-WIFI-ESP8266(-DEV) 2M 1M
SparkFun Thing 512k 64k
SweetPea ESP-210 4M 1M, 3M
WeMos D1 & D1 mini 4M 1M, 3M
ESPDuino 4M 1M, 3M

Note: to use any of file system functions in the sketch, add the following include to the sketch:

#include "FS.h"

File system limitations

The filesystem implementation for ESP8266 had to accomodate the constraints of the chip, among which its limited RAM. SPIFFS was selected because it is designed for small systems, but that comes at the cost of some simplifications and limitations.

First, behind the scenes, SPIFFS does not support directories, it just stores a “flat” list of files. But contrary to traditional filesystems, the slash character '/' is allowed in filenames, so the functions that deal with directory listing (e.g. openDir("/website")) basically just filter the filenames and keep the ones that start with the requested prefix (/website/). Practically speaking, that makes little difference though.

Second, there is a limit of 32 chars in total for filenames. One '\0' char is reserved for C string termination, so that leaves us with 31 usable characters.

Combined, that means it is advised to keep filenames short and not use deeply nested directories, as the full path of each file (including directories, '/' characters, base name, dot and extension) has to be 31 chars at a maximum. For example, the filename /website/images/bird_thumbnail.jpg is 34 chars and will cause some problems if used, for example in exists() or in case another file starts with the same first 31 characters.

Warning: That limit is easily reached and if ignored, problems might go unnoticed because no error message will appear at compilation nor runtime.

For more details on the internals of SPIFFS implementation, see the SPIFFS readme file.

Uploading files to file system

ESP8266FS is a tool which integrates into the Arduino IDE. It adds a menu item to Tools menu for uploading the contents of sketch data directory into ESP8266 flash file system.

  • Download the tool:
  • In your Arduino sketchbook directory, create tools directory if it doesn’t exist yet
  • Unpack the tool into tools directory (the path will look like <home_dir>/Arduino/tools/ESP8266FS/tool/esp8266fs.jar)
  • Restart Arduino IDE
  • Open a sketch (or create a new one and save it)
  • Go to sketch directory (choose Sketch > Show Sketch Folder)
  • Create a directory named data and any files you want in the file system there
  • Make sure you have selected a board, port, and closed Serial Monitor
  • Select Tools > ESP8266 Sketch Data Upload. This should start uploading the files into ESP8266 flash file system. When done, IDE status bar will display SPIFFS Image Uploaded message.

File system object (SPIFFS)



This method mounts SPIFFS file system. It must be called before any other FS APIs are used. Returns true if file system was mounted successfully, false otherwise.



This method unmounts SPIFFS file system. Use this method before updating SPIFFS using OTA.



Formats the file system. May be called either before or after calling begin. Returns true if formatting was successful.

open, mode)

Opens a file. path should be an absolute path starting with a slash (e.g. /dir/filename.txt). mode is a string specifying access mode. It can be one of “r”, “w”, “a”, “r+”, “w+”, “a+”. Meaning of these modes is the same as for fopen C function.

r      Open text file for reading.  The stream is positioned at the
       beginning of the file.

r+     Open for reading and writing.  The stream is positioned at the
       beginning of the file.

w      Truncate file to zero length or create text file for writing.
       The stream is positioned at the beginning of the file.

w+     Open for reading and writing.  The file is created if it does
       not exist, otherwise it is truncated.  The stream is
       positioned at the beginning of the file.

a      Open for appending (writing at end of file).  The file is
       created if it does not exist.  The stream is positioned at the
       end of the file.

a+     Open for reading and appending (writing at end of file).  The
       file is created if it does not exist.  The initial file
       position for reading is at the beginning of the file, but
       output is always appended to the end of the file.

Returns File object. To check whether the file was opened successfully, use the boolean operator.

File f ="/f.txt", "w");
if (!f) {
    Serial.println("file open failed");



Returns true if a file with given path exists, false otherwise.



Opens a directory given its absolute path. Returns a Dir object.



Deletes the file given its absolute path. Returns true if file was deleted successfully.


SPIFFS.rename(pathFrom, pathTo)

Renames file from pathFrom to pathTo. Paths must be absolute. Returns true if file was renamed successfully.


FSInfo fs_info;;

Fills FSInfo structure with information about the file system. Returns true is successful, false otherwise.

Filesystem information structure

struct FSInfo {
    size_t totalBytes;
    size_t usedBytes;
    size_t blockSize;
    size_t pageSize;
    size_t maxOpenFiles;
    size_t maxPathLength;

This is the structure which may be filled using FS::info method. - totalBytes — total size of useful data on the file system - usedBytes — number of bytes used by files - blockSize — SPIFFS block size - pageSize — SPIFFS logical page size - maxOpenFiles — max number of files which may be open simultaneously - maxPathLength — max file name length (including one byte for zero termination)

Directory object (Dir)

The purpose of Dir object is to iterate over files inside a directory. It provides three methods: next(), fileName(), and openFile(mode).

The following example shows how it should be used:

Dir dir = SPIFFS.openDir("/data");
while ( {
    File f = dir.openFile("r");
} returns true while there are files in the directory to iterate over. It must be called before calling fileName and openFile functions.

openFile method takes mode argument which has the same meaning as for function.

File object and dir.openFile functions return a File object. This object supports all the functions of Stream, so you can use readBytes, findUntil, parseInt, println, and all other Stream methods.

There are also some functions which are specific to File object.

seek, mode)

This function behaves like fseek C function. Depending on the value of mode, it moves current position in a file as follows:

  • if mode is SeekSet, position is set to offset bytes from the beginning.
  • if mode is SeekCur, current position is moved by offset bytes.
  • if mode is SeekEnd, position is set to offset bytes from the end of the file.

Returns true if position was set successfully.



Returns the current position inside the file, in bytes.



Returns file size, in bytes.


String name =;

Returns file name, as const char*. Convert it to String for storage.



Close the file. No other operations should be performed on File object after close function was called.