Sketch specification
This is the specification for Arduino sketches.
The programs that run on Arduino boards are called "sketches". This term was inherited from Processing, upon which the Arduino IDE and the core API were based.
Sketch folders and files¶
The sketch root folder name and code file names must start with a basic letter (A
-Z
or a
-z
) or number (0
-9
),
followed by basic letters, numbers, underscores (_
), dots (.
) and dashes (-
). The maximum length is 63 characters.
Support for names starting with a number was added in Arduino IDE 1.8.4.
Sketch root folder¶
Because many Arduino sketches only contain a single .ino file, it's easy to think of that file as the sketch. However, it is the folder that is the sketch. The reason is that sketches may consist of multiple code files and the folder is what groups those files into a single program.
Primary sketch file¶
Every sketch must contain a .ino
file with a file name matching the sketch root folder name.
.pde
is also supported but deprecated and will be removed in the future, using the .ino
extension is strongly
recommended.
Additional code files¶
Sketches may consist of multiple code files.
The following extensions are supported:
- .ino - Arduino language files.
- .pde - Alternate extension for Arduino language files. This file extension is also used by Processing sketches. .ino
is recommended to avoid confusion.
.pde
extension is deprecated and will be removed in the future. - .cpp - C++ files.
- .c - C Files.
- .S - Assembly language files.
- .h - Header files.
- .tpp, .ipp - Header files (available from Arduino CLI 0.19.0).
For information about how each of these files and other parts of the sketch are used during compilation, see the Sketch build process documentation.
src
subfolder¶
The contents of the src
subfolder are compiled recursively. Unlike the code files in the sketch root folder, these
files are not shown as tabs in the IDEs.
This is useful for files you don't want to expose to the sketch user via the IDE's interface. It can be used to bundle libraries with the sketch in order to make it a self-contained project.
Arduino language files under the src
folder are not supported.
- In Arduino IDE 1.6.5-r5 and older, no recursive compilation was done.
- In Arduino IDE 1.6.6 - 1.6.9, recursive compilation was done of all subfolders of the sketch folder.
- In Arduino IDE 1.6.10 and newer, recursive compilation is limited to the
src
subfolder of the sketch folder.
data
subfolder¶
The data
folder is used to add additional files to the sketch, which will not be compiled.
Files added to the sketch via the Arduino IDE's Sketch > Add File... are placed in the data
folder.
The Arduino IDE's File > Save As... only copies the code files in the sketch root folder and the full contents of
the data
folder, so any non-code files outside the data
folder are stripped.
Metadata¶
Arduino CLI and Arduino Web Editor use a file named sketch.json, located in the sketch root folder, to store sketch metadata.
The cpu
key contains the board configuration information. This can be set via
arduino-cli board attach
or by selecting a board in the Arduino Web Editor
while the sketch is open. With this configuration set, it is not necessary to specify the --fqbn
or --port
flags to
the arduino-cli compile
or arduino-cli upload
commands when compiling or uploading the sketch.
The included_libs
key defines the library versions the Arduino Web Editor uses when the sketch is compiled. This is
Arduino Web Editor specific because all versions of all the Library Manager libraries are pre-installed in Arduino Web
Editor, while only one version of each library may be installed when using the other Arduino development software.
Secrets¶
Arduino Web Editor has a
"Secret tab" feature
that makes it easy to share sketches without accidentally exposing sensitive data (e.g., passwords or tokens). The
Arduino Web Editor automatically generates macros for any identifier in the sketch which starts with SECRET_
and
contains all uppercase characters.
When you download a sketch from Arduino Web Editor that contains a Secret tab, the empty #define
directives for the
secrets are in a file named arduino_secrets.h, with an #include
directive to that file at the top of the primary
sketch file. This is hidden when viewing the sketch in Arduino Web Editor.
Documentation¶
Image and text files in common formats which are present in the sketch root folder are displayed in tabs in the Arduino Web Editor.
Sketch file structure example¶
Foo
|_ arduino_secrets.h
|_ Abc.ino
|_ Def.cpp
|_ Def.h
|_ Foo.ino
|_ Ghi.c
|_ Ghi.h
|_ Jkl.h
|_ Jkl.S
|_ sketch.json
|_ data
| |_ Schematic.pdf
|_ src
|_ SomeLib
|_ library.properties
|_ src
|_ SomeLib.h
|_ SomeLib.cpp
Sketchbook¶
The Arduino IDE provides a "sketchbook" folder (analogous to Arduino CLI's "user directory"). In addition to being the place where user libraries and manually installed platforms are installed, the sketchbook is a convenient place to store sketches. Sketches in the sketchbook folder appear under the Arduino IDE's File > Sketchbook menu. However, there is no requirement to store sketches in the sketchbook folder.
Library/Boards Manager links¶
A URI in a comment in the form http://librarymanager#SEARCH_TERM
will open a search for SEARCH_TERM in
Library Manager when
clicked in the Arduino IDE.
A URI in a comment in the form http://boardsmanager#SEARCH_TERM
will open a search for SEARCH_TERM in
Boards Manager when clicked in the Arduino IDE.
This can be used to offer the user an easy way to install dependencies of the sketch.
This feature is only available when using the Arduino IDE, so be sure to provide supplementary documentation to help the users of other development software install the sketch dependencies.
This feature was added in Arduino IDE 1.6.9.
Example¶
// install the Arduino SAMD Boards platform to add support for your MKR WiFi 1010 board
// if using the Arduino IDE, click here: http://boardsmanager#SAMD
// install the WiFiNINA library via Library Manager
// if using the Arduino IDE, click here: http://librarymanager#WiFiNINA
#include <WiFiNINA.h>