This is the specification for the Arduino library format, to be used with Arduino IDE 1.5.x onwards.

- rev.1 has been implemented starting with Arduino IDE version 1.5.3 (now superseded by rev.2)
- rev.2 will be implemented starting from Arduino IDE version 1.5.6
- rev.2.1 will be implemented starting from Arduino IDE version 1.6.10
- rev.2.2 will be implemented starting from Arduino IDE version 1.8.10

This new library format is intended to be used in tandem with **Library Manager**, available since Arduino IDE 1.6.2.
The Library Manager allows users to automatically download and install libraries needed in their projects, with an easy
to use graphical interface in the
[Arduino IDE](https://docs.arduino.cc/software/ide-v1/tutorials/installing-libraries#using-the-library-manager)/[Arduino IDE 2.0](https://docs.arduino.cc/software/ide-v2/tutorials/ide-v2-installing-a-library#installing-a-library)
and
[Arduino Web Editor](https://create.arduino.cc/projecthub/Arduino_Genuino/getting-started-with-arduino-web-editor-on-various-platforms-4b3e4a#toc-libraries-and-the-arduino-web-editor-11)
as well as [`arduino-cli lib`](commands/arduino-cli_lib.md).

More information about how Library Manager works is available
[here](https://github.com/arduino/Arduino/wiki/Library-Manager-FAQ).

Arduino development software supports multiple microcontroller architectures (e.g. AVR, SAM, etc), meaning that
libraries may need to work on multiple architectures. The new 1.5 library format doesn’t contain special support for
cross-architecture libraries, but it does provide a preprocessor based mechanism for libraries to target sections of
code to specific architectures.

## See also

- [Arduino library style guide](https://www.arduino.cc/en/Reference/APIStyleGuide)
- [Library dependency resolution process documentation](sketch-build-process.md#dependency-resolution)

## 1.5 library format (rev. 2.2)

### Library metadata

The most significant addition to the format is the ability to add information about the library itself through a
properties file called **library.properties**.

This file allows the _Library Manager_ to search and install a library and its dependencies in an easy and automated
way. It must be located in the root of the library folder.

#### library.properties file format

The library.properties file is a key=value properties list. Every field in this file is UTF-8 encoded. Unless noted
otherwise below, **all fields are required**. The available fields are:

- **name** - the name of the library. Library names must contain only basic letters (A-Z or a-z) and numbers (0-9),
  spaces ( ), underscores (\_), dots (.) and dashes (-). They must start with a letter or number. They must contain at
  least one letter. Note that libraries with a `name` value starting with `Arduino` will no longer be allowed
  [addition to the Library Manager index](https://github.com/arduino/Arduino/wiki/Library-Manager-FAQ) as these names
  are now reserved for official Arduino libraries.
- **version** - version of the library. Version should be [semver](http://semver.org/) compliant. 1.2.0 is correct; 1.2
  is accepted; r5, 003, 1.1c are invalid
- **author** - name/nickname of the authors and their email addresses (not mandatory) separated by comma (,)
- **maintainer** - name and email of the maintainer
- **sentence** - a sentence explaining the purpose of the library
- **paragraph** - a longer description of the library. The value of **sentence** will be prepended to this, so you
  should start by writing the second sentence here
- **category** - (defaults to `Uncategorized`) allowed values:
  - Display
  - Communication
  - Signal Input/Output
  - Sensors
  - Device Control
  - Timing
  - Data Storage
  - Data Processing
  - Other
- **url** - the URL of the library project, for a person to visit. For example, the library's GitHub page. This is used
  for the "More info" links in Library Manager
- **architectures** - (defaults to `*`) a comma separated list of architectures supported by the library. If the library
  doesn’t contain architecture specific code use `*` to match all architectures. This field is used as one factor in
  determining priority when multiple libraries match an `#include` directive and to provide a warning message when the
  library is compiled for a board of an architecture that doesn't match any on the list.
- **depends** - **(available from Arduino IDE 1.8.10/Arduino CLI 0.7.0)** (optional) a comma-separated list of
  dependencies (libraries that are needed to build the current library). The Arduino IDE's Library Manager will offer to
  install the dependencies during installation of the library.
  [`arduino-cli lib install`](commands/arduino-cli_lib_install.md) will automatically install the dependencies. Since
  spaces are allowed in the `name` of a library, but not commas, you can refer to libraries containing spaces in the
  name without ambiguity for example:<br> `depends=Very long library name, Another library with long-name`
- **dot_a_linkage** - **(available from Arduino IDE 1.6.0 / arduino-builder 1.0.0-beta13)** (optional) when set to
  `true`, the library will be compiled using a .a (archive) file. First, all source files are compiled into .o files as
  normal. Then instead of including all .o
    <!-- prettier-ignore -->
  files in the linker command directly, all .o files are saved into a .a file, which is then included in the linker
  command. [1.5 format library folder structure](#layout-of-folders-and-files) is required.
- **includes** - **(available from Arduino IDE 1.6.10)** (optional) a comma separated list of files of the library to be
  added to the sketch as `#include <...>` lines. This property is used with the "Include library" command in the Arduino
  IDE. If the `includes` property is missing, all the header files (.h) on the root source folder are included.
- **precompiled** - **(available from Arduino IDE 1.8.6/arduino-builder 1.4.0)** (optional) enables support for .a
  (archive) and .so (shared object) files. See the ["Precompiled binaries"](#precompiled-binaries) section for
  documentation of the required location in the library for these files. The static library should be linked as an
  ldflag. The **precompiled** field has two supported values, which control how any source files in the library are
  handled:
  - true - Source files are always compiled. This is useful for "mixed" libraries, such as those that contain both open
    source code and the precompiled binary of a closed source component. Support for "mixed" libraries was inadvertently
    lost in Arduino IDE 1.8.12/arduino-builder 1.5.2/Arduino CLI 0.8.0, and returned in Arduino IDE
    1.8.13/arduino-builder 1.5.3/Arduino CLI 0.11.0.
  - full - **(available from Arduino IDE 1.8.13/arduino-builder 1.5.3/Arduino CLI 0.11.0)** If the library provides a
    precompiled library for the board being compiled for, the source files will not be compiled. If no precompiled
    library was provided for the selected board, source files are compiled as a fallback. This is useful for
    precompiling the library to reduce compilation time for specific target hardware, but also providing support for
    arbitrary boards by compiling the library on demand.
- **ldflags** - **(available from Arduino IDE 1.8.6/arduino-builder 1.4.0)** (optional) the linker flags to be added.
  Ex: `ldflags=-lm`

Example:

```
name=WebServer
version=1.0.0
author=Cristian Maglie <c.maglie@example.com>, Pippo Pluto <pippo@example.com>
maintainer=Cristian Maglie <c.maglie@example.com>
sentence=A library that makes coding a Webserver a breeze.
paragraph=Supports HTTP1.1 and you can do GET and POST.
category=Communication
url=http://example.com/
architectures=avr
includes=WebServer.h
depends=ArduinoHttpClient
```

### Layout of folders and files

Each folder has a specific purpose (sources, examples, documentation, etc). Folders not covered in this specification
may be added as needed to future revisions.

#### Library Root folder

The library root folder name 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.

#### Source code

For libraries intended to be used with Arduino IDE 1.5.x+ only, the source code resides in the **src** folder. For
example:

```
Servo/src/Servo.h
Servo/src/Servo.cpp
```

The source code found in **src** folder and _all its subfolders_ is compiled and linked in the user’s sketch. Only the
_src_ folder is added to the include search path (both when compiling the sketch and the library). When the user imports
a library into their sketch (from the Arduino IDE's "Sketch > Include Library" menu or the Arduino Web Editor's
"Include" button), the default behavior (configurable via the
[library.properties `includes` field](#libraryproperties-file-format)) is for an `#include` statement to be added for
all header (.h) files in the src/ directory (but not its subfolders). As a result, these header files form something of
a de facto interface to your library; in general, the only header files in the root src/ folder should be those that you
want to expose to the user's sketch and plan to maintain compatibility with in future versions of the library. Place
internal header files in a subfolder of the src/ folder.

For backward compatibility with Arduino IDE 1.0.x, the library author may opt to place source code into the root folder,
instead of the folder called **src**. In this case the 1.0 library format is applied and the source code is searched
from the **library root folder** and the **utility** folder, for example:

```
Servo/Servo.h
Servo/Servo.cpp
Servo/utility/ServoTimers.h
Servo/utility/ServoTimers.cpp
```

This will allow existing 1.0 format libraries to compile under Arduino IDE 1.5.x+ as well, and vice-versa. If a library
only needs to run on Arduino IDE 1.5.x+, we recommend placing all source code in the src/ folder. If a library requires
recursive compilation of nested source folders, its code must be in the src/ folder (since Arduino IDE 1.0.x doesn’t
support recursive compilation, backwards compatibility wouldn’t be possible anyway).

##### Library layout

![Library layout](img/library-layout.svg)

| Layout        | root compiled | `src` compiled | `utility` compiled |
| ------------- | ------------- | -------------- | ------------------ |
| **recursive** | no            | recursively    | no                 |
| **flat**      | yes           | no             | yes                |

#### Precompiled binaries

The `precompiled` field of [library.properties](#libraryproperties-file-format) enables support for the use of
precompiled libraries. This requires providing .a (archive) or .so (shared object) files which are compiled for a
particular processor architecture. The target architecture of the files is indicated by folder names.

The binaries must be located at `src/{build.mcu}`, where `{build.mcu}` is the architecture name of the target the file
was compiled for. Ex: `cortex-m3` for the Arduino Due.

The filenames of the compiled binaries should start with `lib` (e.g., `libFoo.a`).

**(available from Arduino IDE 1.8.12/arduino-builder 1.5.2/Arduino CLI 0.8.0)** The floating point ABI configuration of
ARM core microcontrollers may be adjusted via compiler flags. An extra subfolder level can be used to provide files
compiled for a specific floating point configuration: `src/{build.mcu}/{build.fpu}-{build.float-abi}`, where
`{build.fpu}` is the value of the `-mfpu` compiler flag and `{build.float-abi}` is the value of the `-mfloat-abi`
compiler flag. **(available from Arduino IDE 1.8.13/arduino-builder 1.5.3/Arduino CLI 0.11.0)** If floating point
configuration flags are used but no folder matching that configuration is found, `src/{build.mcu}` is used as a
fallback.

Below is an example library `src` folder structure that provides:

- Header file containing the declarations for the library API.
- Source file to use as a fallback for other architectures (`precompiled=full` mode).
- Archive file for the ARM Cortex M0+ architecture of the Arduino SAMD boards.
- Archive file for the ARM Cortex M4 architecture of the Arduino Nano 33 BLE, as a fallback for backwards compatibility
  with Arduino development software before the floating point configuration support was added.
- Archive file for the ARM Cortex M4 architecture of the Arduino Nano 33 BLE, compiled for the
  `-mfloat-abi=softfp -mfpu=fpv4-sp-d16` floating point ABI configuration.

```
Servo/src/Servo.h
Servo/src/Servo.cpp
Servo/src/cortex-m0plus/libServo.a
Servo/src/cortex-m4/libServo.a
Servo/src/cortex-m4/fpv4-sp-d16-softfp/libServo.a
```

#### Library Examples

Library examples must be placed in the **examples** folder. Note that the **examples** folder name must be written
exactly like that (with lower case letters).

```
Servo/examples/...
```

Sketches contained inside the examples folder will be shown in the Examples menu of the Arduino IDE and Arduino Web
Editor.

More information:

- [Arduino sketch specification](sketch-specification.md)
- [Style guide for Arduino examples](https://www.arduino.cc/en/Reference/StyleGuide)

#### Extra documentation

An **extras** folder can be used by the developer to put documentation or other items to be bundled with the library.
Remember that files placed inside this folder will increase the size of the library, so putting a 20MB PDF in a library
that weights a few kilobytes may not be such a good idea.

The content of the _extras_ folder is totally ignored by the Arduino development software; you are free to put anything
inside.

### Keywords

A list of keywords for the library may be specified in a file named keywords.txt located in the root of the library
folder. When a keyword of any installed library is used in a sketch the Arduino IDE colors it.

```
Servo/keywords.txt
```

An example keywords.txt file:

```
# Syntax Coloring Map For ExampleLibrary

# Datatypes (KEYWORD1)
Test	KEYWORD1

# Methods and Functions (KEYWORD2)
doSomething	KEYWORD2

# Instances (KEYWORD2)

# Constants (LITERAL1)
```

This file would cause the Arduino IDE to highlight `Test` as a data type, and `doSomething` as a method / function.

#### keywords.txt format

keywords.txt is formatted in four fields which are separated by a single true tab (not spaces):

```
KEYWORD	KEYWORD_TOKENTYPE	REFERENCE_LINK	RSYNTAXTEXTAREA_TOKENTYPE
```

It is permitted to leave a field empty.

##### `KEYWORD_TOKENTYPE`

| KEYWORD_TOKENTYPE | Use for    | Theme property                 |
| ----------------- | ---------- | ------------------------------ |
| `KEYWORD1`        | datatypes  | `editor.data_type.style`       |
| `KEYWORD2`        | functions  | `editor.function.style`        |
| `KEYWORD3`        | structures | `editor.function.style`        |
| `LITERAL1`        | constants  | `editor.reserved_word_2.style` |
| `LITERAL2`        | ?          | `editor.function.style`        |

##### `REFERENCE_LINK`

This field specifies the [Arduino Language Reference](https://www.arduino.cc/reference/en) page to open via the Arduino
IDE's **Right Click > Find in Reference** or **Help > Find in Reference** when the cursor is on that keyword. Generally
it does not make sense to define the `REFERENCE_LINK` field for 3rd party library keywords since they are not likely to
be in the Arduino Language Reference.

##### `RSYNTAXTEXTAREA_TOKENTYPE`

In Arduino IDE 1.6.5 and newer this field overrides `KEYWORD_TOKENTYPE`. In previous IDE versions the
`RSYNTAXTEXTAREA_TOKENTYPE` field is ignored and `KEYWORD_TOKENTYPE` is used instead.

| RSYNTAXTEXTAREA_TOKENTYPE | Theme property                 | KEYWORD_TOKENTYPE equivalent |
| ------------------------- | ------------------------------ | ---------------------------- |
| `RESERVED_WORD`           | `editor.reserved_word.style`   | `KEYWORD3`                   |
| `RESERVED_WORD_2`         | `editor.reserved_word_2.style` | `LITERAL1`                   |
| `DATA_TYPE`               | `editor.data_type.style`       | `KEYWORD1`                   |
| `PREPROCESSOR`            | `editor.preprocessor.style`    | `KEYWORD3`                   |
| `LITERAL_BOOLEAN`         | `editor.literal_boolean.style` | `LITERAL1`                   |

### Development flag file

Normally the Arduino IDE treats the contents of the library folder as read-only. This is to prevent users from
accidentally modifying example sketches. During the library development process you may want to edit example sketches in
place using the Arduino IDE. With Arduino IDE 1.6.6 and newer, the read-only behavior can be disabled by adding a file
named .development to the root of the library folder. A [library.properties](#libraryproperties-file-format) file must
also be present. The [Library Manager indexer](https://github.com/arduino/Arduino/wiki/Library-Manager-FAQ) will not
pick up releases that contain a .development file so be sure not to push this file to your remote repository.

### A complete example

A hypothetical library named "Servo" that adheres to the specification follows:

```
Servo/
Servo/library.properties
Servo/keywords.txt
Servo/src/
Servo/src/Servo.h
Servo/src/Servo.cpp
Servo/src/ServoTimers.h
Servo/examples/
Servo/examples/Sweep/Sweep.ino
Servo/examples/Pot/Pot.ino
Servo/extras/
Servo/extras/Servo_Connectors.pdf
```

## Working with multiple architectures

Libraries placed in the `libraries` subfolder of the sketchbook folder (AKA "user directory") will be made available for
all boards, which may include multiple different processor architectures. To provide architecture-specific code or
optimizations, library authors can use the `ARDUINO_ARCH_XXX` preprocessor macro (`#define`), where XXX is the name of
the architecture (as determined by the name of the
[architecture folder](platform-specification.md#hardware-folders-structure) of the board's platform). For example,
`ARDUINO_ARCH_AVR` will be defined when compiling for AVR-based boards.

An example:

```cpp
#if defined(ARDUINO_ARCH_AVR)
  // AVR-specific code
#elif defined(ARDUINO_ARCH_SAM)
  // SAM-specific code
#else
  // generic, non-platform specific code
#endif
```

Alternatively, if a library only works on certain architectures, you can provide an explicit error message (instead of
allowing the compilation to fail in a difficult to understand way):

```cpp
#if defined(ARDUINO_ARCH_AVR)
  // AVR-specific code
#elif defined(ARDUINO_ARCH_SAM)
  // SAM-specific code
#else
  #error “This library only supports boards with an AVR or SAM processor.”
#endif
```

## Old library format (pre-1.5)

In order to support old libraries (from Arduino IDE 1.0.x), the Arduino IDE/Pro IDE and Arduino CLI will also compile
libraries missing a library.properties metadata file. As a result, these libraries should behave as they did in Arduino
IDE 1.0.x, although they will be available for all boards, including non-AVR ones (which wouldn’t have been present in
Arduino IDE 1.0.x).
