What causes "'Missing '' + p + '' from library'" in arduino/Arduino?

Open /library.properties and add the missing key reported in the message (name, version, author, maintainer, sentence, paragraph, url...)

arduino/Arduino · error · IOException

'Missing '' + p + '' from library'

Error message

'Missing '' + p + '' from library'

What it means

UserLibrary.create() checks that library.properties declares every key in Constants.LIBRARY_MANDATORY_PROPERTIES (name, version, author, maintainer, sentence, paragraph etc.). If any mandatory property is absent from the parsed properties map, an IOException naming the missing key is thrown so the library cannot be loaded.

Source

Thrown at arduino-core/src/processing/app/packages/UserLibrary.java:97

    // Library sanity checks
    // ---------------------

    // Compatibility with 1.5 rev.1 libraries:
    // "email" field changed to "maintainer"
    if (!properties.containsKey("maintainer") && properties.containsKey("email")) {
      properties.put("maintainer", properties.get("email"));
    }

    // Compatibility with 1.5 rev.1 libraries:
    // "arch" folder no longer supported
    File archFolder = new File(libFolder, "arch");
    if (archFolder.isDirectory())
      throw new IOException("'arch' folder is no longer supported! See http://goo.gl/gfFJzU for more information");

    // Check mandatory properties
    for (String p : Constants.LIBRARY_MANDATORY_PROPERTIES)
      if (!properties.containsKey(p))
        throw new IOException("Missing '" + p + "' from library");

    // Check layout
    LibraryLayout layout;
    File srcFolder = new File(libFolder, "src");

    if (srcFolder.exists() && srcFolder.isDirectory()) {
      // Layout with a single "src" folder and recursive compilation
      layout = LibraryLayout.RECURSIVE;
    } else {
      // Layout with source code on library's root and "utility" folders
      layout = LibraryLayout.FLAT;
    }

    // Warn if root folder contains development leftovers
    File[] files = libFolder.listFiles();
    if (files == null) {
      throw new IOException("Unable to list files of library in " + libFolder);
    }

View on GitHub (pinned to a0df6e0e83)

Solutions

Open <library>/library.properties and add the missing key reported in the message (name, version, author, maintainer, sentence, paragraph, url...)
Validate the manifest with the Arduino library-spec checklist or arduino-lint
Re-download/reinstall the library if library.properties was truncated or corrupted
Run the arduino-library-manager formatter (Arduino IDE: convert legacy library) to generate a complete library.properties

Example fix

// before
name=MyLib
version=1.0.0
// after (complete manifest)
name=MyLib
version=1.0.0
author=Jane Doe
maintainer=Jane Doe <jane@example.com>
sentence=Does something useful.
paragraph=Longer description.
url=https://example.com/mylib

Defensive patterns

Strategy: validation

Validate before calling

Properties props = new Properties();
props.load(new FileInputStream(new File(libFolder, "library.properties")));
for (String p : Constants.LIBRARY_MANDATORY_PROPERTIES)
  if (!props.containsKey(p)) throw new IllegalStateException("Missing " + p);

Try / catch

try {
  UserLibrary lib = UserLibrary.create(libFolder);
} catch (IOException e) {
  if (e.getMessage().startsWith("Missing '")) {
    String key = e.getMessage().replace("Missing '", "").replace("' from library", "");
    // append key to library.properties
  }
}

Prevention

Keep a template library.properties with all mandatory fields
Run arduino-lint or the IDE's library spec checker before publishing
Never hand-edit library.properties without re-validating it

When it happens

Trigger: Calling UserLibrary.create(libFolder) on a library whose library.properties (or.properties file for flat layouts) is missing one or more mandatory keys, or where the properties file could not be parsed into those keys.

Common situations: Hand-writing a library.properties and forgetting a required field like 'sentence' or 'maintainer'; converting a legacy library to 1.5 format with an incomplete manifest; corrupted or truncated library.properties after a bad download.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

AI-assisted analysis of arduino/Arduino@a0df6e0e83 (2026-09-06). Data as JSON: /api/errors/42eb34c85142540e. Report an issue: GitHub.