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Directory capabilities

Directory capabilities allow components to connect to a directory provided by another component.

For information on directories that are isolated per-component, see storage capabilities.

Providing directory capabilities

To provide a directory capability, a component must declare the capability and route it from self. The component hosts the directory capability in its outgoing directory .

To define the capability, add a capabilities declaration for it:

{
    capabilities: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/published-data",
        },
    ],
}

This defines a capability hosted by this component whose outgoing directory path is /published-data, and whose maximum usable rights are "read-only".

Routing directory capabilities

Components route directory capabilities by exposing them to their parent and offering them to their children.

For more details on how the framework routes component capabilities, see capability routing.

Exposing

Exposing a directory capability gives the component's parent access to that capability:

{
    expose: [
        {
            directory: "data",
            from: "self",
        },
    ],
}

You may optionally specify:

Offering

Offering a storage capability gives a child component access to that capability:

{
    offer: [
        {
            directory: "data",
            from: "parent",
            to: [ "#child-a", "#child-b" ],
        },
    ],
}

You may optionally specify:

Consuming directory capabilities

To consume a storage capability, the component must request the capability and open the corresponding path in its namespace .

To request the capability, add a use declaration for it:

{
    use: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/data",
        },
    ],
}

This populates the component's namespace with a directory at the provided path containing the shared directory contents.

You must specify rights. You may optionally specify subdir.

Directory capability rights

Directory rights enable components to control access to directories as they are routed throughout the system. Directory rights are applied as follows:

  • capabilities: Required. Provides the base set of rights available for the directory. Any rights specified in a use, offer, or expose must be a subset of what is declared here.
  • use: Required. Describes the access rights requested by the consuming component.
  • offer: Optional. Modified rights available to the destination component. Rights are inherited from the offer source if not present.
  • expose: Optional. Modified rights available to the destination component. Rights are inherited from the expose source if not present.

The rights field can contain any combination of the following fuchsia.io.Rights tokens:

rights: [
  "connect",
  "enumerate",
  "traverse",
  "read_bytes",
  "write_bytes",
  "execute_bytes",
  "update_attributes",
  "get_attributes",
  "modify_directory",
]

The framework provides a simplified form for declaring rights using aliases. Each alias represents the combination of FIDL rights tokens to provide common read, write, or execute access:

Alias FIDL rights
r* connect, enumerate, traverse, read_bytes, get_attributes
w* connect, enumerate, traverse, write_bytes, update_attributes, modify_directory
x* connect, enumerate, traverse, execute_bytes
rw* connect, enumerate, traverse, read_bytes, write_bytes, get_attributes, update_attributes, modify_directory
rx* connect, enumerate, traverse, read_bytes, execute_bytes, get_attributes

The rights field may only contain one alias. Additional FIDL rights may be appended as long as they do not duplicate rights expressed by the alias.

Example

Consider the following example where component A requests read-write access to the data directory:

// A.cml
{
    use: [
        {
            directory: "data",
            rights: ["rw*"],
            path: "/data",
        },
    ],
}

However, the parent component B offers the directory data to component A with only read-only rights. In this case the routing fails and data wouldn't be present in A's namespace.

// B.cml
{
    capabilities: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/published-data",
        },
    ],
    offer: [
        {
            directory: "data",
            from: "self",
            to: [ "#A" ],
        },
    ],
}

Subdirectories

You may expose, offer, or use a subdirectory of a directory capability:

{
    offer: [
        {
            directory: "data",
            from: "parent",
            to: [ "#child-a", "#child-b" ],
            subdir: "children",
        },
    ],
}

Renaming directories

You may expose or offer a directory capability by a different name:

{
    offer: [
        {
            directory: "data",
            from: "#child-a",
            to: [ "#child-b" ],
            as: "a-data",
        },
    ],
}

Framework directories

A framework directory is a directory provided by the component framework. Any component may use these capabilities by setting framework as the source without an accompanying offer from its parent. Fuchsia supports the following framework directories:

  • hub: Allows a component to perform runtime introspection of itself and its children.
{
    use: [
        {
            directory: "hub",
            from: "framework",
            rights: ["r*"],
            path: "/hub",
        },
    ],
}