# Advanced Attributes

Derivations can declare some infrequently used optional attributes.

## Inputs

  - [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\
    This attribute allows builders access to the references graph of
    their inputs. The attribute is a list of inputs in the Nix store
    whose references graph the builder needs to know. The value of
    this attribute should be a list of pairs `[ name1 path1 name2
    path2 ...  ]`. The references graph of each *pathN* will be stored
    in a text file *nameN* in the temporary build directory. The text
    files have the format used by `nix-store --register-validity`
    (with the deriver fields left empty). For example, when the
    following derivation is built:

    ```nix
    derivation {
      ...
      exportReferencesGraph = [ "libfoo-graph" libfoo ];
    };
    ```

    the references graph of `libfoo` is placed in the file
    `libfoo-graph` in the temporary build directory.

    `exportReferencesGraph` is useful for builders that want to do
    something with the closure of a store path. Examples include the
    builders in NixOS that generate the initial ramdisk for booting
    Linux (a `cpio` archive containing the closure of the boot script)
    and the ISO-9660 image for the installation CD (which is populated
    with a Nix store containing the closure of a bootable NixOS
    configuration).

  - [`passAsFile`]{#adv-attr-passAsFile}\
    A list of names of attributes that should be passed via files rather
    than environment variables. For example, if you have

    ```nix
    passAsFile = ["big"];
    big = "a very long string";
    ```

    then when the builder runs, the environment variable `bigPath`
    will contain the absolute path to a temporary file containing `a
    very long string`. That is, for any attribute *x* listed in
    `passAsFile`, Nix will pass an environment variable `xPath`
    holding the path of the file containing the value of attribute
    *x*. This is useful when you need to pass large strings to a
    builder, since most operating systems impose a limit on the size
    of the environment (typically, a few hundred kilobyte).

  - [`__structuredAttrs`]{#adv-attr-structuredAttrs}\
    If the special attribute `__structuredAttrs` is set to `true`, the other derivation
    attributes are serialised into a file in JSON format. The environment variable
    `NIX_ATTRS_JSON_FILE` points to the exact location of that file both in a build
    and a [`nix-shell`](../command-ref/nix-shell.md). This obviates the need for
    [`passAsFile`](#adv-attr-passAsFile) since JSON files have no size restrictions,
    unlike process environments.

    It also makes it possible to tweak derivation settings in a structured way; see
    [`outputChecks`](#adv-attr-outputChecks) for example.

    As a convenience to Bash builders,
    Nix writes a script that initialises shell variables
    corresponding to all attributes that are representable in Bash. The
    environment variable `NIX_ATTRS_SH_FILE` points to the exact
    location of the script, both in a build and a
    [`nix-shell`](../command-ref/nix-shell.md). This includes non-nested
    (associative) arrays. For example, the attribute `hardening.format = true`
    ends up as the Bash associative array element `${hardening[format]}`.

    > **Warning**
    >
    > If set to `true`, other advanced attributes such as [`allowedReferences`](#adv-attr-allowedReferences), [`allowedRequisites`](#adv-attr-allowedRequisites),
    [`disallowedReferences`](#adv-attr-disallowedReferences) and [`disallowedRequisites`](#adv-attr-disallowedRequisites), maxSize, and maxClosureSize.
    will have no effect.

## Output checks

See the [corresponding section in the derivation output page](@docroot@/store/derivation/outputs/index.md).

  - [`allowedReferences`]{#adv-attr-allowedReferences}\
    The optional attribute `allowedReferences` specifies a list of legal
    references (dependencies) of the output of the builder. For example,