# Content-Addressing File System Objects

For many operations, Nix needs to calculate [a content addresses](@docroot@/glossary.md#gloss-content-address) of [a file system object][file system object] (FSO).
Usually this is needed as part of
[content addressing store objects](../store-object/content-address.md),
since store objects always have a root file system object.
But some command-line utilities also just work on "raw" file system objects, not part of any store object.

Every content addressing scheme Nix uses ultimately involves feeding data into a [hash function](https://en.wikipedia.org/wiki/Hash_function), and getting back an opaque fixed-size digest which is deemed a content address.
The various *methods* of content addressing thus differ in how abstract data (in this case, a file system object and its descendants) are fed into the hash function.

## Serialising File System Objects { #serial }

The simplest method is to serialise the entire file system object tree into a single binary string, and then hash that binary string, yielding the content address.
In this section we describe the currently-supported methods of serialising file system objects.

### Flat { #serial-flat }

A single file object can just be hashed by its contents.
This is not enough information to encode the fact that the file system object is a file,
but if we *already* know that the FSO is a single non-executable file by other means, it is sufficient.

Because the hashed data is just the raw file, as is, this choice is good for compatibility with other systems.
For example, Unix commands like `sha256sum` or `sha1sum` will produce hashes for single files that match this.

### Nix Archive (NAR) { #serial-nix-archive }

For the other cases of [file system objects][file system object], especially directories with arbitrary descendants, we need a more complex serialisation format.
Examples of such serialisations are the ZIP and TAR file formats.
However, for our purposes these formats have two problems:

- They do not have a canonical serialisation, meaning that given an FSO, there can
be many different serialisations.
  For instance, TAR files can have variable amounts of padding between archive members;
  and some archive formats leave the order of directory entries undefined.
  This would be bad because we use serialisation to compute cryptographic hashes over file system objects, and for those hashes to be useful as a content address or for integrity checking, uniqueness is crucial.
  Otherwise, correct hashes would report false mismatches, and the store would fail to find the content.

- They store more information than we have in our notion of FSOs, such as time stamps.
  This can cause FSOs that Nix should consider equal to hash to different values on different machines, just because the dates differ.

- As a practical consideration, the TAR format is the only truly universal format in the Unix environment.