# 5.7 Defaults _Peios / Peios technical reference / PGSS / Applications_ > Which application opens a type of file when several could — the person's choice, else the machine's — where each is kept in the registry, how a consumer finds the default from them, and who may set them. A person may choose which application opens a type of file, and so may whoever administers the machine, for everyone on it. Each such **choice** is one value in the registry. The **default** for a media type is the application the choices give it, by the rules of this section, and it is what a consumer that picks among the candidates (§5.6) opens a file with. ## 5.7.1 Where choices are kept There are two places, each a registry key: | Key | Whose choices | |---|---| | `CurrentUser\Generic\Applications\Defaults` | The person's. The kernel routes `CurrentUser\` to the caller's own `Users\\`, so each person has their own. | | `Machine\Generic\Applications\Defaults` | The machine's: everyone's on it, for whatever they have not chosen themselves. | Each value of either key is one choice: | Part | Is | |---|---| | Name | A media type pattern (§5.2): the type the choice is for, `image/png`, or a top-level type and `*`, `image/*`. | | Type | `REG_SZ`. | | Data | The id (§5.3) of the application chosen, in UTF-8, with or without a terminating NUL. | The registry compares value names ignoring case, as media types are compared (§5.6), so one key holds at most one choice for a type however it is spelt. A writer SHOULD write the name in lowercase. A consumer reads the name ignoring case, and the data with leading and trailing whitespace removed. A value whose name is not a media type pattern, whose type is not `REG_SZ`, or whose data is not in the form of an id is no choice. A consumer passes over it, SHOULD say on its own log which value it passed over and why, and goes on to the rest of the key. [*applications.defaults.malformed-value-is-no-choice] A key that is absent has no choices, and neither has one the consumer may not read. Neither is an error. Anything in either key other than its values, such as a subkey, is reserved (§5.8) and a consumer MUST ignore it. The person's choices are those of the person the file is opened for. A consumer that runs as that person reads them through `CurrentUser\`. A consumer that runs as anyone else MUST NOT read its own `CurrentUser\` for them, since that is its own key and not the person's; it reads the person's `Users\\Generic\Applications\Defaults` if it may, and otherwise uses the machine's choices alone. [*applications.defaults.person-is-whom-the-file-is-opened-for] ## 5.7.2 Finding the default The default for a file whose media type is `type/subtype` is found by trying these choices in order and taking the first that is usable: 1. the person's choice named `type/subtype` [*applications.defaults.order.person-exact] 2. the person's choice named `type/*` [*applications.defaults.order.person-pattern] 3. the machine's choice named `type/subtype` [*applications.defaults.order.machine-exact] 4. the machine's choice named `type/*` [*applications.defaults.order.machine-pattern] A choice is **usable** for the file when the catalogue has a usable declaration for the id it names (§5.3, §5.4), and that application is a candidate to open the file (§5.6). A choice that is not usable is passed over and the next is tried. It is never an error: an application that has been removed, or that no longer says it opens the type, is simply no longer anybody's default, and nothing need be done about the choice that named it. A consumer MAY say on its own log that it passed one over. [*applications.defaults.unusable-choice-is-passed-over] When no choice is usable, the type has no default, and a consumer that picks uses its own rule among the candidates (§5.6). A choice selects among the candidates and never adds one. A consumer MUST NOT open a file with an application that is not a candidate for it, whatever a choice says. [*applications.defaults.a-choice-adds-no-candidate] > [!NOTE] > The person's choices come before the machine's whichever is the more > particular. A person who chooses one application for `image/*` means > every image, including the types the machine has chosen something > else for; the machine's choices are what a person has not decided, > not what overrules them. A consumer reads the choices when it picks, or remembers them under the rule of §5.4 for a remembered catalogue: it MUST notice when either key changes, or MUST say, in its own documentation, how stale a remembered choice may be. ## 5.7.3 Setting them A person sets their own choices by writing values in their own key, and clears one by deleting its value. Whoever may write the machine's key sets the machine's in the same way. A writer creates each key of the path that is not there yet, one level at a time: the registry creates no key on the way to another. Who may write each key is that key's security descriptor, and nothing else. A system MUST let every person who can log on to a desktop read the machine's key, and SHOULD let only administrators write it, as a Peios system's `Machine` hive does from its root: SYSTEM and Administrators have full control, and Authenticated Users read. A person's own key is theirs by the descriptor of their part of the `Users` hive. A program that offers to set choices, such as a settings application, MUST find out whether it may write a key by asking the registry: by opening the key for writing values, or, where the key is not there yet, the nearest key above it that is for creating subkeys. It MUST NOT infer the answer from the groups the person is in, since the descriptor, and not membership, is what the registry checks. [*applications.defaults.a-writer-asks-the-registry] A writer SHOULD offer, for a type, only the applications that are candidates for it, and for a pattern `type/*`, only those that say they open `type/*`. It is not an error to write anything else: a choice that is not usable is passed over when it is read. > [!NOTE] > Any program running as a person can change that person's choices, as > it can change anything else of theirs. This section does not try to > prevent it, since a program running as the person could as easily do > whatever it would have done by being the default. What it does > prevent is a choice opening a file with something that is not a > candidate, or with something a package did not install.