This document defines general terms that have a specific meaning in a FIDL context. To learn more about specific FIDL topics, refer to the FIDL traihead
A member of a declaration is an individual element belonging to a declaration, i.e. a declaration is comprised of zero, one, or many members.
For instance, consider the Mode bits declaration:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/misc.test.fidl" region_tag="mode" %}
Both READ and WRITE are members.
When referring to members of structs or tables, we can more specifically refer to these members as fields.
When referring to members of a union, we can more specifically refer to these members as variants.
For example, consider the Command union declaration:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples.docs/misc.test.fidl" region_tag="command" %}
The two variants are create_resource and release_resource.
Furthermore, the selected variant of an instance of a union is the current value held by the union at that moment.
The tag is the target language variant discriminator, i.e. the specific construct in a target language that is used to indicate the selected variant of a union. For example, consider the following TypeScript representation of the Command union:
enum CommandTag { Create, Release, } interface Command = { tag: CommandTag, data: CreateResource | ReleaseResource, }
The tag of Command is Command.tag and has type CommandTag. The actual values and type representing each variant of Command are up to the implementation.
Note that some languages will not require a tag. For example, some languages use pattern matching to branch on the variant of a union instead of having an explicit tag value.
The ordinal is the on the wire variant discriminator, i.e. the value used to indicate the variant of a union in the FIDL wire format. The ordinals are explicitly specified in the FIDL definition (in this example, 1 for create_resource and 2 for release_resource).
Encoding refers to the process of serializing values from a target language into the FIDL wire format.
For the C family of bindings (HLCPP, LLCPP), encode can have a more specific meaning of taking bytes matching the layout of the FIDL wire format and patching pointers and handles by replacing them with FIDL_ALLOC_PRESENT/FIDL_ALLOC_ABSENT or FIDL_HANDLE_PRESENT/FIDL_HANDLE_ABSENT in-place, moving handles into an out-of-band handle table.
Decoding refers to the process of deserializing values from raw bytes in the FIDL wire format into a value in a target language.
For the C family of bindings (HLCPP, LLCPP), decode can have a more specific meaning of taking bytes matching the layout of the FIDL wire format and patching pointers and handles by replacing FIDL_ALLOC_PRESENT/FIDL_ALLOC_ABSENT or FIDL_HANDLE_PRESENT/FIDL_HANDLE_ABSENT with the “real” pointer/handle values in-place, moving handles out of an out-of-band handle table.
Validation is the process of checking if constraints from the FIDL definition are satisfied for a given value. Validation occurs both when encoding a value before being sent, or when decoding a value after receiving it. Example constraints are vector bounds, handle constraints, and the valid encoding of a string as UTF-8.
For methods with error types specified:
DoWork() -> (Data result) error uint32
The result type refers to the entire message that would be received by a server for this method, i.e. the union that consists of either a result of Data or an error of uint32. The error type in this case is uint32, whereas Data can be referred to as either the response type or the success type.
