This document is a specification of the Fuchsia Interface Definition Language (FIDL) message format.
See Overview for more information about FIDL's overall purpose, goals, and requirements, as well as links to related documents.
This section provides requisite background information for the concepts used throughout the description.
A FIDL message is a collection of data.
The message is a contiguous structure consisting of a single in-line primary object followed by zero or more out-of-line secondary objects.
Objects are stored in traversal order, and are subject to padding.
The first object is called the primary object. It is a structure of fixed size whose type and size are known from the context.
The primary object may refer to secondary objects (such as in the case of strings, vectors, unions, and so on) if additional variable-sized or optional data is required.
Secondary objects are stored out-of-line in traversal order.
Both primary and secondary objects are 8-byte aligned, and are stored without gaps (other than those required for alignment).
Together, a primary object and its secondary objects are called a message.
A transactional FIDL message (transactional message) is used to send data from one application to another.
The roles of the applications (e.g. client vs server) are not relevant to the formatting of the data.
As we will see in the transactional messages section below, a transactional message is composed of a header message optionally followed by a body message.
The traversal order of a message is determined by a recursive depth-first walk of all of the objects it contains, as obtained by following the chain of references.
Given the following structure:
struct Cart { vector<Item> items; }; struct Item { Product product; uint32 quantity; }; struct Product { string sku; string name; string? description; uint32 price; };
The depth-first traversal order for a Cart message is defined by the following pseudo-code:
visit Cart:
for each Item in Cart.items vector data:
visit Item.product:
visit Product.sku
visit Product.name
visit Product.description
visit Product.price
visit Item.quantity
The same message content can be expressed in one of two forms: encoded and decoded. These have the same size and overall layout, but differ in terms of their representation of pointers (memory addresses) or handles (capabilities).
FIDL is designed such that encoding and decoding of messages can occur in place in memory.
Message encoding is canonical — there is exactly one encoding for a given message.
An encoded message has been prepared for transfer to another process: it does not contain pointers (memory addresses) or handles (capabilities).
During encoding...
The resulting encoded message and handle vector can then be sent to another process using zx_channel_write() or a similar IPC mechanism. There are additional constraints on this kind of IPC; see transactional messages.
Note that the handle vector is not stored as part of the message, it's sent separately (also known as "out-of-band, not to be confused with out-of-line). For example, the zx_channel_write() function takes two sets of data pointers; one for the message, and one for the handle vector. The message data pointer will contain all of the in-line and out-of-line data, and the handle vector pointer will contain the handles.
A decoded message has been prepared for use within a process's address space: it may contain pointers (memory addresses) or handles (capabilities).
During decoding...
The resulting decoded message is ready to be consumed directly from memory.
Objects may also contain inlined objects which are aggregated within the body of the containing object, such as embedded structs and fixed-size arrays of structs.
In the following example, the Region structure contains a vector of Rect structures, with each Rect consisting of two Points. Each Point consists of an x and y value.
struct Region { vector<Rect> rects; }; struct Rect { Point top_left; Point bottom_right; }; struct Point { uint32 x, y; };
Examining the objects in traversal order means that we start with the Region structure — it's the primary object.
The rects member is a vector, so its contents are stored out-of-line. This means that the vector content immediately follows the Region object.
Each Rect struct contains two Points, which are stored in-line (because there are a fixed number of them), and each of the Points' primitive data types (x and y) are also stored in-line. The reason is the same; there is a fixed number of the member types.
We use in-line storage when the size of the subordinate object is fixed, and out-of-line when it's variable (including optional).
In this section, we illustrate the encodings for all FIDL objects.
The following primitive types are supported:
| Category | Types |
|---|---|
| Boolean | bool |
| Signed integer | int8, int16, int32, int64 |
| Unsigned integer | uint8, uint16, uint32, uint64 |
| IEEE 754 floating-point | float32, float64 |
| strings | (not a primitive, see Strings below) |
Number types are suffixed with their size in bits.
The Boolean type, bool, is stored as a single byte, and has only the value 0 or 1.
All floating point values represent valid IEEE 754 bit patterns.
Bit fields and enumerations are stored as their underlying primitive type (e.g., uint32).
A handle is a 32-bit integer, but with special treatment. When encoded for transfer, the handle's on-wire representation is replaced with a present / not-present indication, and the handle itself is stored in a separate handle vector. When decoded, the handle presence indication is replaced with zero (if not present) or a valid handle (if present).
The handle value itself is not transferred from one application to another. In this respect, handles are like pointers; they reference a context that‘s unique to each application. Handles are moved from one application’s context to the other's.
The value zero can be used to indicate a nullable handle is null[1].
Aggregate objects serve as containers of other objects. They may store that data in-line or out-of-line, depending on their type.
Arrays are denoted:
array<T>:N: where T can be any FIDL type (including an array) and N is the number of elements in the array.
vector<T>:40 for a maximum 40 element vector.size: 64-bit unsigned number of elementsdata: 64-bit presence indication or pointer to out-of-line element datadata indicates presence of content:0: vector is nullUINTPTR_MAX: vector is non-null, data is the next out-of-line objectdata is a pointer to content:0: vector is null<valid pointer>: vector is non-null, data is at indicated memory addressVectors are denoted as follows:
vector<T>: non-nullable vector of element type T (validation error occurs if null data is encountered)vector<T>?: nullable vector of element type Tvector<T>:N, vector<T>:N?: vector with maximum length of N elementsT can be any FIDL type.
Strings are implemented as a vector of uint8 bytes, with the constraint that the bytes MUST be valid UTF-8.
A structure contains a sequence of typed fields.
Internally, the structure is padded so that all members are aligned to the largest alignment requirement of all members. Externally, the structure is aligned on an 8-byte boundary, and may therefore contain final padding to meet that requirement.
Here are some examples.
A struct with an int32 and an int8 field has an alignment of 4 bytes (due to the int32), and a size of 8 bytes (3 bytes of padding after the int8):
A struct with a bool and a string field has an alignment of 8 bytes (due to the string) and a size of 24 bytes (7 bytes of padding after the bool): 
Keep in mind that a string is really just a special case of
vector<uint8>.
A struct with a bool and two uint8 fields has an alignment of 1 byte and a size of 3 bytes (no padding!):
Note that a structure can be:
uint8 with the value zero.Storage of a structure depends on whether it is nullable at point of reference.
0: reference is nullUINTPTR_MAX: reference is non-null, structure content is the next out-of-line object0: reference is null<valid pointer>: reference is non-null, structure content is at indicated memory addressStructs are denoted by their declared name (e.g. Circle) and nullability:
Point: non-nullable PointColor?: nullable ColorThe following example illustrates:
Point),Color)struct Circle { bool filled; Point center; // Point will be stored in-line float32 radius; Color? color; // Color will be stored out-of-line bool dashed; }; struct Point { float32 x, y; }; struct Color { float32 r, g, b; };
The Color content is padded to the 8 byte secondary object alignment boundary. Going through the layout in detail:
bool filled, occupies one byte, but requires three bytes of padding because of the next member, which has a 4-byte alignment requirement.Point center member is an example of a non-nullable struct. As such, its content (the x and y 32-bit floats) are inlined, and the entire thing consumes 8 bytes.radius is a 32-bit item, requiring 4 byte alignment. Since the next available location is already on a 4 byte alignment boundary, no padding is required.Color? color member is an example of a nullable structure. Since the color data may or may not be present, the most efficient way of handling this is to keep a pointer to the structure as the in-line data. That way, if the color member is indeed present, the pointer points to its data (or, in the case of the encoded format, indicates “is present”), and the data itself is stored out-of-line (after the data for the Circle structure). If the color member is not present, the pointer is NULL (or, in the encoded format, indicates “is not present” by storing a zero).bool dashed doesn‘t require any special alignment, so it goes next. Now, however, we’ve reached the end of the object, and all objects must be 8-byte aligned. That means we need an additional 7 bytes of padding.color follows the Circle data structure, and contains three 32-bit float values (r, g, and b); they require 4 byte alignment and so can follow each other without padding. But, just as in the case of the Circle object, we require the object itself to be 8-byte aligned, so 4 bytes of padding are required.Overall, this structure takes 48 bytes.
By moving the bool dashed to be immediately after the bool filled, though, you can realize significant space savings [2]:
bool values are “packed” together within what would have been wasted space.bools or 8-bit integers.color pointer; everything is perfectly aligned on an 8 byte boundary.The structure now takes 40 bytes.
While
fidlccould automatically pack structs, like Rust, we chose not to do that in order to simplify ABI compatibility changes.
Storage of a union depends on whether it is nullable at point of reference.
0: reference is nullUINTPTR_MAX: reference is non-null, union content is the next out-of-line object0: reference is null<valid pointer>: reference is non-null, union content is at indicated memory addressUnions are denoted by their declared name (e.g. Pattern) and nullability:
Pattern: non-nullable PatternPattern?: nullable PatternThe following example shows how unions are laid out according to their options.
struct Paint { Pattern fg; Pattern? bg; }; union Pattern { Color color; Texture texture; }; struct Color { float32 r, g, b; }; struct Texture { string name; };
When laying out Pattern, space is first allotted to the tag (4 bytes), then to the selected option.
An envelope is a container for out-of-line data, used internally by tables and extensible unions. It is not exposed to the FIDL lanuage.
It has a fixed, 24 byte format, and is not nullable:
An envelope can, however, point to empty content. In that case, num_bytes, num_handles, and the pointer will all be zero.
Furthermore, because num_bytes represents the size of an object, it's always a multiple of 8, regardless of the actual amount of data that it points to.
Having num_bytes and num_handles allows us to skip unknown envelope content.
Tables are denoted by their declared name (e.g., Value), and are not nullable:
Value: non-nullable ValueThe following example shows how tables are laid out according to their fields.
table Value { 1: int16 command; 2: Circle data; 3: float64 offset; };
0 ordinal, and an empty envelope.xunions are denoted by their declared name (e.g. Value) and nullability:
Value: non-nullable ValueValue?: nullable ValueThe following example shows how xunions are laid out according to their fields.
xunion Value { int16 command; Circle data; float64 offset; };
In a transactional message, there is always a header, followed by an optional body.
Both the header and body are FIDL messages, as defined above; that is, a collection of data.
The header has the following form:
zx_txid_t txid, transaction ID (32 bits)txids with the high bit set are reserved for use by zx_channel_write()txids with the high bit unset are reserved for use by userspace0 for txid is reserved for messages which do not require a response from the other side.txid allocationuint32 reserved0, reserved for future use, must be zerouint32 flags, all unused bits must be set to zerouint32 ordinalThere are three kinds of transactional messages:
We'll use the following interface for the next few examples:
protocol Calculator { Add(int32 a, int32 b) -> (int32 sum); Divide(int32 dividend, int32 divisor) -> (int32 quotient, int32 remainder); Clear(); -> OnError(uint32 status_code); };
The Add() and Divide() methods illustrate both the method request (sent from the client to the server), and a method reponse (sent from the server back to the client).
The Clear() method is an example of a method request that does not have a body.
It‘s not correct to say it has an “empty” body: that would imply that there’s a body following the header. In the case of Clear(), there is no body, there is only a header.
The client of an interface sends method request messages to the server in order to invoke the method.
The server sends method reponse messages to the client to indicate completion of a method invocation and to provide a (possibly empty) result.
Only two-way method requests which are defined to provide a (possibly empty) result in the protocol declaration will elicit a method response. One-way method requests must not produce a method response.
A method response message provides the result associated with a prior method request. The body of the message contains the method results as if they were packed in a struct.
Here we see that the answer to 912 / 43 is 21 with a remainder of 9. Note the txid value of 1 — this identifies the transaction. The ordinal value of 2 indicates the method — in this case, the Divide() method.
Below, we see that 123 + 456 is 579. Here, the txid value is now 2 — this is simply the next transaction number assigned to the transaction. The ordinal is 1, indicating Add(), and note that the result requires 4 bytes of padding in order to make the body object have a size that's a multiple of 8 bytes.
And finally, the Clear() method is different than the Add() and Divide() in two important ways:
txid is zero).
An example of an event is the OnError() event in our Calculator.
The server sends an unsolicited event request to the client to indicate that an asynchronous event occurred, as specified by the protocol declaration.
In the Calculator example, we can imagine that an attempt to divide by zero would cause the OnError() event to be sent with a “divide by zero” status code prior to the connection being closed. This allows the client to distinguish between the connection being closed due to an error, as opposed to for other reasons (such as the calculator process terminating abnormally).
Notice how the txid is zero (indicating this is not part of a transaction), and ordinal is 4 (indicating the OnError() method).
The body contains the event arguments as if they were packed in a struct, just as with method result messages. Note that the body is padded to maintain 8-byte alignment.
An epitaph is a message with ordinal 0xFFFFFFFF. A server may send an epitaph as the last message prior to closing the connection, to provide an indication of why the connection is being closed. No further messages may be sent through the channel after the epitaph. Epitaphs are not sent from clients to servers.
The epitaph contains an error status. The error status of the epitaph is stored in the reserved uint32 of the message header. The reserved word is treated as being of type zx_status_t: negative numbers are reserved for system error codes, positive numbers are reserved for application error codes, and ZX_OK is used to indicate normal connection closure. The message is otherwise empty.
sizeof(T) denotes the size in bytes for an object of type T.
alignof(T) denotes the alignment factor in bytes to store an object of type T.
FIDL primitive types are stored at offsets in the message which are a multiple of their size in bytes. Thus for primitives T, alignof(T) == sizeof(T). This is called natural alignment. It has the nice property of satisfying typical alignment requirements of modern CPU architectures.
FIDL complex types, such as structs and arrays, are stored at offsets in the message which are a multiple of the maximum alignment factor of all of their fields. Thus for complex types T, alignof(T) == max(alignof(F:T)) over all fields F in T. It has the nice property of satisfying typical C structure packing requirements (which can be enforced using packing attributes in the generated code). The size of a complex type is the total number of bytes needed to store its members properly aligned plus padding up to the type's alignment factor.
FIDL primary and secondary objects are aligned at 8-byte offsets within the message, regardless of their contents. The primary object of a FIDL message starts at offset 0. Secondary objects, which are the only possible referent of pointers within the message, always start at offsets which are a multiple of 8. (So all pointers within the message point at offsets which are a multiple of 8.)
FIDL in-line objects (complex types embedded within primary or secondary objects) are aligned according to their type. They are not forced to 8 byte alignment.
Notes:
array<T>:N, an array with N elements of type T) or implictly (a table consisting of 7 elements would have N=7).sizeof(T) in the vector entry below is in_line_sizeof(T) + out_of_line_sizeof(T).table entry below is the maximum ordinal of present field.struct entry below, the padding refers to the required padding to make the struct aligned to the widest element. For example, struct{uint32;uint8} has 3 bytes of padding, which is different than the padding to align to 8 bytes boundaries.| Type(s) | Size (in-line) | Size (out-of-line) | Alignment |
|---|---|---|---|
bool | 1 | 0 | 1 |
int8, uint8 | 1 | 0 | 1 |
int16, uint16 | 2 | 0 | 2 |
int32, uint32, float32 | 4 | 0 | 4 |
int64, uint64, float64 | 8 | 0 | 8 |
enum, bits | (underlying type) | 0 | (underlying type) |
handle, et al. | 4 | 0 | 4 |
array<T>:N | sizeof(T) * N | 0 | alignof(T) |
vector, et al. | 16 | N * sizeof(T) | 8 |
struct | sum(sizeof(fields)) + padding | 0 | 8 |
struct? | 8 | sum(sizeof(fields)) + padding | 8 |
union | 4 + max(sizeof(fields)) + padding | 0 | max(all fields) |
union? | 8 | 4 + max(sizeof(fields)) + padding | 8 |
envelope | 16 | sizeof(field) | 8 |
table | 16 | M * sizeof(envelope) + sum(aligned_to_8(sizeof(present fields)) | 8 |
xunion, xunion? | 24 | sizeof(selected variant) | 8 |
The handle entry above refers to all flavors of handles, specifically handle, handle?, handle<H>, handle<H>?, Protocol, Protocol?, request<Protocol>, and request<Protocol>?.
Similarly, the vector entry above refers to all flavors of vectors, specifically vector<T>, vector<T>?, vector<T>:N, vector<T>:N?, string, string?, string:N, and string:N?.
The creator of a message must fill all alignment padding gaps with zeros.
The consumer of a message must verify that padding contains zeros (and generate an error if not).
FIDL arrays, vectors, structures, tables, unions, and xunions enable the construction of recursive messages. Left unchecked, processing excessively deep messages could lead to resource exhaustion of the consumer.
For safety, the maximum recursion depth for all FIDL messages is limited to 32 levels of nested complex objects. The FIDL validator must enforce this by keeping track of the current nesting level during message validation.
Complex objects are arrays, vectors, structures, tables, unions, or xunions which contain pointers or handles which require fix-up. These are precisely the kinds of objects for which encoding tables must be generated. See C Language Bindings for information about encoding tables. Therefore, limiting the nesting depth of complex objects has the effect of limiting the recursion depth for traversal of encoding tables.
Formal definition:
The purpose of message validation is to discover wire format errors early before they have a chance to induce security or stability problems.
Message validation is required when decoding messages received from a peer to prevent bad data from propagating beyond the service entry point.
Message validation is optional but recommended when encoding messages to send to a peer in order to help localize violated integrity constraints.
To minimize runtime overhead, validation should generally be performed as part of a single pass message encoding or decoding process, such that only a single traversal is needed. Since messages are encoded in depth-first traversal order, traversal exhibits good memory locality and should therefore be quite efficient.
For simple messages, validation may be very trivial, amounting to no more than a few size checks. While programmers are encouraged to rely on their FIDL bindings library to validate messages on their behalf, validation can also be done manually if needed.
Conformant FIDL bindings must check all of the following integrity constraints:
Defining the zero handle to mean “there is no handle” means it is safe to default-initialize wire format structures to all zeros. Zero is also the value of the ZX_HANDLE_INVALID constant.
Read The Lost Art of Structure Packing for an in-depth treatise on the subject.