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FIDL wire format specification

For more information about FIDL's overall purpose, goals, and requirements, see the concepts overview.

Core Concepts


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.


Primary and Secondary Objects

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.

Messages for transactions

A transactional FIDL message (transactional message) is used to send data from one application to another.

Note: The roles of the applications (e.g. client vs server) are not relevant to the formatting of the data.

The transactional messages section, describes how a transactional message is composed of a header message optionally followed by a body message.

Traversal Order

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.description
            visit Product.price
        visit Item.quantity

Dual Forms: Encoded vs Decoded

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.


Encoded Messages

An encoded message has been prepared for transfer to another process: it does not contain pointers (memory addresses) or handles (capabilities).

During encoding...

  • All pointers to sub-objects within the message are replaced with flags which indicate whether their referent is present or not-present,
  • All handles within the message are extracted to an associated handle vector and replaced with flags which indicate whether their referent is present or not-present.

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: 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.

Decoded Messages

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:

  • All pointers to sub-objects within the message are reconstructed using the encoded present and not-present flags.
  • All handles within the message are restored from the associated handle vector using the encoded present and not-present flags.

The resulting decoded message is ready to be consumed directly from memory.

Inlined Objects

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).

Type Details

In this section, we illustrate the encodings for all FIDL objects.


  • Value stored in little-endian format.
  • Packed with natural alignment.
    • Each m-byte primitive is stored on an m-byte boundary.
  • Not nullable.

The following primitive types are supported:

Signed integerint8, int16, int32, int64
Unsigned integeruint8, uint16, uint32, uint64
IEEE 754 floating-pointfloat32, float64
strings(not a primitive, see Strings)

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.



Enums and Bits

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 and 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

Aggregate objects serve as containers of other objects. They may store that data in-line or out-of-line, depending on their type.


  • Fixed length sequence of homogeneous elements.
  • Packed with natural alignment of their elements.
    • Alignment of array is the same as the alignment of its elements.
    • Each subsequent element is aligned on element's alignment boundary.
  • The stride of the array is exactly equal to the size of the element (which includes the padding required to satisfy element alignment constraints).
  • Not nullable.
  • There is no special case for arrays of bools. Each bool element takes one byte as usual.

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. Note: An array's size MUST be no more than 232-1. For additional details, see FTP-059.



  • Variable-length sequence of homogeneous elements.
  • Nullable; null vectors and empty vectors are distinct.
  • Can specify a maximum size, e.g. vector<T>:40 for a maximum 40 element vector.
  • Stored as a 16 byte record consisting of:
    • size: 64-bit unsigned number of elements Note: A vector's size MUST be no more than 232-1. For additional details, see FTP-059.
    • data: 64-bit presence indication or pointer to out-of-line element data
  • When encoded for transfer, data indicates presence of content:
    • 0: vector is null
    • UINTPTR_MAX: vector is non-null, data is the next out-of-line object
  • When decoded for consumption, data is a pointer to content:
    • 0: vector is null
    • <valid pointer>: vector is non-null, data is at indicated memory address
  • There is no special case for vectors of bools. Each bool element takes one byte as usual.

Vectors 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 T
  • vector<T>:N, vector<T>:N?: vector with maximum length of N elements

T 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):


Note: 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!):


A structure can be:

  • empty — it has no fields. Such a structure is 1 byte in size, with an alignment of 1 byte, and is exactly equivalent to a structure containing a uint8 with the value zero.
  • non-nullable — the structure's contents are stored in-line.
  • nullable — the structure's contents are stored out-of-line and accessed through an indirect reference.

Storage of a structure depends on whether it is nullable at point of reference.

  • Non-nullable structures:
    • Contents are stored in-line within their containing type, enabling very efficient aggregate structures to be constructed.
    • The structure layout does not change when inlined; its fields are not repacked to fill gaps in its container.
  • Nullable structures:
    • Contents are stored out-of-line and accessed through an indirect reference.
    • When encoded for transfer, stored reference indicates presence of structure:
      • 0: reference is null
      • UINTPTR_MAX: reference is non-null, structure content is the next out-of-line object
    • When decoded for consumption, stored reference is a pointer:
      • 0: reference is null
      • <valid pointer>: reference is non-null, structure content is at indicated memory address

Structs are denoted by their declared name (e.g. Circle) and nullability:

  • Point: non-nullable Point
  • Color?: nullable Color

The following example illustrates:

  • Structure layout (order, packing, and alignment)
  • A non-nullable structure (Point)
  • A nullable structure (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:


  1. The first member, bool filled, occupies one byte, but requires three bytes of padding because of the next member, which has a 4-byte alignment requirement.
  2. The 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.
  3. 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.
  4. The 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).
  5. The 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.
  6. The out-of-line data for 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]:


  1. The two bool values are “packed” together within what would have been wasted space.
  2. The padding is reduced to two bytes — this would be a good place to add a 16-bit value, or some more bools or 8-bit integers.
  3. Notice how there's no padding required after the color pointer; everything is perfectly aligned on an 8 byte boundary.

The structure now takes 40 bytes.

Note: While fidlc could automatically pack structs, like Rust, we chose not to do that in order to simplify ABI compatibility changes.


An envelope is a container for out-of-line data, used internally by tables and extensible unions. It is not exposed to the FIDL language.

It has a fixed, 16 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.


  • Record type consisting of the number of elements and a pointer.
  • Pointer points to an array of envelopes, each of which contains one element.
  • Each element is associated with an ordinal.
  • Ordinals are sequential, gaps incur an empty envelope cost and hence are discouraged.

Tables are denoted by their declared name (e.g., Value), and are not nullable:

  • Value: non-nullable Value

The following example shows how tables are laid out according to their fields.

table Value {
    1: int16 command;
    2: Circle data;
    3: float64 offset;



  • Record type consisting of an ordinal and an envelope.
  • Ordinal indicates member selection, and is represented with a uint64.
  • Each element is associated with a user specified ordinal.
  • Ordinals are sequential. Unlike tables, gaps in ordinals do not incur a wire format space cost.
  • Nullable unions are represented with a 0 ordinal, and an empty envelope.
  • Empty unions are not allowed.

unions are denoted by their declared name (e.g. Value) and nullability:

  • Value: non-nullable Value
  • Value?: nullable Value

The following example shows how unions are laid out according to their fields.

union Value {
    1: int16 command;
    2: Circle data;
    3: float64 offset;


Transactional Messages

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 userspace
    • A value of 0 for txid is reserved for messages which do not require a response from the other side. Note: For more details on txid allocation, see zx_channel_call().
  • uint8[3] flags, MUST NOT be checked by bindings. These flags can be used to enable soft transitions of the wire format. See Header Flags for a description of the current flag definitions.
  • uint8 magic number, determines if two wire formats are compatible.
  • uint64 ordinal
    • The zero ordinal is invalid.
    • Ordinals with the most significant bit set are reserved for control messages and future expansion.
    • Ordinals without the most significant bit set indicate method calls and responses.

There are three kinds of transactional messages:

  • method requests,
  • method responses, and
  • event requests.

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);
    -> OnError(uint32 status_code);

The Add() and Divide() methods illustrate both the method request (sent from the client to the server), and a method response (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.

Method Request Messages

The client of an interface sends method request messages to the server in order to invoke the method.

Method Response Messages

The server sends method response 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:

  • it does not have a body (that is, it consists solely of the header), and
  • it does not solicit a response from the interface (txid is zero).


Event Requests

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.

Epitaph (Control Message Ordinal 0xFFFFFFFF)

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.


Size and Alignment

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.



  • N indicates the number of elements, whether stated explicitly (as in array<T>:N, an array with N elements of type T) or implicitly (a table consisting of 7 elements would have N=7).
  • The out-of-line size is always padded to 8 bytes. We indicate the content size below, excluding the padding.
  • sizeof(T) in the vector entry below is
    in_line_sizeof(T) + out_of_line_sizeof(T).
  • M in the table entry below is the maximum ordinal of present field.
  • In the 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
int8, uint8101
int16, uint16202
int32, uint32, float32404
int64, uint64, float64808
enum, bits(underlying type)0(underlying type)
handle, et al.404
array<T>:Nsizeof(T) * N0alignof(T)
vector, et al.16N * sizeof(T)8
structsum(sizeof(fields)) + padding08
struct?8sum(sizeof(fields)) + padding8
table16M * sizeof(envelope) + sum(aligned_to_8(sizeof(present fields))8
union, union?24sizeof(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).

Maximum Recursion Depth

FIDL arrays, vectors, structures, tables, and unions 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, or unions 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 message body is defined to be at nesting level 0.
  • Each time the validator encounters a complex object, it increments the nesting level, recursively validates the object, then decrements the nesting level.
  • If at any time the nesting level exceeds 31, a validation error is raised and validation terminates.


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:

  • The total size of the message including all of its out-of-line sub-objects exactly equals the total size of the buffer that contains it. All sub-objects are accounted for.
  • The total number of handles referenced by the message exactly equals the total size of the handle table. All handles are accounted for.
  • The maximum recursion depth for complex objects is not exceeded.
  • All enum values fall within their defined range.
  • All union tag values fall within their defined range.
  • Encoding only:
    • All pointers to sub-objects encountered during traversal refer precisely to the next buffer position where a sub-object is expected to appear. As a corollary, pointers never refer to locations outside of the buffer.
  • Decoding only:
    • All present and not-present flags for referenced sub-objects hold the value 0 or UINTPTR_MAX only.
    • All present and not-present flags for referenced handles hold the value 0 or UINT32_MAX only.

Header Flags


BitCurrent UsagePast Usages
7 (MSB)Unused


BitCurrent UsagePast Usages
7 (MSB)Unused


BitCurrent UsagePast Usages
7 (MSB)Unused

Footnote 1

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.

Footnote 2

Read The Lost Art of Structure Packing for an in-depth treatise on the subject.