zircon/vdso/channel.fidl

// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
library zx;

alias HandleOp = uint32;

// TODO(scottmg): ZX_HANDLE_OP_xyz here.

type HandleInfo = resource struct {
    handle Handle;
    type ObjType;
    rights Rights;
    unused uint32;
};

// TODO(https://fxbug.dev/42061412): We cannot yet conveniently express this.
type ChannelCallArgs = resource struct {};

type HandleDisposition = resource struct {
    operation HandleOp;
    handle Handle;
    type ObjType;
    rights Rights;
    result Status;
};

// TODO(https://fxbug.dev/42061412): We cannot yet conveniently express this.
type ChannelCallEtcArgs = resource struct {};

@transport("Syscall")
closed protocol Channel {
    /// ## Summary
    ///
    /// Create a channel.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_create(uint32_t options,
    ///                               zx_handle_t* out0,
    ///                               zx_handle_t* out1);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_channel_create()` creates a channel, a bi-directional
    /// datagram-style message transport capable of sending raw data bytes
    /// as well as handles from one side to the other.
    ///
    /// Two handles are returned on success, providing access to both sides
    /// of the channel.  Messages written to one handle may be read from
    /// the opposite.
    ///
    /// The handles will have these rights:
    ///
    ///  - `ZX_RIGHT_TRANSFER`: allowing them to be sent to another process via
    ///    [`zx_channel_write()`].
    ///  - `ZX_RIGHT_WAIT`: allowing one to wait for its signals.
    ///  - `ZX_RIGHT_INSPECT`
    ///  - `ZX_RIGHT_READ`: allowing messages to be read from them.
    ///  - `ZX_RIGHT_WRITE`: allowing messages to be written to them.
    ///  - `ZX_RIGHT_SIGNAL`
    ///  - `ZX_RIGHT_SIGNAL_PEER`
    ///
    /// ## Rights
    ///
    /// Caller job policy must allow `ZX_POL_NEW_CHANNEL`.
    ///
    /// ## Return value
    ///
    /// `zx_channel_create()` returns `ZX_OK` on success. In the event
    /// of failure, a negative error value is returned.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_INVALID_ARGS`  *out0* or *out1* is an invalid pointer or NULL or
    /// *options* is any value other than 0.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// ## See also
    ///
    ///  - [`zx_channel_call()`]
    ///  - [`zx_channel_read()`]
    ///  - [`zx_channel_write()`]
    ///  - [`zx_handle_close()`]
    ///  - [`zx_handle_replace()`]
    ///  - [`zx_object_wait_async()`]
    ///  - [`zx_object_wait_many()`]
    ///  - [`zx_object_wait_one()`]
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_handle_close()`]: handle_close.md
    /// [`zx_handle_replace()`]: handle_replace.md
    /// [`zx_object_wait_async()`]: object_wait_async.md
    /// [`zx_object_wait_many()`]: object_wait_many.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    strict Create(struct {
        options uint32;
    }) -> (resource struct {
        out0 Handle;
        out1 Handle;
    }) error Status;

    /// ## Summary
    ///
    /// Read a message from a channel.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_read(zx_handle_t handle,
    ///                             uint32_t options,
    ///                             void* bytes,
    ///                             zx_handle_t* handles,
    ///                             uint32_t num_bytes,
    ///                             uint32_t num_handles,
    ///                             uint32_t* actual_bytes,
    ///                             uint32_t* actual_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_channel_read()` attempts to read the first message from the channel
    /// specified by *handle* into the provided *bytes* and/or *handles* buffers.
    ///
    /// The parameters *num_bytes* and *num_handles* are used to specify the size of the
    /// respective read buffers. *num_bytes* is a count of bytes, and
    /// *num_handles* is a count of elements of type `zx_handle_t`.
    ///
    /// The length of *bytes*, in bytes, is stored in the location pointed to by
    /// *actual_bytes*.  The number of handles is stored in the location pointed to by
    /// *actual_handles*.  Either *actual_bytes* or *actual_handles* may be NULL, in
    /// which case they will be ignored.
    ///
    /// Channel messages may contain both byte data and handle payloads and may
    /// only be read in their entirety.  Partial reads are not possible.
    ///
    /// The *bytes* buffer is written before the *handles* buffer. In the event of
    /// overlap between these two buffers, the contents written to *handles*
    /// will overwrite the portion of *bytes* it overlaps.
    ///
    /// When communicating to an untrusted party over a channel, it is recommended that
    /// the [`zx_channel_read_etc()`] form is used and each handle type
    /// and rights are validated against the expected values.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_READ`.
    ///
    /// ## Return value
    ///
    /// Returns `ZX_OK` on success. If non-NULL, the locations pointed to by
    /// *actual_bytes* and *actual_handles* contain the exact number of bytes and count
    /// of handles read.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE`  *handle* is not a valid handle.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *handle* is not a channel handle.
    ///
    /// `ZX_ERR_INVALID_ARGS`  If any of *bytes*, *handles*, *actual_bytes*, or
    /// *actual_handles* are non-NULL and an invalid pointer.
    ///
    /// `ZX_ERR_ACCESS_DENIED`  *handle* does not have `ZX_RIGHT_READ`.
    ///
    /// `ZX_ERR_SHOULD_WAIT`  The channel contained no messages to read and the other side of the
    /// channel is open.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The channel contained no messages to read and the other side of the
    /// channel is closed.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_BUFFER_TOO_SMALL`  The provided *bytes* or *handles* buffers
    /// are too small (in which case, the minimum sizes necessary to receive
    /// the message will be written to *actual_bytes* and *actual_handles*,
    /// provided they are non-NULL). If *options* has `ZX_CHANNEL_READ_MAY_DISCARD`
    /// set, then the message is discarded.
    ///
    /// ## See also
    ///
    ///  - [`zx_channel_call()`]
    ///  - [`zx_channel_create()`]
    ///  - [`zx_channel_read_etc()`]
    ///  - [`zx_channel_write()`]
    ///  - [`zx_channel_write_etc()`]
    ///  - [`zx_handle_close()`]
    ///  - [`zx_handle_replace()`]
    ///  - [`zx_object_wait_async()`]
    ///  - [`zx_object_wait_many()`]
    ///  - [`zx_object_wait_one()`]
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read_etc()`]: channel_read_etc.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_channel_write_etc()`]: channel_write_etc.md
    /// [`zx_handle_close()`]: handle_close.md
    /// [`zx_handle_replace()`]: handle_replace.md
    /// [`zx_object_wait_async()`]: object_wait_async.md
    /// [`zx_object_wait_many()`]: object_wait_many.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    @handle_unchecked
    strict Read(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        @out
        @voidptr
        bytes experimental_pointer<byte>;
        @out
        handles experimental_pointer<Handle>;
        num_bytes uint32;
        num_handles uint32;
    }) -> (resource struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    /// ## Summary
    ///
    /// Read a message from a channel.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_read_etc(zx_handle_t handle,
    ///                                 uint32_t options,
    ///                                 void* bytes,
    ///                                 zx_handle_info_t* handles,
    ///                                 uint32_t num_bytes,
    ///                                 uint32_t num_handles,
    ///                                 uint32_t* actual_bytes,
    ///                                 uint32_t* actual_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// See [`zx_channel_read()`] for a full description.
    ///
    /// Both forms of read behave the same except that [`zx_channel_read()`] returns an
    /// array of raw `zx_handle_t` handle values while `zx_channel_read_etc()` returns
    /// an array of `zx_handle_info_t` structures of the form:
    ///
    /// ```
    /// typedef struct {
    ///     zx_handle_t handle;     // handle value
    ///     zx_obj_type_t type;     // type of object, see ZX_OBJ_TYPE_
    ///     zx_rights_t rights;     // handle rights
    ///     uint32_t unused;        // set to zero
    /// } zx_handle_info_t;
    /// ```
    ///
    /// When communicating to an untrusted party over a channel, it is recommended
    /// that the `zx_channel_read_etc()` form is used and each handle type and rights
    /// are validated against the expected values.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_READ`.
    ///
    /// ## Return value
    ///
    /// Both forms of read return `ZX_OK` on success, if *actual_bytes*
    /// and *actual_handles* (if non-NULL), contain the exact number of bytes
    /// and count of handles read.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE`  *handle* is not a valid handle.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *handle* is not a channel handle.
    ///
    /// `ZX_ERR_INVALID_ARGS`  If any of *bytes*, *handles*, *actual_bytes*, or
    /// *actual_handles* are non-NULL and an invalid pointer.
    ///
    /// `ZX_ERR_ACCESS_DENIED`  *handle* does not have `ZX_RIGHT_READ`.
    ///
    /// `ZX_ERR_SHOULD_WAIT`  The channel contained no messages to read and the other side of the
    /// channel is open.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The channel contained no messages to read and the other side of the
    /// channel is closed.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_BUFFER_TOO_SMALL`  The provided *bytes* or *handles* buffers
    /// are too small (in which case, the minimum sizes necessary to receive
    /// the message will be written to *actual_bytes* and *actual_handles*,
    /// provided they are non-NULL). If *options* has `ZX_CHANNEL_READ_MAY_DISCARD`
    /// set, then the message is discarded.
    ///
    /// ## Notes
    ///
    /// *num_handles* and *actual_handles* are counts of the number of elements
    /// in the *handles* array, not its size in bytes.
    ///
    /// ## See also
    ///
    ///  - [`zx_channel_call()`]
    ///  - [`zx_channel_create()`]
    ///  - [`zx_channel_read()`]
    ///  - [`zx_channel_write()`]
    ///  - [`zx_channel_write_etc()`]
    ///  - [`zx_handle_close()`]
    ///  - [`zx_handle_replace()`]
    ///  - [`zx_object_wait_async()`]
    ///  - [`zx_object_wait_many()`]
    ///  - [`zx_object_wait_one()`]
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_channel_write_etc()`]: channel_write_etc.md
    /// [`zx_handle_close()`]: handle_close.md
    /// [`zx_handle_replace()`]: handle_replace.md
    /// [`zx_object_wait_async()`]: object_wait_async.md
    /// [`zx_object_wait_many()`]: object_wait_many.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    strict ReadEtc(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        @out
        @voidptr
        bytes experimental_pointer<byte>;
        @out
        handles experimental_pointer<HandleInfo>;
        num_bytes uint32;
        num_handles uint32;
    }) -> (resource struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    /// ## Summary
    ///
    /// Write a message to a channel.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_write(zx_handle_t handle,
    ///                              uint32_t options,
    ///                              const void* bytes,
    ///                              uint32_t num_bytes,
    ///                              const zx_handle_t* handles,
    ///                              uint32_t num_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_channel_write()` attempts to write a message of *num_bytes*
    /// bytes and *num_handles* handles to the channel specified by
    /// *handle*.  The pointers *handles* and *bytes* may be NULL if their
    /// respective sizes are zero.
    ///
    /// On success, all *num_handles* of the handles in the *handles* array
    /// are attached to the message and will become available to the reader
    /// of that message from the opposite end of the channel.
    ///
    /// All handles are discarded and no longer available to the caller, on
    /// success or failure. Use [`zx_channel_write_etc()`] if handles need
    /// to be preserved by the sender.
    ///
    /// It is invalid to include *handle* (the handle of the channel being written
    /// to) in the *handles* array (the handles being sent in the message).
    ///
    /// The maximum number of handles that may be sent in a message is
    /// `ZX_CHANNEL_MAX_MSG_HANDLES`, which is 64.
    ///
    /// The maximum number of bytes that may be sent in a message is
    /// `ZX_CHANNEL_MAX_MSG_BYTES`, which is 65536.
    ///
    /// Messages are drained by [`zx_channel_read()`] or [`zx_channel_read_etc()`].
    /// Failure to drain the messages in a timely fashion can cause excessive kernel
    /// memory to be used, which might generate an exception. See
    /// [ipc limits](/docs/concepts/kernel/ipc_limits.md) for details.
    ///
    /// ### ZX_CHANNEL_WRITE_USE_IOVEC option
    ///
    /// When the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified, `bytes` is
    /// interpreted as an array of `zx_channel_iovec_t`, specifying slices of bytes to
    /// sequentially copy to the message in order. `num_bytes` specifies the number of
    /// `zx_channel_iovec_t` array elements in `bytes`.
    ///
    /// ```c
    /// typedef struct zx_channel_iovec {
    ///   const void* buffer;      // User-space bytes.
    ///   uint32_t capacity;       // Number of bytes.
    ///   uint32_t reserved;       // Reserved.
    /// } zx_channel_iovec_t;
    /// ```
    ///
    /// There can be at most `ZX_CHANNEL_MAX_MSG_IOVEC` or `8192`
    /// `zx_channel_iovec_t` elements of the `bytes` array with the sum of `capacity`
    /// across all `zx_channel_iovec_t` not exceeding `ZX_CHANNEL_MAX_MSG_BYTES` or
    /// `65536` bytes. `buffer` need not be aligned and it may only be `NULL` if
    /// `capacity` is zero. `reserved` must be set to zero.
    ///
    /// Either all `zx_channel_iovec_t` are copied and the message is sent, or none
    /// are copied and the message is not sent. Usage for sending handles is unchanged.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_WRITE`.
    ///
    /// Every entry of *handles* must have `ZX_RIGHT_TRANSFER`.
    ///
    /// ## Return value
    ///
    /// `zx_channel_write()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE` `handle` is not a valid handle, any element in
    /// `handles` is not a valid handle, or there are repeated handles among the
    /// handles in the `handles` array.
    ///
    /// `ZX_ERR_WRONG_TYPE` `handle` is not a channel handle.
    ///
    /// `ZX_ERR_INVALID_ARGS` `bytes` is an invalid pointer, `handles`
    /// is an invalid pointer, or `options` contains an invalid option bit.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_INVALID_ARGS` will be produced if the `buffer` field contains an
    /// invalid pointer or if the reserved field is non-zero.
    ///
    /// `ZX_ERR_NOT_SUPPORTED` `handle` was found in the `handles` array.
    /// A handle to the channel performing the write cannot be included in the
    /// `handles` array. In other words a channel handle cannot be written to its own channel.
    /// Fix the error by making sure that `handle` is not in the `handles` array.
    ///
    /// `ZX_ERR_ACCESS_DENIED` `handle` does not have `ZX_RIGHT_WRITE` or
    /// any element in `handles` does not have `ZX_RIGHT_TRANSFER`.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The other side of the channel is closed.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_OUT_OF_RANGE`  `num_bytes` or `num_handles` are larger than
    /// `ZX_CHANNEL_MAX_MSG_BYTES` or `ZX_CHANNEL_MAX_MSG_HANDLES` respectively.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_OUT_OF_RANGE` will be produced if `num_bytes` is larger than
    /// `ZX_CHANNEL_MAX_MSG_IOVEC` or the sum of the iovec capacities exceeds
    /// `ZX_CHANNEL_MAX_MSG_BYTES`.
    ///
    /// ## Notes
    ///
    /// *num_handles* is a count of the number of elements in the *handles*
    /// array, not its size in bytes.
    ///
    /// The byte size limitation on messages is not yet finalized.
    ///
    /// ## See also
    ///
    ///  - [`zx_channel_call()`]
    ///  - [`zx_channel_create()`]
    ///  - [`zx_channel_read()`]
    ///  - [`zx_channel_read_etc()`]
    ///  - [`zx_channel_write_etc()`]
    ///  - [`zx_handle_close()`]
    ///  - [`zx_handle_replace()`]
    ///  - [`zx_object_wait_async()`]
    ///  - [`zx_object_wait_many()`]
    ///  - [`zx_object_wait_one()`]
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_read_etc()`]: channel_read_etc.md
    /// [`zx_channel_write_etc()`]: channel_write_etc.md
    /// [`zx_handle_close()`]: handle_close.md
    /// [`zx_handle_replace()`]: handle_replace.md
    /// [`zx_object_wait_async()`]: object_wait_async.md
    /// [`zx_object_wait_many()`]: object_wait_many.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    strict Write(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        @voidptr
        @size32
        bytes vector<byte>:CHANNEL_MAX_MSG_BYTES;
        @release
        @size32
        handles vector<Handle>:CHANNEL_MAX_MSG_HANDLES;
    }) -> () error Status;

    /// ## Summary
    ///
    /// Write a message to a channel.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_write_etc(zx_handle_t handle,
    ///                                  uint32_t options,
    ///                                  const void* bytes,
    ///                                  uint32_t num_bytes,
    ///                                  zx_handle_disposition_t* handles,
    ///                                  uint32_t num_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// Like [`zx_channel_write()`] it attempts to write a message of *num_bytes*
    /// bytes and *num_handles* handles to the channel specified by *handle*, but in
    /// addition it will perform operations for the handles that are being
    /// transferred with *handles* being an array of `zx_handle_disposition_t`:
    ///
    /// ```
    /// typedef struct zx_handle_disposition {
    ///     zx_handle_op_t operation;
    ///     zx_handle_t handle;
    ///     zx_obj_type_t type;
    ///     zx_rights_t rights;
    ///     zx_status_t result;
    /// } zx_handle_disposition_t;
    /// ```
    /// In zx_handle_disposition_t, *handle* is the source handle to be operated on,
    /// *rights* is the desired final rights (not a mask) and *result* must be set
    /// to `ZX_OK`. All source handles must have `ZX_RIGHT_TRANSFER`, but
    /// it can be removed in *rights* so that it is not available to the message
    /// receiver.
    ///
    /// *type* is used to perform validation of the object type that the caller
    /// expects *handle* to be. It can be *ZX_OBJ_TYPE_NONE* to skip validation
    /// checks or one of `zx_obj_type_t` defined types.
    ///
    /// The operation applied to *handle* is one of:
    ///
    /// *   `ZX_HANDLE_OP_MOVE` This is equivalent to first issuing [`zx_handle_replace()`] then
    ///      [`zx_channel_write()`]. The source handle is always closed.
    ///
    /// *   `ZX_HANDLE_OP_DUPLICATE` This is equivalent to first issuing [`zx_handle_duplicate()`]
    ///     then [`zx_channel_write()`]. The source handle always remains open and accessible to the
    ///     caller.
    ///
    /// *handle* will be transferred with capability *rights* which can be `ZX_RIGHT_SAME_RIGHTS`
    /// or a reduced set of rights, or `ZX_RIGHT_NONE`. In addition, this operation allows removing
    /// `ZX_RIGHT_TRANSFER` in *rights* so that capability is not available for the receiver.
    ///
    /// If any operation fails, the error code for that source handle is written to *result*, and the
    /// first failure is made available in the return value for `zx_channel_write_etc()`. All
    /// operations in the *handles* array are attempted, even if one or more operations fail.
    ///
    /// All operations for each entry must succeed for the message to be written. On success, handles
    /// are attached to the message and will become available to the reader of that message from the
    /// opposite end of the channel.
    ///
    /// It is invalid to include *handle* (the handle of the channel being written to) in the
    /// *handles* array (the handles being sent in the message).
    ///
    /// The maximum number of handles that may be sent in a message is `ZX_CHANNEL_MAX_MSG_HANDLES`,
    /// which is 64.
    ///
    /// The maximum number of bytes that may be sent in a message is `ZX_CHANNEL_MAX_MSG_BYTES`,
    /// which is 65536.
    ///
    /// Messages are drained by [`zx_channel_read()`] or [`zx_channel_read_etc()`]. Failure to drain the
    /// messages in a timely fashion can cause excessive kernel memory to be used, which might generate an
    /// exception. See [ipc limits](/docs/concepts/kernel/ipc_limits.md) for details.
    ///
    /// ### ZX_CHANNEL_WRITE_USE_IOVEC option
    ///
    /// When the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified, `bytes` is
    /// interpreted as an array of `zx_channel_iovec_t`, specifying slices of bytes to
    /// sequentially copy to the message in order. `num_bytes` specifies the number of
    /// `zx_channel_iovec_t` array elements in `bytes`.
    ///
    /// ```c
    /// typedef struct zx_channel_iovec {
    ///   const void* buffer;      // User-space bytes.
    ///   uint32_t capacity;       // Number of bytes.
    ///   uint32_t reserved;       // Reserved.
    /// } zx_channel_iovec_t;
    /// ```
    ///
    /// There can be at most `ZX_CHANNEL_MAX_MSG_IOVEC` or `8192`
    /// `zx_channel_iovec_t` elements of the `bytes` array with the sum of `capacity`
    /// across all `zx_channel_iovec_t` not exceeding `ZX_CHANNEL_MAX_MSG_BYTES` or
    /// `65536` bytes. `buffer` need not be aligned and it may only be `NULL` if
    /// `capacity` is zero. `reserved` must be set to zero.
    ///
    /// Either all `zx_channel_iovec_t` are copied and the message is sent, or none
    /// are copied and the message is not sent. Usage for sending handles is unchanged.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_WRITE`.
    ///
    /// Every entry of *handles* must have `ZX_RIGHT_TRANSFER`.
    ///
    /// ## Return value
    ///
    /// `zx_channel_write_etc()` returns `ZX_OK` on success.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE`  *handle* is not a valid handle, any source handle in
    /// *handles* is not a valid handle, or there are repeated handles
    /// in the *handles* array if `ZX_HANDLE_OP_DUPLICATE` flags is not present.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *handle* is not a channel handle, or any source handle
    /// in *handles* did not match the object type *type*.
    ///
    /// `ZX_ERR_INVALID_ARGS`  *bytes* is an invalid pointer, *handles*
    /// is an invalid pointer, or *options* is nonzero, or *operation* is not
    /// one of ZX_HANDLE_OP_MOVE or ZX_HANDLE_OP_DUPLICATE, or any source
    /// handle in *handles\[i\]->handle* did not have the rights specified in
    /// *whandle\[i\]->rights*.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_INVALID_ARGS` will be produced if the *buffer* field contains an
    /// invalid pointer or if the reserved field is non-zero.
    ///
    /// `ZX_ERR_NOT_SUPPORTED`  *handle* is included in the *handles* array.
    ///
    /// `ZX_ERR_ACCESS_DENIED`  *handle* does not have `ZX_RIGHT_WRITE` or
    /// any source handle in *handles* does not have `ZX_RIGHT_TRANSFER`, or
    /// any source handle in *handles* does not have `ZX_RIGHT_DUPLICATE` when
    /// `ZX_HANDLE_OP_DUPLICATE` operation is specified.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The other side of the channel is closed.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_OUT_OF_RANGE`  *num_bytes* or *num_handles* are larger than
    /// `ZX_CHANNEL_MAX_MSG_BYTES` or `ZX_CHANNEL_MAX_MSG_HANDLES` respectively.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_OUT_OF_RANGE` will be produced if *num_bytes* is larger than
    /// `ZX_CHANNEL_MAX_MSG_IOVEC` or the sum of the iovec capacities exceeds
    /// `ZX_CHANNEL_MAX_MSG_BYTES`.
    ///
    /// ## Notes
    ///
    /// If the caller removes the `ZX_RIGHT_TRANSFER` to a handle attached
    /// to a message, the reader of the message will receive a handle that cannot
    /// be written to any other channel, but still can be using according to its
    /// rights and can be closed if not needed.
    ///
    /// ## See also
    ///
    ///  - [`zx_channel_call()`]
    ///  - [`zx_channel_create()`]
    ///  - [`zx_channel_read()`]
    ///  - [`zx_channel_read_etc()`]
    ///  - [`zx_channel_write()`]
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_read_etc()`]: channel_read_etc.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_handle_duplicate()`]: handle_duplicate.md
    /// [`zx_handle_replace()`]: handle_replace.md
    strict WriteEtc(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        @voidptr
        @size32
        bytes vector<byte>:CHANNEL_MAX_MSG_BYTES;
        @inout
        @size32
        handles vector<HandleDisposition>:CHANNEL_MAX_MSG_HANDLES;
    }) -> () error Status;

    @internal
    strict CallNoretry(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        deadline Time;
        args ChannelCallArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    @internal
    strict CallFinish(resource struct {
        deadline Time;
        args ChannelCallArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    // TODO(scottmg): Express "All wr_handles of args must have ZX_RIGHT_TRANSFER."
    /// ## Summary
    ///
    /// Send a message to a channel and await a reply.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_call(zx_handle_t handle,
    ///                             uint32_t options,
    ///                             zx_time_t deadline,
    ///                             const zx_channel_call_args_t* args,
    ///                             uint32_t* actual_bytes,
    ///                             uint32_t* actual_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_channel_call()` is like a combined [`zx_channel_write()`], [`zx_object_wait_one()`],
    /// and [`zx_channel_read()`], with the addition of a feature where a transaction id at
    /// the front of the message payload *bytes* is used to match reply messages with send
    /// messages, enabling multiple calling threads to share a channel without any additional
    /// userspace bookkeeping.
    ///
    /// The write and read phases of this operation behave like [`zx_channel_write()`] and
    /// [`zx_channel_read()`] with the difference that their parameters are provided via the
    /// `zx_channel_call_args_t` structure.
    ///
    /// The first four bytes of the written and read back messages are treated as a
    /// transaction ID of type `zx_txid_t`.  The kernel generates a txid for the
    /// written message, replacing that part of the message as read from userspace.
    /// The kernel generated txid will be between 0x80000000 and 0xFFFFFFFF, and will
    /// not collide with any txid from any other `zx_channel_call()` in progress against
    /// this channel endpoint.  If the written message has a length of fewer than four
    /// bytes, an error is reported.
    ///
    /// When the outbound message is written, simultaneously an interest is registered
    /// for inbound messages of the matching txid.
    ///
    /// *deadline* may be automatically adjusted according to the job's [timer slack]
    /// policy.
    ///
    /// While the slack-adjusted *deadline* has not passed, if an inbound message
    /// arrives with a matching txid, instead of being added to the tail of the general
    /// inbound message queue, it is delivered directly to the thread waiting in
    /// `zx_channel_call()`.
    ///
    /// If such a reply arrives after the slack-adjusted *deadline* has passed, it will
    /// arrive in the general inbound message queue, cause `ZX_CHANNEL_READABLE` to be
    /// signaled, etc.
    ///
    /// Inbound messages that are too large to fit in *rd_num_bytes* and *rd_num_handles*
    /// are discarded and `ZX_ERR_BUFFER_TOO_SMALL` is returned in that case.
    ///
    /// As with [`zx_channel_write()`], the handles in *handles* are always consumed by
    /// `zx_channel_call()` and no longer exist in the calling process.
    ///
    /// ### ZX_CHANNEL_WRITE_USE_IOVEC option
    ///
    /// When the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified, `wr_bytes` is
    /// interpreted as an array of `zx_channel_iovec_t`, specifying slices of bytes to
    /// sequentially copy to the message in order. `num_wr_bytes` specifies the number
    /// of `zx_channel_iovec_t` array elements in `wr_bytes`.
    ///
    /// ```c
    /// typedef struct zx_channel_iovec {
    ///   const void* buffer;      // User-space bytes.
    ///   uint32_t capacity;       // Number of bytes.
    ///   uint32_t reserved;       // Reserved.
    /// } zx_channel_iovec_t;
    /// ```
    ///
    /// There can be at most `ZX_CHANNEL_MAX_MSG_IOVEC` or `8192`
    /// `zx_channel_iovec_t` elements of the `wr_bytes` array with the sum of
    /// `capacity` across all `zx_channel_iovec_t` not exceeding
    /// `ZX_CHANNEL_MAX_MSG_BYTES` or `65536` bytes. `buffer` need not be aligned and
    /// it may only be `NULL` if `capacity` is zero. `reserved` must be set to zero.
    ///
    /// Either all `zx_channel_iovec_t` are copied and the message is sent, or none
    /// are copied and the message is not sent. Usage for sending handles is unchanged.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_READ` and have `ZX_RIGHT_WRITE`.
    ///
    /// All wr_handles of *args* must have `ZX_RIGHT_TRANSFER`.
    ///
    /// ## Return value
    ///
    /// `zx_channel_call()` returns `ZX_OK` on success and the number of bytes and
    /// count of handles in the reply message are returned via *actual_bytes* and
    /// *actual_handles*, respectively.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE`  *handle* is not a valid handle, any element in
    /// *handles* is not a valid handle, or there are duplicates among the handles
    /// in the *handles* array.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *handle* is not a channel handle.
    ///
    /// `ZX_ERR_INVALID_ARGS`  any of the provided pointers are invalid or null,
    /// or *wr_num_bytes* is less than four, or *options* is nonzero.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_INVALID_ARGS` will be produced if the *buffer* field contains an
    /// invalid pointer or if the reserved field is non-zero.
    ///
    /// `ZX_ERR_ACCESS_DENIED`  *handle* does not have `ZX_RIGHT_WRITE` or
    /// any element in *handles* does not have `ZX_RIGHT_TRANSFER`.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The other side of the channel was closed or became
    /// closed while waiting for the reply.
    ///
    /// `ZX_ERR_CANCELED`  *handle* was invalidated (e.g., closed) while waiting for a reply.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_OUT_OF_RANGE`  *wr_num_bytes* or *wr_num_handles* are larger than the
    /// largest allowable size for channel messages.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_OUT_OF_RANGE` will be produced if *num_bytes* is larger than
    /// `ZX_CHANNEL_MAX_MSG_IOVEC` or the sum of the iovec capacities exceeds
    /// `ZX_CHANNEL_MAX_MSG_BYTES`.
    ///
    /// `ZX_ERR_BUFFER_TOO_SMALL`  *rd_num_bytes* or *rd_num_handles* are too small
    /// to contain the reply message.
    ///
    /// `ZX_ERR_NOT_SUPPORTED`  one of the handles in *handles* was *handle*
    /// (the handle to the channel being written to).
    ///
    /// ## Notes
    ///
    /// The facilities provided by `zx_channel_call()` can interoperate with message dispatchers
    /// using [`zx_channel_read()`] and [`zx_channel_write()`] directly, provided the following rules
    /// are observed:
    ///
    /// 1. A server receiving synchronous messages via [`zx_channel_read()`] should ensure that the
    /// txid of incoming messages is reflected back in outgoing responses via [`zx_channel_write()`]
    /// so that clients using `zx_channel_call()` can correctly route the replies.
    ///
    /// 2. A client sending messages via [`zx_channel_write()`] that will be replied to should ensure
    /// that it uses txids between 0 and 0x7FFFFFFF only, to avoid colliding with other threads
    /// communicating via `zx_channel_call()`.
    ///
    /// If a `zx_channel_call()` returns due to `ZX_ERR_TIMED_OUT`, if the server eventually replies,
    /// at some point in the future, the reply *could* match another outbound request (provided about
    /// 2^31 `zx_channel_call()`s have happened since the original request.  This syscall is designed
    /// around the expectation that timeouts are generally fatal and clients do not expect to continue
    /// communications on a channel that is timing out.
    ///
    /// ## See also
    ///
    ///  - [timer slack]
    ///  - [`zx_channel_create()`]
    ///  - [`zx_channel_read()`]
    ///  - [`zx_channel_write()`]
    ///  - [`zx_handle_close()`]
    ///  - [`zx_handle_replace()`]
    ///  - [`zx_object_wait_async()`]
    ///  - [`zx_object_wait_many()`]
    ///  - [`zx_object_wait_one()`]
    ///
    /// [timer slack]: /docs/concepts/kernel/timer_slack.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_handle_close()`]: handle_close.md
    /// [`zx_handle_replace()`]: handle_replace.md
    /// [`zx_object_wait_async()`]: object_wait_async.md
    /// [`zx_object_wait_many()`]: object_wait_many.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    @blocking
    @vdsocall
    strict Call(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        deadline Time;
        args ChannelCallArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    @internal
    strict CallEtcNoretry(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        deadline Time;
        @inout
        args ChannelCallEtcArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    @internal
    strict CallEtcFinish(resource struct {
        deadline Time;
        @inout
        args ChannelCallEtcArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;

    /// ## Summary
    ///
    /// Send a message to a channel and await a reply.
    ///
    /// ## Declaration
    ///
    /// ```c
    /// #include <zircon/syscalls.h>
    ///
    /// zx_status_t zx_channel_call_etc(zx_handle_t handle,
    ///                                 uint32_t options,
    ///                                 zx_time_t deadline,
    ///                                 zx_channel_call_etc_args_t* args,
    ///                                 uint32_t* actual_bytes,
    ///                                 uint32_t* actual_handles);
    /// ```
    ///
    /// ## Description
    ///
    /// `zx_channel_call_etc()` writes a request to a channel and blocks until it
    /// receives a response. It is an extension of [`zx_channel_call()`] that
    /// incorporates the functionality of [`zx_channel_write_etc()`] and
    /// [`zx_channel_read_etc()`] for write and read phases, instead of the more basic
    /// [`zx_channel_write()`] and  [`zx_channel_read()`]. See [`zx_channel_call()`] for
    /// a full description of channel calls.
    ///
    /// The effect of a call to `zx_channel_call_etc()` is similar to performing a
    /// sequence of calls to [`zx_channel_write_etc()`], [`zx_object_wait_one()`] and
    /// [`zx_channel_read_etc()`] in that order. However, a key difference is that
    /// `zx_channel_call_etc()` will wait for an incoming message matches the outgoing
    /// message's transaction id. The arguments that would be supplied to
    /// `zx_channel_read_etc()` and `zx_channel_write_etc()` are instead specified with
    /// `zx_channel_call_etc_args_t`:
    ///
    /// ```
    /// typedef struct {
    ///     const void* wr_bytes;
    ///     zx_handle_disposition_t* wr_handles;
    ///     void *rd_bytes;
    ///     zx_handle_info_t* rd_handles;
    ///     uint32_t wr_num_bytes;
    ///     uint32_t wr_num_handles;
    ///     uint32_t rd_num_bytes;
    ///     uint32_t rd_num_handles;
    /// } zx_channel_call_etc_args_t;
    /// ```
    ///
    /// ### ZX_CHANNEL_WRITE_USE_IOVEC option
    ///
    /// When the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified, `wr_bytes` is
    /// interpreted as an array of `zx_channel_iovec_t`, specifying slices of bytes to
    /// sequentially copy to the message in order. `num_wr_bytes` specifies the number
    /// of `zx_channel_iovec_t` array elements in `wr_bytes`.
    ///
    /// ```c
    /// typedef struct zx_channel_iovec {
    ///   const void* buffer;      // User-space bytes.
    ///   uint32_t capacity;       // Number of bytes.
    ///   uint32_t reserved;       // Reserved.
    /// } zx_channel_iovec_t;
    /// ```
    ///
    /// There can be at most `ZX_CHANNEL_MAX_MSG_IOVEC` or `8192`
    /// `zx_channel_iovec_t` elements of the `wr_bytes` array with the sum of
    /// `capacity` across all `zx_channel_iovec_t` not exceeding
    /// `ZX_CHANNEL_MAX_MSG_BYTES` or `65536` bytes. `buffer` need not be aligned and
    /// it may only be `NULL` if `capacity` is zero. `reserved` must be set to zero.
    ///
    /// Either all `zx_channel_iovec_t` are copied and the message is sent, or none
    /// are copied and the message is not sent. Usage for sending handles is unchanged.
    ///
    /// ## Rights
    ///
    /// *handle* must be of type `ZX_OBJ_TYPE_CHANNEL` and have `ZX_RIGHT_READ` and have `ZX_RIGHT_WRITE`.
    ///
    /// All wr_handles of *args* must have `ZX_RIGHT_TRANSFER`.
    ///
    /// ## Return value
    ///
    /// `zx_channel_call_etc()` returns `ZX_OK` on success and the number of bytes and
    /// count of handles in the reply message are returned via *actual_bytes* and
    /// *actual_handles*, respectively.
    ///
    /// ## Errors
    ///
    /// `ZX_ERR_BAD_HANDLE`  *handle* is not a valid handle, any element in
    /// *handles* is not a valid handle, or there are duplicates among the handles
    /// in the *handles* array.
    ///
    /// `ZX_ERR_WRONG_TYPE`  *handle* is not a channel handle, or any source
    /// handle in *wr_handles* did not match the object type type.
    ///
    /// `ZX_ERR_INVALID_ARGS`  any of the provided pointers are invalid or null,
    /// or *wr_num_bytes* is less than four, or *options* is nonzero, or any source
    /// handle in *wr_handles\[i\]->handle* did not have the rights specified in
    /// *wr_handle\[i\]->rights*.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_INVALID_ARGS` will be produced if the *buffer* field contains an
    /// invalid pointer or if the reserved field is non-zero.
    ///
    /// `ZX_ERR_ACCESS_DENIED`  *handle* does not have `ZX_RIGHT_WRITE` or
    /// any element in *handles* does not have `ZX_RIGHT_TRANSFER`.
    ///
    /// `ZX_ERR_PEER_CLOSED`  The other side of the channel was closed or became
    /// closed while waiting for the reply.
    ///
    /// `ZX_ERR_CANCELED`  *handle* was invalidated (e.g., closed) while waiting for a reply.
    ///
    /// `ZX_ERR_NO_MEMORY`  Failure due to lack of memory.
    /// There is no good way for userspace to handle this (unlikely) error.
    /// In a future build this error will no longer occur.
    ///
    /// `ZX_ERR_OUT_OF_RANGE`  *wr_num_bytes* or *wr_num_handles* are larger than the
    /// largest allowable size for channel messages.
    /// If the `ZX_CHANNEL_WRITE_USE_IOVEC` option is specified,
    /// `ZX_ERR_OUT_OF_RANGE` will be produced if *num_bytes* is larger than
    /// `ZX_CHANNEL_MAX_MSG_IOVEC` or the sum of the iovec capacities exceeds
    /// `ZX_CHANNEL_MAX_MSG_BYTES`.
    ///
    /// `ZX_ERR_BUFFER_TOO_SMALL`  *rd_num_bytes* or *rd_num_handles* are too small
    /// to contain the reply message.
    ///
    /// `ZX_ERR_NOT_SUPPORTED`  one of the handles in *handles* was *handle*
    /// (the handle to the channel being written to).
    ///
    /// [`zx_channel_call()`]: channel_call.md
    /// [`zx_channel_create()`]: channel_create.md
    /// [`zx_channel_read()`]: channel_read.md
    /// [`zx_channel_read_etc()`]: channel_read_etc.md
    /// [`zx_channel_write()`]: channel_write.md
    /// [`zx_channel_write_etc()`]: channel_write_etc.md
    /// [`zx_object_wait_one()`]: object_wait_one.md
    @blocking
    @vdsocall
    // TODO(scottmg): Express "All wr_handles of args must have ZX_RIGHT_TRANSFER."
    strict CallEtc(resource struct {
        handle Handle:CHANNEL;
        options uint32;
        deadline Time;
        @inout
        args ChannelCallEtcArgs;
    }) -> (struct {
        actual_bytes uint32;
        actual_handles uint32;
    }) error Status;
};