Write a message to a channel.


#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);


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 for details.


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.

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.


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.


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.


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