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 no longer accessible to the caller's process -- they are attached to the message and will become available to the reader of that message from the opposite end of the channel. On any failure, all handles are discarded rather than transferred.

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 which may be sent in a message is ZX_CHANNEL_MAX_MSG_HANDLES, which is 64.

The maximum number of bytes which may be sent in a message is ZX_CHANNEL_MAX_MSG_BYTES, which is 65536.


handle must be of type ZX_OBJ_TYPE_CHANNEL and have ZX_RIGHT_WRITE.

Every entry of handles must have ZX_RIGHT_TRANSFER.


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 duplicates 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 is nonzero.

ZX_ERR_NOT_SUPPORTED handle was found in the handles array, or one of the handles in handles was handle (the handle to the channel being written to).

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 the largest allowable size for channel messages.


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.