Google is committed to advancing racial equity for Black communities. See how.



Send a message to a channel and await a reply.


#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);


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;


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.

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.


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.


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.


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 closed while waiting for a reply. TODO( Transferring a channel with pending calls currently leads to undefined behavior. With the current implementation, transferring such a channel does not interrupt the pending calls, as it does not close the underlying channel endpoint. Programs should be aware of this behavior, but they must not rely on it.

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).