Send a message to a channel and await a reply.
#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);
zx_channel_call() is like a combined
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
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
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.
zx_channel_write(), the handles in handles are always consumed by
zx_channel_call() and no longer exist in the calling process.
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() returns ZX_OK on success and the number of bytes and
count of handles in the reply message are returned via actual_bytes and
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.
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(ZX-4233): 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.
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).
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.
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() 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
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.
- timer slack
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Last updated 2019-11-21.