channel_call - 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);
channel_call() is like a combined channel_write(), object_wait_one(), and 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 channel_write() and 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 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.
While 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 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.
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.
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.
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.
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).
The facilities provided by channel_call() can interoperate with message dispatchers using channel_read() and channel_write() directly, provided the following rules are observed:
A server receiving synchronous messages via channel_read() should ensure that the txid of incoming messages is reflected back in outgoing responses via channel_write() so that clients using channel_call() can correctly route the replies.
A client sending messages via 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 channel_call().
If a 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 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.