xs_sendmsg(3) send a message part on a socket (zero-copy)


int xs_sendmsg (void *socket, xs_msg_t *msg, int flags);


The xs_sendmsg() function shall queue the message referenced by the msg argument to be sent to the socket referenced by the socket argument. The flags argument is a combination of the flags defined below:


Specifies that the operation should be performed in non-blocking mode. If the message cannot be queued on the socket, the xs_sendmsg() function shall fail with errno set to EAGAIN.


Specifies that the message being sent is a multi-part message, and that further message parts are to follow. Refer to the section regarding multi-part messages below for a detailed description.

The xs_msg_t structure passed to xs_sendmsg() is nullified during the call. If you want to send the same message to multiple sockets you have to copy it using (e.g. using xs_msg_copy()).


A successful invocation of xs_sendmsg() does not indicate that the message has been transmitted to the network, only that it has been queued on the socket and Crossroads have assumed responsibility for the message.

Multi-part messages

A Crossroads message is composed of 1 or more message parts. Each message part is an independent xs_msg_t in its own right. Crossroads ensure atomic delivery of messages; peers shall receive either all message parts of a message or none at all. The total number of message parts is unlimited except by available memory.

An application that sends multipart messages must use the XS_SNDMORE flag when sending each data part except the final one.


The xs_sendmsg() function shall return number of bytes in the message if successful. Otherwise it shall return -1 and set errno to one of the values defined below.



Non-blocking mode was requested and the message cannot be sent at the moment.


The xs_sendmsg() operation is not supported by this socket type.


The xs_sendmsg() operation cannot be performed on this socket at the moment due to the socket not being in the appropriate state. This error may occur with socket types that switch between several states, such as XS_REP. See the messaging patterns section of xs_socket(3) for more information.


The context associated with the specified socket was terminated.


The provided socket was invalid.


The operation was interrupted by delivery of a signal before the message was sent.


Invalid message.


Filling in a message and sending it to a socket.

/* Create a new message, allocating 6 bytes for message content */
xs_msg_t msg;
int rc = xs_msg_init_size (&msg, 6);
assert (rc == 0);
/* Fill in message content with 'AAAAAA' */
memset (xs_msg_data (&msg), 'A', 6);
/* Send the message to the socket */
rc = xs_sendmsg (socket, &msg, 0);
assert (rc == 6);

Sending a multi-part message.

/* Send a multi-part message consisting of three parts to socket */
rc = xs_sendmsg (socket, &part1, XS_SNDMORE);
rc = xs_sendmsg (socket, &part2, XS_SNDMORE);
/* Final part; no more parts to follow */
rc = xs_sendmsg (socket, &part3, 0);


This man page was written by Martin Sustrik <m[blue][email protected]m[][1]>, Martin Lucina <m[blue][email protected]m[][2]> and Pieter Hintjens <m[blue][email protected]m[][3]>.