The message API is reached through its own header, alongside the examples' support header:

#include <vws/message.h>

#include "vwsx.h"

The connection is opened with the same helper as the basic client:

vws_cnx* cnx = vwsx_connect(URI);

A message is created with vrtql_msg_new() and then populated:

vrtql_msg* request = vrtql_msg_new();
vrtql_msg_set_header(request, "action", "echo");
vrtql_msg_set_content(request, "hello in a vrtql_msg");

vrtql_msg_set_header() adds an entry to the headers map, which is free for application use, and vrtql_msg_set_content() sets the payload. (A vrtql_msg also has a routing map, set with vrtql_msg_set_routing(), which this program does not use.) The wire format — MessagePack or JSON — is carried in the message's format field and auto-detected on receipt, so the two may be mixed on one connection.

The message is sent with vrtql_msg_send():

if (vrtql_msg_send(cnx, request) < 0)
{
    vwsx_die("vrtql_msg_send() failed");
}

It serialises the message and writes it to the connection, returning the bytes written or a value less than zero on error. vrtql_msg_send() does not take ownership of the request, so the program frees it later.

The reply is read with vrtql_msg_recv(), which returns a deserialised message or NULL on timeout, and its content is printed:

vrtql_msg* reply = vrtql_msg_recv(cnx);
if (reply == NULL)
{
    vwsx_die("vrtql_msg_recv() returned NULL");
}

printf("client-message: reply content (%zu bytes): %s\n",
       vrtql_msg_get_content_size(reply),
       vrtql_msg_get_content(reply));

vrtql_msg_get_content() returns the reply's payload and vrtql_msg_get_content_size() its length. Both the reply and the request are then released with vrtql_msg_free(), and the connection is closed and freed:

vrtql_msg_free(reply);
vrtql_msg_free(request);

vws_disconnect(cnx);
vws_cnx_free(cnx);