Building a Simple Client

You should already have a basic server from the previous tutorials. open62541 provides both a server- and clientside API, so creating a client is as easy as creating a server. Copy the following into a file myClient.c:

#include <open62541/client_config_default.h>
#include <open62541/client_highlevel.h>
#include <open62541/plugin/log_stdout.h>

#include <stdlib.h>

int main(void) {
    UA_Client *client = UA_Client_new();
    UA_StatusCode retval = UA_Client_connect(client, "opc.tcp://localhost:4840");
    if(retval != UA_STATUSCODE_GOOD) {
        return (int)retval;

    /* Read the value attribute of the node. UA_Client_readValueAttribute is a
     * wrapper for the raw read service available as UA_Client_Service_read. */
    UA_Variant value; /* Variants can hold scalar values and arrays of any type */

    /* NodeId of the variable holding the current time */
    retval = UA_Client_readValueAttribute(client, nodeId, &value);

    if(retval == UA_STATUSCODE_GOOD &&
       UA_Variant_hasScalarType(&value, &UA_TYPES[UA_TYPES_DATETIME])) {
        UA_DateTime raw_date = *(UA_DateTime *);
        UA_DateTimeStruct dts = UA_DateTime_toStruct(raw_date);
        UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND, "date is: %u-%u-%u %u:%u:%u.%03u\n",
          , dts.month, dts.year, dts.hour, dts.min, dts.sec, dts.milliSec);

    /* Clean up */
    UA_Client_delete(client); /* Disconnects the client internally */
    return EXIT_SUCCESS;

Compilation is similar to the server example.

$ gcc -std=c99 open62541.c myClient.c -o myClient

In a MinGW environment, the Winsock library must be added.

$ gcc -std=c99 open62541.c myClient.c -lws2_32 -o myClient.exe

Further tasks

  • Try to connect to some other OPC UA server by changing opc.tcp://localhost:4840 to an appropriate address (remember that the queried node is contained in any OPC UA server).
  • Try to set the value of the variable node (ns=1,i=”the.answer”) containing an Int32 from the example server (which is built in Building a Simple Server) using “UA_Client_write” function. The example server needs some more modifications, i.e., changing request types. The answer can be found in “examples/client.c”.