Call an OPC UA method in C

The code

#include <open62541/client_config_default.h>
#include <open62541/client_highlevel.h>
#include <stdio.h>

int main(void) {
    UA_Client *client = UA_Client_new();
    UA_ClientConfig_setDefault(UA_Client_getConfig(client));
    if (UA_Client_connect(client, "opc.tcp://localhost:4840") != UA_STATUSCODE_GOOD) {
        UA_Client_delete(client); return 1;
    }

    UA_Double input = 16.0;
    UA_Variant inArg;
    UA_Variant_init(&inArg);
    UA_Variant_setScalar(&inArg, &input, &UA_TYPES[UA_TYPES_DOUBLE]);

    size_t outSize = 0;
    UA_Variant *outArgs = NULL;

    UA_StatusCode rc = UA_Client_call(client,
        UA_NODEID_STRING(1, "Methods"),        /* object that owns the method */
        UA_NODEID_STRING(1, "Methods/sqrt"),   /* the method itself */
        1, &inArg, &outSize, &outArgs);

    if (rc == UA_STATUSCODE_GOOD) {
        printf("outputs: %zu\n", outSize);
        for (size_t i = 0; i < outSize; i++)
            if (outArgs[i].type == &UA_TYPES[UA_TYPES_DOUBLE])
                printf("  %.1f\n", *(UA_Double *)outArgs[i].data);
        UA_Array_delete(outArgs, outSize, &UA_TYPES[UA_TYPES_VARIANT]);
    } else {
        printf("call failed: %s\n", UA_StatusCode_name(rc));
    }

    UA_Client_disconnect(client);
    UA_Client_delete(client);
    return 0;
}

What it prints

outputs: 1
  4.0

Notes

• The output array is yours to free. UA_Array_delete with UA_TYPES_VARIANT, exactly as above: this leaks quietly otherwise.
• You need both the object node id and the method node id. Browse the object to find the method.
• Input argument types must match the declared InputArguments, in order. Read that property first if you are unsure.
• Check the Executable attribute if a method exists but refuses to run.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.