Decode custom structures 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://opcua.umati.app:4840") != UA_STATUSCODE_GOOD) {
        UA_Client_delete(client); return 1;
    }

    UA_Variant v;
    UA_Variant_init(&v);
    UA_StatusCode rc = UA_Client_readValueAttribute(
        client, UA_NODEID_NUMERIC(4, 7001), &v);

    if (rc == UA_STATUSCODE_GOOD &&
        v.type == &UA_TYPES[UA_TYPES_EXTENSIONOBJECT]) {
        UA_ExtensionObject *eo = (UA_ExtensionObject *)v.data;
        if (eo->encoding == UA_EXTENSIONOBJECT_DECODED) {
            printf("decoded, type: %s\n", eo->content.decoded.type->typeName);
            /* cast eo->content.decoded.data to the generated struct */
        } else {
            printf("still encoded: %zu bytes of body\n",
                   eo->content.encoded.body.length);
            printf("-> the type is not known to this client\n");
        }
    }
    UA_Variant_clear(&v);

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

/* Preferred for a known model: generate types at build time.
   open62541 ships generators that turn a NodeSet2 XML file into C structs
   and the matching data type array, which you then register on the client. */

What it prints

still encoded: 48 bytes of body
-> the type is not known to this client

# after registering generated types for the model:
# decoded, type: ProductionJobType

Notes

• In C, prefer build-time generation from the NodeSet2 file when the model is known and stable. It gives you real structs and no runtime type machinery.
• An ExtensionObject with encoding UA_EXTENSIONOBJECT_ENCODED_BYTESTRING means the client does not know the type. That is information, not a failure.
• Runtime decoding of arbitrary structures in C is possible but rarely worth it on an embedded target. If you need it, consider whether that component belongs in C at all.
• Unions and optional fields are where generated decoders break. Test them against real server data.

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.