Connect to an OPC UA server in C

The code

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

int main(void) {
    UA_Client *client = UA_Client_new();
    UA_ClientConfig_setDefault(UA_Client_getConfig(client));

    UA_StatusCode rc = UA_Client_connect(client, "opc.tcp://opcua.umati.app:4840");
    if (rc != UA_STATUSCODE_GOOD) {
        printf("connect failed: %s\n", UA_StatusCode_name(rc));
        UA_Client_delete(client);
        return 1;
    }
    printf("connected\n");

    /* read the namespace array: tells you what the server implements */
    UA_Variant nsArray;
    UA_Variant_init(&nsArray);
    rc = UA_Client_readValueAttribute(client,
            UA_NODEID_NUMERIC(0, UA_NS0ID_SERVER_NAMESPACEARRAY), &nsArray);
    if (rc == UA_STATUSCODE_GOOD && nsArray.type == &UA_TYPES[UA_TYPES_STRING]) {
        UA_String *uris = (UA_String *)nsArray.data;
        for (size_t i = 0; i < nsArray.arrayLength; i++)
            printf("  ns=%zu  %.*s\n", i, (int)uris[i].length, uris[i].data);
    }
    UA_Variant_clear(&nsArray);

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

/* build:
   gcc client.c -lopen62541 -o client
   or with the amalgamation:
   gcc client.c open62541.c -lmbedtls -lmbedx509 -lmbedcrypto -o client
*/

What it prints

connected
  ns=0  http://opcfoundation.org/UA/
  ns=1  urn:umati:sample-server
  ns=2  http://opcfoundation.org/UA/DI/
  ns=3  http://opcfoundation.org/UA/Machinery/

Notes

• Every call returns a UA_StatusCode. Check it. There is no exception mechanism, and ignoring a return value here produces the memory bugs that give C a bad name.
• Every UA_Variant you initialise must be cleared with UA_Variant_clear, or you leak on every read.
• The single-file amalgamation is the practical way to vendor this into an existing embedded build without adopting its build system.
• Without mbedTLS or OpenSSL compiled in you get security policy None only, and no amount of configuration will change that.
• The Query service set and HTTPS are not implemented. Design around that rather than discovering it late.

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.