Write a value to an OPC UA server in C

The code

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

/* umati is read-only. Run a local server:
     ./examples/server_ctt   (from an open62541 build)   */
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_NodeId target = UA_NODEID_STRING(1, "the.answer");

    /* check what the node expects before writing */
    UA_NodeId dataType;
    UA_NodeId_init(&dataType);
    UA_StatusCode rc = UA_Client_readDataTypeAttribute(client, target, &dataType);
    if (rc == UA_STATUSCODE_GOOD)
        printf("declared datatype: ns=%d;i=%d\n",
               dataType.namespaceIndex, dataType.identifier.numeric);
    UA_NodeId_clear(&dataType);

    UA_Int32 value = 42;
    UA_Variant v;
    UA_Variant_init(&v);
    UA_Variant_setScalar(&v, &value, &UA_TYPES[UA_TYPES_INT32]);

    rc = UA_Client_writeValueAttribute(client, target, &v);
    printf("write: %s\n", UA_StatusCode_name(rc));

    /* read back - a Good status does not prove the value stuck */
    UA_Variant back;
    UA_Variant_init(&back);
    if (UA_Client_readValueAttribute(client, target, &back) == UA_STATUSCODE_GOOD &&
        back.type == &UA_TYPES[UA_TYPES_INT32])
        printf("read back: %d\n", *(UA_Int32 *)back.data);
    UA_Variant_clear(&back);

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

What it prints

declared datatype: ns=0;i=6
write: Good
read back: 42

Notes

• UA_Variant_setScalar does not copy: the variant points at your stack variable. Keep it alive until the write returns, or use UA_Variant_setScalarCopy.
• Read the DataType attribute. UA_TYPES_INT32 is not UA_TYPES_INT16, and the server will refuse the mismatch.
• Read back in tests. Some servers, including the umati sample server, accept writes and discard them.
• For arrays use UA_Variant_setArray and match ValueRank. A scalar into an array node is a type mismatch.

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.