SDK · C

OPC UA in C

Project	open62541
Version	1.5.8
Licence	MPL 2.0
Install	apt install libopen62541-dev, or vendor the single-file amalgamation

What to know before you start

• MPL-2.0 file-level copyleft: your own files stay yours, modifications to theirs do not. Much easier to live with than LGPL for embedded work.
• The single-file amalgamation is the pragmatic way to vendor it into an existing embedded build.
• Crypto comes from mbedTLS or OpenSSL. Without one of them you get no security policies beyond None.
• The Query service set and HTTPS are not implemented. If your design needs either, design differently.
• An example server built with it has been certified for a standard server profile: note the pattern: the example is certified, not the library.
• Commercial support from o6 Automation, who employ the core maintainers.

Recipes

The same twelve tasks, in this stack, pinned to the version above. Each one runs against a free public server.

Task	What it does
Connect to an OPC UA server in C	Create a client, apply the default configuration, then connect. open62541 returns a UA_StatusCode from everything: check it every
Browse an OPC UA address space in C	Build a UA_BrowseRequest with a filtered UA_BrowseDescription and walk the results. Filtering in the request matters more in C tha
Read a value from an OPC UA server in C	The high-level API gives you UA_Client_readValueAttribute for a bare value; use a full UA_ReadRequest when you need the status cod
Write a value to an OPC UA server in C	Build a variant with the scalar type the node declares and write it. Read the DataType attribute first rather than assuming, and c
Subscribe to data changes in C	Create a subscription, add a monitored item with a callback, then drive the client's event loop so notifications get processed. Th
Call an OPC UA method in C	UA_Client_call takes the owning object, the method node and an input argument array, and hands back an output array you must free.
Set up OPC UA certificates in C	Load a DER certificate and private key, pass them plus your trust list to the encrypted client configuration helper, and set the A
Decode custom structures in C	open62541 can read a server's DataTypeDefinitions at runtime, but in C the pragmatic path for a known model is to generate types a
Read historical data in C	open62541 exposes historical reads through UA_Client_HistoryRead_raw with a callback per batch. Bound the window, check the server
Subscribe to alarms and events in C	Create an event monitored item on the Server object with an explicit select clause. In C you get the field values back as a positi
Publish with OPC UA PubSub in C	open62541 implements Part 14, which makes it a genuine option for deterministic publishing from an embedded target. Build a connec
Handle errors and reconnect in C	open62541's client keeps its own state machine. Call UA_Client_run_iterate in a loop, watch the channel and session state, and let

Last verified 7 September 2026 against OPC UA 1.05.07. Version read from the project release tags. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.