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Automated build of open62541 in alpine linux container
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License change: LGPL (Static Linking Exception) -> MPLv2

We are planning to change the license of open62541 project. It should be licensed under the Mozilla Public license v2 instead of our old LGPL with static linking exception. The MPLv2 license offers a broader vantage than the LGPL, as it is a standard license, thus legally easier to accept and more compatible with the juristic matters. Our intentions of changing our licensing is to expand and have a wider acceptance especially from corporates. For questions please refer to issue #846.


open62541 ( is an open source and free implementation of OPC UA (OPC Unified Architecture). open62541 is a C-based library (linking with C++ projects is possible) with all necessary tools to implement dedicated OPC UA clients and servers, or to integrate OPC UA-based communication into existing applications. The library is distributed as a single pair of header and source files, that can be easily dropped into your project. An example server and client implementation can be found in the /examples directory or further down on this page.

open62541 is licensed under the LGPL with a static linking exception. So the open62541 library can be used in projects that are not open source. However, changes to the open62541 library itself need to published under the same license. The plugins, as well as the server and client examples are in the public domain (CC0 license). They can be reused under any license and changes do not have to be published.


For a complete list of features check: open62541 Features

open62541 implements the OPC UA binary protocol stack as well as a client and server SDK. It currently supports the Micro Embedded Device Server Profile plus some additional features. The final server binaries can be well under 100kb, depending on the size of the information model.

  • Communication Stack
    • OPC UA binary protocol
    • Chunking (splitting of large messages)
    • Exchangeable network layer (plugin) for using custom networking APIs (e.g. on embedded targets)
  • Information model
    • Support for all OPC UA node types (including method nodes)
    • Support for adding and removing nodes and references also at runtime.
    • Support for inheritance and instantiation of object- and variable-types (custom constructor/destructor, instantiation of child nodes)
  • Subscriptions
    • Support for subscriptions/monitoreditems for data change notifications
    • Very low resource consumption for each monitored value (event-based server architecture)
  • Code-Generation
    • Support for generating data types from standard XML definitions
    • Support for generating server-side information models (nodesets) from standard XML definitions

Features still missing in the 0.2 release are:

  • Encryption
  • Access control for individual nodes
  • Events (notifications emitted by objects, data change notifications are implemented)
  • Event-loop (background tasks) and asynchronous service requests in the client

Using open62541

A general introduction to OPC UA and the open62541 documentation can be found at
Past releases of the library can be downloaded at
To use the latest improvements, download a nightly build of the single-file distribution (the entire library merged into a single source and header file) from Nightly builds of MSVC binaries of the library are available here.

For discussion and help, you can use

Contribute to open62541

As an open source project, we invite new contributors to help improve open62541. Issue reports, bugfixes and new features are very welcome. Note that there are ways to begin contributing without deep knowledge of the OPC UA standard:

Example Server Implementation

Compile the examples with the single-file distribution open62541.h/.c header and source file.
Using the GCC compiler, just run gcc -std=c99 <server.c> open62541.c -o server.

#include <signal.h>
#include "open62541.h"

UA_Boolean running = true;
void signalHandler(int sig) {
    running = false;

int main(int argc, char** argv)
    signal(SIGINT, signalHandler); /* catch ctrl-c */

    /* Create a server with one network layer listening on port 4840 */
    UA_ServerConfig config = UA_ServerConfig_standard;
    UA_ServerNetworkLayer nl = UA_ServerNetworkLayerTCP(UA_ConnectionConfig_standard, 4840);
    config.networkLayers = &nl;
    config.networkLayersSize = 1;
    UA_Server *server = UA_Server_new(config);

    /* Add a variable node */
    /* 1) Define the node attributes */
    UA_VariableAttributes attr;
    attr.displayName = UA_LOCALIZEDTEXT("en_US", "the answer");
    UA_Int32 myInteger = 42;
    UA_Variant_setScalar(&attr.value, &myInteger, &UA_TYPES[UA_TYPES_INT32]);

    /* 2) Define where the node shall be added with which browsename */
    UA_NodeId newNodeId = UA_NODEID_STRING(1, "the.answer");
    UA_NodeId parentReferenceNodeId = UA_NODEID_NUMERIC(0, UA_NS0ID_ORGANIZES);
    UA_NodeId variableType = UA_NODEID_NULL; /* take the default variable type */
    UA_QualifiedName browseName = UA_QUALIFIEDNAME(1, "the answer");

    /* 3) Add the node */
    UA_Server_addVariableNode(server, newNodeId, parentNodeId, parentReferenceNodeId,
                              browseName, variableType, attr, NULL, NULL);

    /* Run the server loop */
    UA_StatusCode status = UA_Server_run(server, &running);
    return status;

Example Client Implementation

#include <stdio.h>
#include "open62541.h"

int main(int argc, char *argv[])
    /* Create a client and connect */
    UA_Client *client = UA_Client_new(UA_ClientConfig_standard);
    UA_StatusCode status = UA_Client_connect(client, "opc.tcp://localhost:4840");
    if(status != UA_STATUSCODE_GOOD) {
        return status;

    /* Read the value attribute of the node. UA_Client_readValueAttribute is a
     * wrapper for the raw read service available as UA_Client_Service_read. */
    UA_Variant value; /* Variants can hold scalar values and arrays of any type */
    status = UA_Client_readValueAttribute(client, UA_NODEID_STRING(1, "the.answer"), &value);
    if(status == UA_STATUSCODE_GOOD &&
       UA_Variant_hasScalarType(&value, &UA_TYPES[UA_TYPES_INT32])) {
        printf("the value is: %i\n", *(UA_Int32*);

    /* Clean up */
    UA_Client_delete(client); /* Disconnects the client internally */
    return status;
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