Working with Data Types

OPC UA defines a type system for values that can be encoded in the protocol messages. This tutorial shows some examples for available data types and their use. See the section on Data Types for the full definitions.

Basic Data Handling

This section shows the basic interaction patterns for data types. Make sure to compare with the type definitions in types.h.

#include <open62541/plugin/log_stdout.h>
#include <open62541/server.h>
#include <open62541/server_config_default.h>

#include <stdlib.h>

static void
variables_basic(void) {
    /* Int32 */
    UA_Int32 i = 5;
    UA_Int32 j;
    UA_Int32_copy(&i, &j);

    UA_Int32 *ip = UA_Int32_new();
    UA_Int32_copy(&i, ip);

    /* String */
    UA_String s;
    UA_String_init(&s); /* _init zeroes out the entire memory of the datatype */
    char *test = "test";
    s.length = strlen(test); = (UA_Byte*)test;

    UA_String s2;
    UA_String_copy(&s, &s2);
    UA_String_clear(&s2); /* Copying heap-allocated the dynamic content */

    UA_String s3 = UA_STRING("test2");
    UA_String s4 = UA_STRING_ALLOC("test2"); /* Copies the content to the heap */
    UA_Boolean eq = UA_String_equal(&s3, &s4);

    /* Structured Type */
    UA_ReadRequest rr;
    UA_init(&rr, &UA_TYPES[UA_TYPES_READREQUEST]); /* Generic method */
    UA_ReadRequest_init(&rr); /* Shorthand for the previous line */

    rr.requestHeader.timestamp = UA_DateTime_now(); /* Members of a structure */

    rr.nodesToRead = (UA_ReadValueId *)UA_Array_new(5, &UA_TYPES[UA_TYPES_READVALUEID]);
    rr.nodesToReadSize = 5; /* Array size needs to be made known */

    UA_ReadRequest *rr2 = UA_ReadRequest_new();
    UA_copy(&rr, rr2, &UA_TYPES[UA_TYPES_READREQUEST]);


An OPC UA information model is made up of nodes and references between nodes. Every node has a unique NodeId. NodeIds refer to a namespace with an additional identifier value that can be an integer, a string, a guid or a bytestring.

static void
variables_nodeids(void) {
    UA_NodeId id1 = UA_NODEID_NUMERIC(1, 1234);
    id1.namespaceIndex = 3;

    UA_NodeId id2 = UA_NODEID_STRING(1, "testid"); /* the string is static */
    UA_Boolean eq = UA_NodeId_equal(&id1, &id2);

    UA_NodeId id3;
    UA_NodeId_copy(&id2, &id3);

    UA_NodeId id4 = UA_NODEID_STRING_ALLOC(1, "testid"); /* the string is copied
                                                            to the heap */


The datatype Variant belongs to the built-in datatypes of OPC UA and is used as a container type. A variant can hold any other datatype as a scalar (except variant) or as an array. Array variants can additionally denote the dimensionality of the data (e.g. a 2x3 matrix) in an additional integer array.

static void
variables_variants(void) {
    /* Set a scalar value */
    UA_Variant v;
    UA_Int32 i = 42;
    UA_Variant_setScalar(&v, &i, &UA_TYPES[UA_TYPES_INT32]);

    /* Make a copy */
    UA_Variant v2;
    UA_Variant_copy(&v, &v2);

    /* Set an array value */
    UA_Variant v3;
    UA_Double d[9] = {1.0, 2.0, 3.0,
                      4.0, 5.0, 6.0,
                      7.0, 8.0, 9.0};
    UA_Variant_setArrayCopy(&v3, d, 9, &UA_TYPES[UA_TYPES_DOUBLE]);

    /* Set array dimensions */
    v3.arrayDimensions = (UA_UInt32 *)UA_Array_new(2, &UA_TYPES[UA_TYPES_UINT32]);
    v3.arrayDimensionsSize = 2;
    v3.arrayDimensions[0] = 3;
    v3.arrayDimensions[1] = 3;

It follows the main function, making use of the above definitions.

int main(void) {
    return EXIT_SUCCESS;