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coap_attr_get_value(3)
coap_attribute

## NAME

coap_attribute, coap_add_attr, coap_find_attr, coap_attr_get_value - Work with CoAP attributes

## SYNOPSIS

#include <coap3/coap.h>

coap_attr_t *coap_add_attr(coap_resource_t *resource, coap_str_const_t *name, coap_str_const_t *value, int flags);

coap_attr_t *coap_find_attr(coap_resource_t *resource, coap_str_const_t *name);

coap_str_const_t *coap_attr_get_value(coap_attr_t *attribute);

For specific (D)TLS library support, link with -lcoap-3-notls, -lcoap-3-gnutls, -lcoap-3-openssl, -lcoap-3-mbedtls or -lcoap-3-tinydtls. Otherwise, link with -lcoap-3 to get the default (D)TLS library support.

## DESCRIPTION

CoAP Resources on a CoAP Server need to be created, updated etc. The URI in the request packet defines the resource to work with, with possibly the Query referring to a sub-resource.

When resources are configured on the CoAP server, the URI to match against is specified. Callback Handlers are then added to the resource to handle the different request methods.

Adding Attributes allows textual information to be added to the resource which can then be reported back to any client doing a Resource Discovery using a "GET /.well-known/core" request with an optional query. See "RFC6690 5. Examples" for some examples of resource discovery usage. Common attribute names are rt, if, sz, ct, obs, rel, anchor, rev, hreflang, media, title and type. href cannot be an attribute name.

Attributes are automatically deleted when a Resource is deleted.

## FUNCTIONS

Function: coap_add_attr()

The coap_add_attr() function registers a new attribute called name for the resource. The value of the attribute is value which can be NULL.

flags can be zero or more of the following or’d together, which, if set, defines what is to be internally freed off when the attribute is deleted with coap_delete_resource().

COAP_ATTR_FLAGS_RELEASE_NAME | Free off name when attribute is deleted with coap_delete_resource().
COAP_ATTR_FLAGS_RELEASE_VALUE | Free off value when attribute is deleted with coap_delete_resource().

Function: coap_find_attr()

The coap_find_attr() function returns the attribute with the name, if found, associated with resource.

Function: coap_attr_get_value()

The coap_attr_get_value() function returns the value associated with the specified attribute.

## RETURN VALUES

coap_add_attr() returns a pointer to the attribute that was created or NULL if there is a malloc failure.

coap_find_attr() returns a pointer to the first matching attribute or NULL if the name was not found.

coap_attr_get_value() returns a pointer to the value held within the attribute. The pointer can be NULL if the value id NULL, or NULL if attribute does not exist.

## EXAMPLE

Initialize Resources

    #include <coap3/coap.h>

    void hnd_get_index(coap_resource_t *resource, coap_session_t *session,
                       const coap_pdu_t *request,
                       const coap_string_t *query,
                       coap_pdu_t *response);

    void hnd_get_time(coap_resource_t *resource, coap_session_t *session,
                      const coap_pdu_t *request,
                      const coap_string_t *query,
                      coap_pdu_t *response);

    void hnd_put_time(coap_resource_t *resource, coap_session_t *session,
                      const coap_pdu_t *request,
                      const coap_string_t *query,
                      coap_pdu_t *response);

    void hnd_delete_time(coap_resource_t *resource, coap_session_t *session,
                         const coap_pdu_t *request,
                         const coap_string_t *query,
                         coap_pdu_t *response);

    static void
    init_resources(coap_context_t *ctx) {

      coap_resource_t *r;

      /* Create a resource to return general information */
      r = coap_resource_init(NULL, 0);
      coap_register_request_handler(r, COAP_REQUEST_GET, hnd_get_index);

      /* Document resource for '.well-known/core' request */
      coap_add_attr(r, coap_make_str_const("ct"), coap_make_str_const("0"), 0);
      coap_add_attr(r, coap_make_str_const("title"),
                    coap_make_str_const("\"General Info\""), 0);

      coap_add_resource(ctx, r);

      /* Create a resource to return return or update time */
      r = coap_resource_init(coap_make_str_const("time"),
                             COAP_RESOURCE_FLAGS_NOTIFY_CON);
      coap_resource_set_get_observable(r, 1);
      coap_register_request_handler(r, COAP_REQUEST_GET, hnd_get_time);
      coap_register_request_handler(r, COAP_REQUEST_PUT, hnd_put_time);
      coap_register_request_handler(r, COAP_REQUEST_DELETE, hnd_delete_time);

      /* Document resource for 'time' request */
      coap_add_attr(r, coap_make_str_const("ct"), coap_make_str_const("0"), 0);
      coap_add_attr(r, coap_make_str_const("title"),
                    coap_make_str_const("\"Internal Clock\""), 0);
      coap_add_attr(r, coap_make_str_const("rt"), coap_make_str_const("\"secs\""),
                    0);
      coap_add_attr(r, coap_make_str_const("if"), coap_make_str_const("\"clock\""),
                    0);

      coap_add_resource(ctx, r);

    }

## SEE ALSO

coap_resource(3) and coap_handler(3)

## FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

"RFC6690: Constrained RESTful Environments (CoRE) Link Format"

for further information.

## BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net or raise an issue on GitHub at https://github.com/obgm/libcoap/issues

## AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>

coap_attr_get_value(3)