The no-op message is analogous to a ping packet: a packet is sent to the resolver daemon and is simply echoed back. The opcode is intended to allow a client to determine if the server is operational or not.

There are four main functions for the no-op opcode. One render function converts a no-op request structure --- lwres_nooprequest_t --- to the lightweight resolver's canonical format. It is complemented by a parse function that converts a packet in this canonical format to a no-op request structure. Another render function converts the no-op response structure --- lwres_noopresponse_t to the canonical format. This is complemented by a parse function which converts a packet in canonical format to a no-op response structure.

These structures are defined in lwres/lwres.h. They are shown below.

#define LWRES_OPCODE_NOOP       0x00000000U

typedef struct {
        lwres_uint16_t  datalength;
        unsigned char   *data;
} lwres_nooprequest_t;

typedef struct {
        lwres_uint16_t  datalength;
        unsigned char   *data;
} lwres_noopresponse_t;

Although the structures have different types, they are identical. This is because the no-op opcode simply echos whatever data was sent: the response is therefore identical to the request.

lwres_nooprequest_render() uses resolver context ctx to convert no-op request structure req to canonical format. The packet header structure pkt is initialised and transferred to buffer b. The contents of *req are then appended to the buffer in canonical format. lwres_noopresponse_render() performs the same task, except it converts a no-op response structure lwres_noopresponse_t to the lightweight resolver's canonical format.

lwres_nooprequest_parse() uses context ctx to convert the contents of packet pkt to a lwres_nooprequest_t structure. Buffer b provides space to be used for storing this structure. When the function succeeds, the resulting lwres_nooprequest_t is made available through *structp. lwres_noopresponse_parse() offers the same semantics as lwres_nooprequest_parse() except it yields a lwres_noopresponse_t structure.

lwres_noopresponse_free() and lwres_nooprequest_free() release the memory in resolver context ctx that was allocated to the lwres_noopresponse_t or lwres_nooprequest_t structures referenced via structp.