# lwres_noop

## Name

lwres_nooprequest_render, lwres_noopresponse_render, lwres_nooprequest_parse, lwres_noopresponse_parse, lwres_noopresponse_free, lwres_nooprequest_free -- lightweight resolver no-op message handling

## Synopsis

    #include <lwres/lwres.h>

``lwres_result_t lwres_nooprequest_render`(lwres_context_t *ctx, lwres_nooprequest_t *req, lwres_lwpacket_t *pkt, lwres_buffer_t *b);`

``lwres_result_t lwres_noopresponse_render`(lwres_context_t *ctx, lwres_noopresponse_t *req, lwres_lwpacket_t *pkt, lwres_buffer_t *b);`

``lwres_result_t lwres_nooprequest_parse`(lwres_context_t *ctx, lwres_buffer_t *b, lwres_lwpacket_t *pkt, lwres_nooprequest_t **structp);`

``lwres_result_t lwres_noopresponse_parse`(lwres_context_t *ctx, lwres_buffer_t *b, lwres_lwpacket_t *pkt, lwres_noopresponse_t **structp);`

``void lwres_noopresponse_free`(lwres_context_t *ctx, lwres_noopresponse_t **structp);`

``void lwres_nooprequest_free`(lwres_context_t *ctx, lwres_nooprequest_t **structp);`

## DESCRIPTION

These are low-level routines for creating and parsing lightweight resolver no-op request and response messages.

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 lighweight 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`.

## RETURN VALUES

The no-op opcode functions `lwres_nooprequest_render()`, `lwres_noopresponse_render()` `lwres_nooprequest_parse()` and `lwres_noopresponse_parse()` all return LWRES_R_SUCCESS on success. They return LWRES_R_NOMEMORY if memory allocation fails. LWRES_R_UNEXPECTEDEND is returned if the available space in the buffer `b` is too small to accommodate the packet header or the lwres_nooprequest_t and lwres_noopresponse_t structures. `lwres_nooprequest_parse()` and `lwres_noopresponse_parse()` will return LWRES_R_UNEXPECTEDEND if the buffer is not empty after decoding the received packet. These functions will return LWRES_R_FAILURE if `pktflags` in the packet header structure lwres_lwpacket_t indicate that the packet is not a response to an earlier query.

## SEE ALSO

lwres_packet(3)