gensio_mdns(3) Library Functions Manual gensio_mdns(3)

gensio_alloc_mdns, gensio_free_mdns, gensio_mdns_add_service, gensio_mdns_remove_service, gensio_mdns_add_watch, gensio_mdns_remove_watch - Functions for doing mDNS operations

#include <gensio/gensio_mdns.h>


void *userdata);

struct gensio_mdns **m);

void *userdata);

int ipdomain, const char *name, const char *type,
const char *domain, const char *host,
int port, const char *txt[],
struct gensio_mdns_service **s);

enum gensio_mdns_data_state state,
int interface, int ipdomain,
const char *name, const char *type,
const char *domain, const char *host,
const struct gensio_addr *addr, const char *txt[],
void *userdata);

int ipdomain, const char *name, const char *type,
const char *domain, const char *host,
gensio_mdns_watch_cb callback, void *userdata,
struct gensio_mdns_watch **w);

These functions and type deal with mDNS (Multicast DNS) discovery of services on a local network. They can be used to advertise services or find services. These provide a powerful and easy to use interface to mDNS capabilities.

To do these things, you must first use gensio_alloc_mdns to allocate an mdns structure with the standard gensio OS functions structure. You must, of course, do the standard OS functions wait loops and such. When you are finished, you should free the mdns structure with gensio_free_mdns. Note that code may still be in mdns callbacks when the free returns, you have to make sure to wait until the done callback is called to know the code is out of all callbacks. The done callback is optional if you don't care. Also note that the mdns will not be freed (and done not called) until all the services and watches are freed for it.

For the remaining functions, the interface parameter is the system interface number of the network device, and may be -1 to specify all interfaces. The ipdomain parameter is one of:

Do both IPV4 and IPV4
IPV4 only
IPV6 only

The name, type, domain, and host parameters are the standard mDNS fields, see documentation on mDNS for details. The txt parameter is for mDNS text fields, and is a standard argv-like array.

Once you have an mdns structure, you can advertise a service on the net with it using gensio_mdns_add_service. That service will continue to be advertised until you call gensio_mdns_remove_service on it or your program terminates. The domain and host parameters should generally be NULL to take the system defaults. The txt parameter may be NULL if you don't have any.

To find services on the network, you add a watch with gensio_mdns_add_watch. Any service that matches your query will be reported with the callback functions and all fields will be provided in the callback. Once you are done with a watch, you may call gensio_mdns_remove_watch Note that code may still be in mdns callbacks when the remove returns, you have to make sure to wait until the done callback is called to know the code is out of all callbacks. The done callback is optional if you don't care.

The watch callback state has three possible values:

This is a new entry being reported.
An entry that was previously reported as gone away.
This is called one time after the watch is created, it reports that all currently known entries have been reported. New ones may be reported in the future, but those will have been dynamically added later.

In the watch callback, you must make copies of all the strings and addresses passed to you to keep them. Their continued existence is not guaranteed.

In watches, all of the string fields may be NULL, meaning you want to take anything in that field. So if all the strings are NULL and the interface is -1 and the ipdomain is GENSIO_NETTYPE_UNSPEC you will get all of the fields.

The string values to the watch add function may use regular expressions or globs. If the string starts with '%', then the data after it is treated as a regular expression and fields are matched against that. If the string starts with '@', the the data after it is treated as a standard glob. See the regex(7) and glob(7) man pages for details.

If the string starts with '=', an exact comparison is done with the data after it.

If the string starts with a-z0-9_ or a space, then an exact string comparison is done, including the first character.

The behavior of matching for any other starting character is undefined. In general, you should always use '@', '%', or '=' as the starting character of all your query strings to be sure.

Zero is returned on success, or a gensio error on failure.

gensio_err(3), gensio_os_funcs(3), regex(7), glob(7)

15 Oct 2020