ares_gethostbyaddr - Initiate a host query by address
#include <ares.h> typedef void (*ares_host_callback)(void *arg, int status, int timeouts, struct hostent *hostent) void ares_gethostbyaddr(ares_channel_t *channel, const void *addr, int addrlen, int family, ares_host_callback callback, void *arg)
The ares_gethostbyaddr function initiates a host query by address on the name service channel identified by channel. The parameters addr and addrlen give the address as a series of bytes, and family gives the type of address. When the query is complete or has failed, the ares library will invoke callback. Completion or failure of the query may happen immediately, or may happen during a later call to ares_process, ares_destroy or ares_cancel.
The callback argument arg is copied from the ares_gethostbyaddr argument arg. The callback argument status indicates whether the query succeeded and, if not, how it failed. It may have any of the following values:
ARES_SUCCESS The host lookup completed successfully.
ARES_ENOTIMP The ares library does not know how to look up addresses of type family.
ARES_ENOTFOUND The address addr was not found.
ARES_ENOMEM Memory was exhausted.
ARES_ECANCELLED The query was cancelled.
ARES_EDESTRUCTION The name service channel channel is being destroyed; the query will not be completed.
The callback argument timeouts reports how many times a query timed out during the execution of the given request.
On successful completion of the query, the callback argument hostent points to a struct hostent containing the name of the host returned by the query. The callback need not and should not attempt to free the memory pointed to by hostent; the ares library will free it when the callback returns. If the query did not complete successfully, hostent will be NULL.
When the associated callback is called, it is called with a channel lock so care must be taken to ensure any processing is minimal to prevent DNS channel stalls.
The callback may be triggered from a different thread than the one which called ares_gethostbyaddr.
For integrators running their own event loops and not using ARES_OPT_EVENT_THREAD, care needs to be taken to ensure any file descriptor lists are updated immediately within the eventloop when notified.
Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
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