.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" SPDX-License-Identifier: MIT
.\"
.TH ARES_GETHOSTBYADDR 3 "24 July 1998"
.SH NAME
ares_gethostbyaddr \- Initiate a host query by address
.SH SYNOPSIS
.nf
#include <ares.h>

typedef void (*ares_host_callback)(void *\fIarg\fP, int \fIstatus\fP,
                                   int \fItimeouts\fP,
                                   struct hostent *\fIhostent\fP)

void ares_gethostbyaddr(ares_channel_t *\fIchannel\fP, const void *\fIaddr\fP,
                        int \fIaddrlen\fP, int \fIfamily\fP,
                        ares_host_callback \fIcallback\fP, void *\fIarg\fP)
.fi
.SH DESCRIPTION
The
.B ares_gethostbyaddr
function initiates a host query by address on the name service channel
identified by
.IR channel .
The parameters
.I addr
and
.I addrlen
give the address as a series of bytes, and
.I family
gives the type of address.  When the query is complete or has failed, the ares
library will invoke \fIcallback\fP.  Completion or failure of the query may
happen immediately, or may happen during a later call to
\fIares_process(3)\fP, \fIares_destroy(3)\fP or \fIares_cancel(3)\fP.
.PP
The callback argument
.I arg
is copied from the
.B ares_gethostbyaddr
argument
.IR arg .
The callback argument
.I status
indicates whether the query succeeded and, if not, how it failed.  It
may have any of the following values:
.TP 19
.B ARES_SUCCESS
The host lookup completed successfully.
.TP 19
.B ARES_ENOTIMP
The ares library does not know how to look up addresses of type
.IR family .
.TP 19
.B ARES_ENOTFOUND
The address
.I addr
was not found.
.TP 19
.B ARES_ENOMEM
Memory was exhausted.
.TP 19
.B ARES_ECANCELLED
The query was cancelled.
.TP 19
.B ARES_EDESTRUCTION
The name service channel
.I channel
is being destroyed; the query will not be completed.
.PP
The callback argument
.I timeouts
reports how many times a query timed out during the execution of the
given request.
.PP
On successful completion of the query, the callback argument
.I hostent
points to a
.B 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
.IR hostent ;
the ares library will free it when the callback returns.  If the query
did not complete successfully,
.I hostent
will be
.BR NULL .
.PP
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 \fIares_gethostbyaddr(3)\fP.

For integrators running their own event loops and not using \fBARES_OPT_EVENT_THREAD\fP,
care needs to be taken to ensure any file descriptor lists are updated immediately
within the eventloop when notified.
.SH SEE ALSO
.BR ares_process (3),
.BR ares_gethostbyname (3)