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1 .\"
2 .\" Copyright 1998 by the Massachusetts Institute of Technology.
3 .\" SPDX-License-Identifier: MIT
4 .\"
5 .TH ARES_GETHOSTBYADDR 3 "24 July 1998"
6 .SH NAME
7 ares_gethostbyaddr \- Initiate a host query by address
8 .SH SYNOPSIS
9 .nf
10 #include <ares.h>
11
12 typedef void (*ares_host_callback)(void *\fIarg\fP, int \fIstatus\fP,
13 int \fItimeouts\fP,
14 struct hostent *\fIhostent\fP)
15
16 void ares_gethostbyaddr(ares_channel_t *\fIchannel\fP, const void *\fIaddr\fP,
17 int \fIaddrlen\fP, int \fIfamily\fP,
18 ares_host_callback \fIcallback\fP, void *\fIarg\fP)
19 .fi
20 .SH DESCRIPTION
21 The
22 .B ares_gethostbyaddr
23 function initiates a host query by address on the name service channel
24 identified by
25 .IR channel .
26 The parameters
27 .I addr
28 and
29 .I addrlen
30 give the address as a series of bytes, and
31 .I family
32 gives the type of address. When the query is complete or has failed, the ares
33 library will invoke \fIcallback\fP. Completion or failure of the query may
34 happen immediately, or may happen during a later call to
35 \fIares_process(3)\fP, \fIares_destroy(3)\fP or \fIares_cancel(3)\fP.
36 .PP
37 The callback argument
38 .I arg
39 is copied from the
40 .B ares_gethostbyaddr
41 argument
42 .IR arg .
43 The callback argument
44 .I status
45 indicates whether the query succeeded and, if not, how it failed. It
46 may have any of the following values:
47 .TP 19
48 .B ARES_SUCCESS
49 The host lookup completed successfully.
50 .TP 19
51 .B ARES_ENOTIMP
52 The ares library does not know how to look up addresses of type
53 .IR family .
54 .TP 19
55 .B ARES_ENOTFOUND
56 The address
57 .I addr
58 was not found.
59 .TP 19
60 .B ARES_ENOMEM
61 Memory was exhausted.
62 .TP 19
63 .B ARES_ECANCELLED
64 The query was cancelled.
65 .TP 19
66 .B ARES_EDESTRUCTION
67 The name service channel
68 .I channel
69 is being destroyed; the query will not be completed.
70 .PP
71 The callback argument
72 .I timeouts
73 reports how many times a query timed out during the execution of the
74 given request.
75 .PP
76 On successful completion of the query, the callback argument
77 .I hostent
78 points to a
79 .B struct hostent
80 containing the name of the host returned by the query. The callback
81 need not and should not attempt to free the memory pointed to by
82 .IR hostent ;
83 the ares library will free it when the callback returns. If the query
84 did not complete successfully,
85 .I hostent
86 will be
87 .BR NULL .
88 .PP
89 When the associated callback is called, it is called with a channel lock so care
90 must be taken to ensure any processing is minimal to prevent DNS channel stalls.
91
92 The callback may be triggered from a different thread than the one which
93 called \fIares_gethostbyaddr(3)\fP.
94
95 For integrators running their own event loops and not using \fBARES_OPT_EVENT_THREAD\fP,
96 care needs to be taken to ensure any file descriptor lists are updated immediately
97 within the eventloop when notified.
98 .SH SEE ALSO
99 .BR ares_process (3),
100 .BR ares_gethostbyname (3)