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200 lines
6.6 KiB
Groff
200 lines
6.6 KiB
Groff
.\" $Id$
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.\"
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.\" Copyright 1998 by the Massachusetts Institute of Technology.
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.\" Copyright (C) 2004-2009 by Daniel Stenberg
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.\"
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.\" Permission to use, copy, modify, and distribute this
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.\" software and its documentation for any purpose and without
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.\" fee is hereby granted, provided that the above copyright
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.\" notice appear in all copies and that both that copyright
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.\" notice and this permission notice appear in supporting
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.\" documentation, and that the name of M.I.T. not be used in
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.\" advertising or publicity pertaining to distribution of the
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.\" software without specific, written prior permission.
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.\" M.I.T. makes no representations about the suitability of
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.\" this software for any purpose. It is provided "as is"
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.\" without express or implied warranty.
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.\"
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.TH ARES_INIT 3 "26 May 2009"
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.SH NAME
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ares_init, ares_init_options \- Initialize a resolver channel
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.SH SYNOPSIS
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.nf
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.B #include <ares.h>
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.PP
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.B int ares_init(ares_channel *\fIchannel\fP)
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.B int ares_init_options(ares_channel *\fIchannel\fP,
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.B struct ares_options *\fIoptions\fP, int \fIoptmask\fP)
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.PP
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.B cc file.c -lcares
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.fi
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.SH DESCRIPTION
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The \fBares_init\fP function initializes a communications channel for name
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service lookups. If it returns successfully, \fBares_init\fP will set the
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variable pointed to by \fIchannel\fP to a handle used to identify the name
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service channel. The caller should invoke
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.BR ares_destroy (3)
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on the handle when the channel is no longer needed.
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.PP
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The \fBares_init_options\fP function also initializes a name service channel,
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with additional options useful for applications requiring more control over
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name service configuration. The \fIoptmask\fP parameter specifies which fields
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in the structure pointed to by \fIoptions\fP are set, as follows:
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.TP 18
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.B ARES_OPT_FLAGS
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.B int \fIflags\fP;
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.br
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Flags controlling the behavior of the resolver. See below for a
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description of possible flag values.
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.TP 18
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.B ARES_OPT_TIMEOUT
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.B int \fItimeout\fP;
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.br
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The number of seconds each name server is given to respond to a query on the
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first try. (After the first try, the timeout algorithm becomes more
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complicated, but scales linearly with the value of \fItimeout\fP.) The
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default is five seconds. This option is being deprecated by
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\fIARES_OPT_TIMEOUTMS\fP starting in c-ares 1.5.2.
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.TP 18
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.B ARES_OPT_TIMEOUTMS
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.B int \fItimeout\fP;
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.br
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The number of milliseconds each name server is given to respond to a query on
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the first try. (After the first try, the timeout algorithm becomes more
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complicated, but scales linearly with the value of \fItimeout\fP.) The
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default is five seconds. Note that this option is specified with the same
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struct field as the former \fIARES_OPT_TIMEOUT\fP, it is but the option bits
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that tell c-ares how to interpret the number. This option was added in c-ares
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1.5.2.
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.TP 18
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.B ARES_OPT_TRIES
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.B int \fItries\fP;
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.br
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The number of tries the resolver will try contacting each name server
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before giving up. The default is four tries.
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.TP 18
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.B ARES_OPT_NDOTS
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.B int \fIndots\fP;
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.br
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The number of dots which must be present in a domain name for it to be
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queried for "as is" prior to querying for it with the default domain
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extensions appended. The default value is 1 unless set otherwise by
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resolv.conf or the RES_OPTIONS environment variable.
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.TP 18
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.B ARES_OPT_PORT
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.B unsigned short \fIport\fP;
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.br
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The port to use for queries (both TCP and UDP), in network byte order.
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The default value is 53 (in network byte order), the standard name
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service port.
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.TP 18
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.B ARES_OPT_SERVERS
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.B struct in_addr *\fIservers\fP;
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.br
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.B int \fInservers\fP;
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.br
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The list of servers to contact, instead of the servers specified in
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resolv.conf or the local named.
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.TP 18
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.B ARES_OPT_DOMAINS
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.B char **\fIdomains\fP;
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.br
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.B int \fIndomains\fP;
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.br
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The domains to search, instead of the domains specified in resolv.conf
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or the domain derived from the kernel hostname variable.
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.TP 18
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.B ARES_OPT_LOOKUPS
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.B char *\fIlookups\fP;
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.br
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The lookups to perform for host queries.
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.I lookups
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should be set to a string of the characters "b" or "f", where "b"
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indicates a DNS lookup and "f" indicates a lookup in the hosts file.
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.TP 18
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.B ARES_OPT_SOCK_STATE_CB
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.B void (*\fIsock_state_cb\fP)(void *data, int s, int read, int write);
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.br
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.B void *\fIsock_state_cb_data\fP;
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.br
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A callback function to be invoked when a socket changes state.
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.I s
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will be passed the socket whose state has changed;
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.I read
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will be set to true if the socket should listen for read events, and
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.I write
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will be set to true if the socket should listen for write events.
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The value of
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.I sock_state_cb_data
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will be passed as the
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.I data
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argument.
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.PP
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The
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.I flags
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field should be the bitwise or of some subset of the following values:
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.TP 23
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.B ARES_FLAG_USEVC
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Always use TCP queries (the "virtual circuit") instead of UDP
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queries. Normally, TCP is only used if a UDP query yields a truncated
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result.
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.TP 23
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.B ARES_FLAG_PRIMARY
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Only query the first server in the list of servers to query.
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.TP 23
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.B ARES_FLAG_IGNTC
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If a truncated response to a UDP query is received, do not fall back
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to TCP; simply continue on with the truncated response.
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.TP 23
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.B ARES_FLAG_NORECURSE
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Do not set the "recursion desired" bit on outgoing queries, so that the name
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server being contacted will not try to fetch the answer from other servers if
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it doesn't know the answer locally. Be aware that ares will not do the
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recursion for you. Recursion must be handled by the application calling ares
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if \fIARES_FLAG_NORECURSE\fP is set.
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.TP 23
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.B ARES_FLAG_STAYOPEN
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Do not close communications sockets when the number of active queries
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drops to zero.
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.TP 23
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.B ARES_FLAG_NOSEARCH
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Do not use the default search domains; only query hostnames as-is or
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as aliases.
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.TP 23
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.B ARES_FLAG_NOALIASES
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Do not honor the HOSTALIASES environment variable, which normally
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specifies a file of hostname translations.
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.TP 23
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.B ARES_FLAG_NOCHECKRESP
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Do not discard responses with the SERVFAIL, NOTIMP, or REFUSED
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response code or responses whose questions don't match the questions
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in the request. Primarily useful for writing clients which might be
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used to test or debug name servers.
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.SH RETURN VALUES
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.I ares_init
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or
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.I ares_init_options
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can return any of the following values:
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.TP 14
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.B ARES_SUCCESS
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Initialization succeeded.
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.TP 14
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.B ARES_EFILE
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A configuration file could not be read.
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.TP 14
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.B ARES_ENOMEM
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The process's available memory was exhausted.
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.TP 14
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.B ARES_ENOTINITIALIZED
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c-ares library initialization not yet performed.
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.SH SEE ALSO
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.BR ares_destroy(3),
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.BR ares_dup(3),
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.BR ares_library_init(3)
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.SH AUTHOR
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Greg Hudson, MIT Information Systems
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.br
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Copyright 1998 by the Massachusetts Institute of Technology.
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.br
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Copyright (C) 2004-2009 by Daniel Stenberg.
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