c-ares/ares_init.3

.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" Copyright (C) 2004-2010 by Daniel Stenberg
.\"
.\" Permission to use, copy, modify, and distribute this
.\" software and its documentation for any purpose and without
.\" fee is hereby granted, provided that the above copyright
.\" notice appear in all copies and that both that copyright
.\" notice and this permission notice appear in supporting
.\" documentation, and that the name of M.I.T. not be used in
.\" advertising or publicity pertaining to distribution of the
.\" software without specific, written prior permission.
.\" M.I.T. makes no representations about the suitability of
.\" this software for any purpose.  It is provided "as is"
.\" without express or implied warranty.
.\"
.TH ARES_INIT 3 "5 March 2010"
.SH NAME
ares_init, ares_init_options \- Initialize a resolver channel
.SH SYNOPSIS
.nf
.B #include <ares.h>
.PP
.B int ares_init(ares_channel *\fIchannelptr\fP)
.B int ares_init_options(ares_channel *\fIchannelptr\fP,
.B 	struct ares_options *\fIoptions\fP, int \fIoptmask\fP)
.PP
.B cc file.c -lcares
.fi
.SH DESCRIPTION
The \fBares_init\fP function initializes a communications channel for name
service lookups.  If it returns successfully, \fBares_init\fP will set the
variable pointed to by \fIchannelptr\fP to a handle used to identify the name
service channel.  The caller should invoke
.BR ares_destroy (3)
on the handle when the channel is no longer needed.
.PP
The \fBares_init_options\fP function also initializes a name service channel,
with additional options useful for applications requiring more control over
name service configuration. The \fIoptmask\fP parameter specifies which fields
in the structure pointed to by \fIoptions\fP are set, as follows:
.TP 18
.B ARES_OPT_FLAGS
.B int \fIflags\fP;
.br
Flags controlling the behavior of the resolver.  See below for a
description of possible flag values.
.TP 18
.B ARES_OPT_TIMEOUT
.B int \fItimeout\fP;
.br
The number of seconds each name server is given to respond to a query on the
first try.  (After the first try, the timeout algorithm becomes more
complicated, but scales linearly with the value of \fItimeout\fP.)  The
default is five seconds. This option is being deprecated by
\fIARES_OPT_TIMEOUTMS\fP starting in c-ares 1.5.2.
.TP 18
.B ARES_OPT_TIMEOUTMS
.B int \fItimeout\fP;
.br
The number of milliseconds each name server is given to respond to a query on
the first try.  (After the first try, the timeout algorithm becomes more
complicated, but scales linearly with the value of \fItimeout\fP.)  The
default is five seconds. Note that this option is specified with the same
struct field as the former \fIARES_OPT_TIMEOUT\fP, it is but the option bits
that tell c-ares how to interpret the number. This option was added in c-ares
1.5.2.
.TP 18
.B ARES_OPT_TRIES
.B int \fItries\fP;
.br
The number of tries the resolver will try contacting each name server
before giving up.  The default is four tries.
.TP 18
.B ARES_OPT_NDOTS
.B int \fIndots\fP;
.br
The number of dots which must be present in a domain name for it to be
queried for "as is" prior to querying for it with the default domain
extensions appended.  The default value is 1 unless set otherwise by
resolv.conf or the RES_OPTIONS environment variable.
.TP 18
.B ARES_OPT_PORT
.B unsigned short \fIport\fP;
.br
The port to use for queries (both TCP and UDP), in network byte order.
The default value is 53 (in network byte order), the standard name
service port.
.TP 18
.B ARES_OPT_SERVERS
.B struct in_addr *\fIservers\fP;
.br
.B int \fInservers\fP;
.br
The list of IPv4 servers to contact, instead of the servers specified in
resolv.conf or the local named. In order to allow specification of either
IPv4 or IPv6 name servers, the
.BR ares_set_servers(3)
function must be used instead.
.TP 18
.B ARES_OPT_DOMAINS
.B char **\fIdomains\fP;
.br
.B int \fIndomains\fP;
.br
The domains to search, instead of the domains specified in resolv.conf
or the domain derived from the kernel hostname variable.
.TP 18
.B ARES_OPT_LOOKUPS
.B char *\fIlookups\fP;
.br
The lookups to perform for host queries.
.I lookups
should be set to a string of the characters "b" or "f", where "b"
indicates a DNS lookup and "f" indicates a lookup in the hosts file.
.TP 18
.B ARES_OPT_SOCK_STATE_CB
.B void (*\fIsock_state_cb\fP)(void *data, int s, int read, int write);
.br
.B void *\fIsock_state_cb_data\fP;
.br
A callback function to be invoked when a socket changes state.
.I s
will be passed the socket whose state has changed;
.I read
will be set to true if the socket should listen for read events, and
.I write
will be set to true if the socket should listen for write events.
The value of
.I sock_state_cb_data
will be passed as the
.I data
argument.
.PP
The
.I flags
field should be the bitwise or of some subset of the following values:
.TP 23
.B ARES_FLAG_USEVC
Always use TCP queries (the "virtual circuit") instead of UDP
queries.  Normally, TCP is only used if a UDP query yields a truncated
result.
.TP 23
.B ARES_FLAG_PRIMARY
Only query the first server in the list of servers to query.
.TP 23
.B ARES_FLAG_IGNTC
If a truncated response to a UDP query is received, do not fall back
to TCP; simply continue on with the truncated response.
.TP 23
.B ARES_FLAG_NORECURSE
Do not set the "recursion desired" bit on outgoing queries, so that the name
server being contacted will not try to fetch the answer from other servers if
it doesn't know the answer locally. Be aware that ares will not do the
recursion for you.  Recursion must be handled by the application calling ares
if \fIARES_FLAG_NORECURSE\fP is set.
.TP 23
.B ARES_FLAG_STAYOPEN
Do not close communications sockets when the number of active queries
drops to zero.
.TP 23
.B ARES_FLAG_NOSEARCH
Do not use the default search domains; only query hostnames as-is or
as aliases.
.TP 23
.B ARES_FLAG_NOALIASES
Do not honor the HOSTALIASES environment variable, which normally
specifies a file of hostname translations.
.TP 23
.B ARES_FLAG_NOCHECKRESP
Do not discard responses with the SERVFAIL, NOTIMP, or REFUSED
response code or responses whose questions don't match the questions
in the request.  Primarily useful for writing clients which might be
used to test or debug name servers.
.SH RETURN VALUES
.I ares_init
or
.I ares_init_options
can return any of the following values:
.TP 14
.B ARES_SUCCESS
Initialization succeeded.
.TP 14
.B ARES_EFILE
A configuration file could not be read.
.TP 14
.B ARES_ENOMEM
The process's available memory was exhausted.
.TP 14
.B ARES_ENOTINITIALIZED
c-ares library initialization not yet performed.
.SH NOTES
When initializing from
.B /etc/resolv.conf,
.BR ares_init (3)
reads the
.I domain
and
.I search
directives to allow lookups of short names relative to the domains
specified. The
.I domain
and
.I search
directives override one another. If more that one instance of either
.I domain
or
.I search
directives is specified, the last occurence wins. For more information,
please see the
.BR resolv.conf (5)
manual page.
.SH SEE ALSO
.BR ares_destroy(3),
.BR ares_dup(3),
.BR ares_library_init(3),
.BR ares_set_servers(3)
.SH AUTHOR
Greg Hudson, MIT Information Systems
.br
Copyright 1998 by the Massachusetts Institute of Technology.
.br
Copyright (C) 2004-2010 by Daniel Stenberg.