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docs/ares_query.3
164 строки5 KB
Brad House
Revert "Mark parameters in callbacks as const" (#1060); document non-mutability instead (#1244)
07 июл 2026, 02:26
07 июл 2026, 02:262c9dc2d
.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" SPDX-License-Identifier: MIT
.\"
.TH ARES_QUERY 3 "24 July 1998"
.SH NAME
ares_query \- Initiate a single-question DNS query
.SH SYNOPSIS
.nf
#include <ares.h>

typedef void (*ares_callback_dnsrec)(void *arg, ares_status_t status,
                                     size_t timeouts,
                                     const ares_dns_record_t *dnsrec);

ares_status_t ares_query_dnsrec(ares_channel_t      *channel,
                                const char          *name,
                                ares_dns_class_t     dnsclass,
                                ares_dns_rec_type_t  type,
                                ares_callback_dnsrec callback,
                                void                *arg,
                                unsigned short      *qid);

typedef void (*ares_callback)(void *arg, int status,
                              int timeouts, unsigned char *abuf,
                              int alen);

void ares_query(ares_channel_t *channel, const char *name,
                int dnsclass, int type,
                ares_callback callback, void *arg);

.fi

.SH DESCRIPTION

The \fBares_query_dnsrec(3)\fP and \fBares_query(3)\fP functions initiate a
single-question DNS query on the name service channel identified by
.IR channel .
The parameter
.I name
gives the query name as a NUL-terminated C string of period-separated
labels optionally ending with a period; periods and backslashes within
a label must be escaped with a backslash.  The parameters
.I dnsclass
and
.I type
give the class and type of the query.

\fBares_query_dnsrec(3)\fP uses the ares \fBares_dns_class_t\fP and
\fBares_dns_rec_type_t\fP defined types.  However, \fBares_query(3)\fP uses
the values defined in \fB<arpa/nameser.h>\fP.

When the query is complete or has failed, the ares library will invoke
.IR callback .
Completion or failure of the query may happen immediately (even before the
return of the function call), or may happen during a later call to
\fBares_process(3)\fP or \fBares_destroy(3)\fP.

If this is called from a thread other than which the main program event loop is
running, care needs to be taken to ensure any file descriptor lists are updated
immediately within the eventloop.  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 argument
.I arg
is copied from the \fBares_query_dnsrec(3)\fP or \fBares_query(3)\fP 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 query completed successfully.
.TP 19
.B ARES_ENODATA
The query completed but contains no answers.
.TP 19
.B ARES_EFORMERR
The query completed but the server claims that the query was
malformatted.
.TP 19
.B ARES_ESERVFAIL
The query completed but the server claims to have experienced a
failure.  (This code can only occur if the
.B ARES_FLAG_NOCHECKRESP
flag was specified at channel initialization time; otherwise, such
responses are ignored at the \fBares_send_dnsrec(3)\fP level.)
.TP 19
.B ARES_ENOTFOUND
The query completed but the queried-for domain name was not found.
.TP 19
.B ARES_ENOTIMP
The query completed but the server does not implement the operation
requested by the query.  (This code can only occur if the
.B ARES_FLAG_NOCHECKRESP
flag was specified at channel initialization time; otherwise, such
responses are ignored at the \fBares_send_dnsrec(3)\fP level.)
.TP 19
.B ARES_EREFUSED
The query completed but the server refused the query.  (This code can
only occur if the
.B ARES_FLAG_NOCHECKRESP
flag was specified at channel initialization time; otherwise, such
responses are ignored at the \fBares_send_dnsrec(3)\fP level.)
.TP 19
.B ARES_EBADNAME
The query name
.I name
could not be encoded as a domain name, either because it contained a
zero-length label or because it contained a label of more than 63
characters.
.TP 19
.B ARES_ETIMEOUT
No name servers responded within the timeout period.
.TP 19
.B ARES_ECONNREFUSED
No name servers could be contacted.
.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.
.TP 19
.B ARES_ENOSERVER
The query will not be completed because no DNS servers were configured on the
channel.
.PP

The callback argument
.I timeouts
reports how many times a query timed out during the execution of the
given request.

If the query completed (even if there was something wrong with it, as
indicated by some of the above error codes), the callback argument
.I dnsrec
or
.I abuf
will be non-NULL, otherwise they will be NULL.

The
.I abuf
buffer is owned by the c-ares library and is only valid for the duration of
the callback.  The callback
.B must not
modify or free it, and must not retain a reference to it after returning; treat
it as read-only.  The parameter is intentionally not declared
.B const
solely to preserve API/ABI compatibility with existing applications.

.SH AVAILABILITY
\fBares_query_dnsrec(3)\fP was introduced in c-ares 1.28.0.

.SH SEE ALSO
.BR ares_process (3),
.BR ares_dns_record (3)