c-ares/docs/ares_query.3

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.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)(void *\fIarg\fP, int \fIstatus\fP,
                              int \fItimeouts\fP, unsigned char *\fIabuf\fP,
                              int \fIalen\fP)

void ares_query(ares_channel_t *\fIchannel\fP, const char *\fIname\fP,
                int \fIdnsclass\fP, int \fItype\fP,
                ares_callback \fIcallback\fP, void *\fIarg\fP)
.fi
.SH DESCRIPTION
The
.B ares_query
function initiates 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 using the values defined in
.BR <arpa/nameser.h> .
When the query is complete or has failed, the ares library will invoke
.IR callback .
Completion or failure of the query may happen immediately, or may
happen during a later call to
.BR ares_process (3)
or
.BR ares_destroy (3).
.PP
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.
.PP
The callback argument
.I arg
is copied from the
.B ares_query
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
.BR ares_send (3)
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
.BR ares_send (3)
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
.BR ares_send (3)
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.
.PP
The callback argument
.I timeouts
reports how many times a query timed out during the execution of the
given request.
.PP
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 abuf
points to a result buffer of length
.IR alen .
If the query did not complete,
.I abuf
will be NULL and
.I alen
will be 0.
.SH SEE ALSO
.BR ares_process (3)
.SH AUTHOR
Greg Hudson, MIT Information Systems
.br
Copyright 1998 by the Massachusetts Institute of Technology.