arlib.3

网络时间协议NTP 源码 版本v4.2.0b 该源码用于linux平台下

.TH arlib 3
.SH NAME
ar_answer, ar_close, ar_delete, ar_gethostbyname, ar_gethostbyaddr,
ar_init, ar_open, ar_timeout - Asynchronous DNS library routines
.SH SYNOPSIS
.nf
.B #include "arlib.h"

.B struct hostent *ar_answer(dataptr, size)
.B char *dataptr;
.B int size;

.B void ar_close();

.B int ar_delete(dataptr, size)
.B char *dataptr;
.B int size;

.B int ar_gethostbyname(name, dataptr, size)
.B char *name;
.B char *dataptr;
.B int size;

.B int ar_gethostbyaddr(name, dataptr, size)
.B char *name;
.B char *dataptr;
.B int size;

.B int ar_init(flags)
.B int flags;

.B int ar_open();

.B long ar_timeout(time, dataptr, size)
.B long time;
.B char *dataptr;
.B int size;
.fi
.SH DESCRIPTION

.PP
   This small library of DNS routines is intended to provide an
asynchronous interface to performing hostname and IP number lookups.
Only lookups of Internet domain are handled as yet.  To use this
set of routines properly, the presence of the
.B "BIND 4.8"
resolve
libraries is required (or any library derived from it).
.PP
   This library should be used in conjunction with
.B select(2)
to wait for
the name server's reply to arrive or the lookup to timeout.
.PP
   To open a fd for talking to the name server, either
.B ar_open()
or
ar_init()
must be used.
.B  ar_open()
 will open either a datagram socket
or a virtual circuit with the name server, depending on the flags
set in the _res structure (see
.B resolv(5)
).  In both cases, if the socket

> i
.B  ar_init()
is
used to both open the socket (as in
.B ar_open()
) and initialize the
queues used by this library.  The values recognized as parameters to
.B ar_init()
are:

.RS
#define ARES_INITLIST   1
.RE
.RS
#define ARES_CALLINIT   2
.RE
.RS
#define ARES_INITSOCK   4
.RE
.RS
#define ARES_INITDEBG   8
.RE

   ARES_INITLIST initializes the list of queries waiting for replies.
ARES_CALLINIT is a flag which when set causes
.B res_init()
to be called.
ARES_INITSOCK will close the current socket if it is open and call
.B ar_open()
to open a new one, returning the fd for that socket.
ARES_INITDEBG sets the RES_DEBUG flag of the
.B _res
structure.
ARES_INITCACH is as yet, unused and is for future use where the library
keeps its own cache of replies.

   To send a query about either a hostname or an IP number,
.B ar_gethostbyname()
and
.B ar_gethostbyaddr()
must be used.  Each takes
either a pointer to the hostname or the IP number respectively for use
when making the query.  In addition to this, both (optionally) can be
passed a pointer to data, dataptr, with the size also passed which can
be used for identifying individual queries.  A copy of the area pointed
to is made if dataptr is non NULL and size is non zero.  These functions
will always return NULL unless the answer to the query is found in
internal caches.  A new flag, RES_CHECKPTR is checked during the
processing of answers for
.B ar_gethostbyname()
which will automatically
cause a reverse lookup to be queued, causing a failure if that reply
differs from the original.

   To check for a query,
.B ar_answer()
is called with a pointer to an  area
of memory which is sufficient to hold what was originally passed via
.B ar_gethostbyname()
or
.B ar_gethostbyaddr()
through dataptr.  If an answer
is found, a pointer to the host information is returned and the data
segment copied if dataptr is non NULL and it was originally passed.  The
size of the copied data is the smaller of the passed size and that of
original data stored.

   To expire old queries,
.B ar_timeout()
is called with the 'current' time
(or the time for which you want to do timeouts for).  If a queue entry
is too old, it will be expired when it has exhausted all available avenues
for lookups and the data segment for the expired query copied into
dataptr.  The size of the copied data is the smaller of the passed size
and that of the original stored data.  Only 1 entry is thus expired with
each call, requiring that it be called immediately after an expiration
to check for others.  In addition to expiring lookups,
.B ar_timeout()
also
triggers resends of queries and the searching of the domain tree for the
host, the latter works from the
.B _res
structure of
.B resolv(5).

   To delete entries from the queue,
.B ar_delete()
can be used and by
passing the pointer and size of the data segment, all queries have their
data segments checked (if present) for an exact match, being deleted if
and only if there is a match.  A NULL pointer passed to ar_deleted()
matches all queries which were called with a NULL dataptr parameter.
The amount of data compared is the smaller of the size passed and that
of the data stored for the queue entry being compared.

   To close a socket opened by
.B ar_open()
,
.B ar_close()
should  be used so
that it is closed and also marked closed within this library.

   
.SH DIAGNOSIS

.B ar_open()
returns -1 if a socket isn't open and could not be opened;
otherwise returns the current fd open or the fd it opened.

.B ar_init()
returns -1 for any errors, the value returned by
.B res_init()
if
.B res_init()
was called, the return value for
.B ar_open()
if that was
called or the current socket open if 0 is passed and a socket is open.
If neither
.B res_init()
or
.B ar_open()
are called and the flags are non-zero, -2 is returned.
 
.B ar_gethostbyaddr()
and
.B ar_gethostbyname()
will always return NULL in this version but may return a pointer to a hostent
structure if a cache is being used and the answer is found in the cache.

.B ar_answer()
returns NULL if the answer is either not found or the
query returned an error and another attempt at a lookup is attempted.
If an answer was found, it returned a pointer to this structure and
the contents of the data segment copied over.

.B ar_timeout()
returns the time when it should be called next or 0 if
there are no queries in the queue to be checked later.  If any queries
are expired, the data segment is copied over if dataptr is non NULL.

.B ar_delete()
returns the number of entries that were found to match
and consequently deleted.

.SH SEE ALSO

gethostbyaddr(3), gethostbyname(3), resolv(5)

.SH FILES
.nf
arlib.h
/usr/include/resolv.h
/usr/include/arpa/nameser.h
/etc/resolv.conf

.SH BUGS

The results of a successful call to ar_answer() destroy the structure
for any previous calls.

.SH AUTHOR

Darren Reed.  Email address: [e-mail]