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.\"
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.\" Copyright 1998 by the Massachusetts Institute of Technology.
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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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.\" SPDX-License-Identifier: MIT
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.\"
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.TH ARES_GETHOSTBYADDR 3 "24 July 1998"
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.SH NAME
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ares_gethostbyaddr \- Initiate a host query by address
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.SH SYNOPSIS
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.nf
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#include <ares.h>
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typedef void (*ares_host_callback)(void *\fIarg\fP, int \fIstatus\fP,
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int \fItimeouts\fP,
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struct hostent *\fIhostent\fP)
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`ares_channel` -> `ares_channel_t *`: don't bury the pointer (#595)
`ares_channel` is defined as `typedef struct ares_channeldata *ares_channel;`. The problem with this, is it embeds the pointer into the typedef, which means an `ares_channel` can never be declared as `const` as if you write `const ares_channel channel`, that expands to `struct ares_channeldata * const ares_channel` and not `const struct ares_channeldata *channel`.
We will now typedef `ares_channel_t` as `typedef struct ares_channeldata ares_channel_t;`, so if you write `const ares_channel_t *channel`, it properly expands to `const struct ares_channeldata *channel`.
We are maintaining the old typedef for API compatibility with existing integrations, and due to typedef expansion this should not even cause any compiler warnings for existing code. There are no ABI implications with this change. I could be convinced to keep existing public functions as `ares_channel` if a sufficient argument exists, but internally we really need make this change for modern best practices.
This change will allow us to internally use `const ares_channel_t *` where appropriate. Whether or not we decide to change any public interfaces to use `const` may require further discussion on if there might be ABI implications (I don't think so, but I'm also not 100% sure what a compiler internally does with `const` when emitting machine code ... I think more likely ABI implications would occur going the opposite direction).
FYI, This PR was done via a combination of sed and clang-format, the only manual code change was the addition of the new typedef, and a couple doc fixes :)
Fix By: Brad House (@bradh352)
1 year ago
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void ares_gethostbyaddr(ares_channel_t *\fIchannel\fP, const void *\fIaddr\fP,
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int \fIaddrlen\fP, int \fIfamily\fP,
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ares_host_callback \fIcallback\fP, void *\fIarg\fP)
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.fi
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.SH DESCRIPTION
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The
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.B ares_gethostbyaddr
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function initiates a host query by address on the name service channel
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identified by
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.IR channel .
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The parameters
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.I addr
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and
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.I addrlen
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give the address as a series of bytes, and
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.I family
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gives the type of address. When the query is complete or has failed, the ares
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library will invoke \fIcallback\fP. Completion or failure of the query may
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happen immediately, or may happen during a later call to
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\fIares_process(3)\fP, \fIares_destroy(3)\fP or \fIares_cancel(3)\fP.
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.PP
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The callback argument
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.I arg
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is copied from the
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.B ares_gethostbyaddr
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argument
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.IR arg .
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The callback argument
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.I status
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indicates whether the query succeeded and, if not, how it failed. It
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may have any of the following values:
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.TP 19
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.B ARES_SUCCESS
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The host lookup completed successfully.
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.TP 19
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.B ARES_ENOTIMP
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The ares library does not know how to look up addresses of type
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.IR family .
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.TP 19
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.B ARES_ENOTFOUND
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The address
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.I addr
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was not found.
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.TP 19
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.B ARES_ENOMEM
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Memory was exhausted.
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.TP 19
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.B ARES_ECANCELLED
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The query was cancelled.
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.TP 19
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.B ARES_EDESTRUCTION
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The name service channel
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.I channel
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is being destroyed; the query will not be completed.
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.PP
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The callback argument
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.I timeouts
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reports how many times a query timed out during the execution of the
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given request.
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.PP
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On successful completion of the query, the callback argument
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.I hostent
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points to a
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.B struct hostent
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containing the name of the host returned by the query. The callback
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need not and should not attempt to free the memory pointed to by
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.IR hostent ;
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the ares library will free it when the callback returns. If the query
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did not complete successfully,
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.I hostent
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will be
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.BR NULL .
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.SH SEE ALSO
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.BR ares_process (3),
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.BR ares_gethostbyname (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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