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<title>Debugging with GDB: Host I/O Packets</title>
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<a name="Host-I_002fO-Packets"></a>
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<p>
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Next: <a href="Interrupts.html#Interrupts" accesskey="n" rel="next">Interrupts</a>, Previous: <a href="Tracepoint-Packets.html#Tracepoint-Packets" accesskey="p" rel="previous">Tracepoint Packets</a>, Up: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="u" rel="up">Remote Protocol</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Host-I_002fO-Packets-1"></a>
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<h3 class="section">E.7 Host I/O Packets</h3>
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<a name="index-Host-I_002fO_002c-remote-protocol"></a>
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<a name="index-file-transfer_002c-remote-protocol"></a>
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<p>The <em>Host I/O</em> packets allow <small>GDB</small> to perform I/O
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operations on the far side of a remote link. For example, Host I/O is
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used to upload and download files to a remote target with its own
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filesystem. Host I/O uses the same constant values and data structure
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layout as the target-initiated File-I/O protocol. However, the
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Host I/O packets are structured differently. The target-initiated
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protocol relies on target memory to store parameters and buffers.
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Host I/O requests are initiated by <small>GDB</small>, and the
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target’s memory is not involved. See <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension">File-I/O Remote Protocol Extension</a>, for more details on the target-initiated protocol.
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</p>
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<p>The Host I/O request packets all encode a single operation along with
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its arguments. They have this format:
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</p>
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<dl compact="compact">
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<dt>‘<samp>vFile:<var>operation</var>: <var>parameter</var>…</samp>’</dt>
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<dd><p><var>operation</var> is the name of the particular request; the target
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should compare the entire packet name up to the second colon when checking
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for a supported operation. The format of <var>parameter</var> depends on
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the operation. Numbers are always passed in hexadecimal. Negative
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numbers have an explicit minus sign (i.e. two’s complement is not
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used). Strings (e.g. filenames) are encoded as a series of
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hexadecimal bytes. The last argument to a system call may be a
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buffer of escaped binary data (see <a href="Overview.html#Binary-Data">Binary Data</a>).
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</p>
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</dd>
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</dl>
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<p>The valid responses to Host I/O packets are:
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</p>
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<dl compact="compact">
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<dt>‘<samp>F <var>result</var> [, <var>errno</var>] [; <var>attachment</var>]</samp>’</dt>
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<dd><p><var>result</var> is the integer value returned by this operation, usually
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non-negative for success and -1 for errors. If an error has occured,
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<var>errno</var> will be included in the result specifying a
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value defined by the File-I/O protocol (see <a href="errno-Values.html#errno-Values">errno Values</a>). For
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operations which return data, <var>attachment</var> supplies the data as a
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binary buffer. Binary buffers in response packets are escaped in the
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normal way (see <a href="Overview.html#Binary-Data">Binary Data</a>). See the individual packet
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documentation for the interpretation of <var>result</var> and
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<var>attachment</var>.
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</p>
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</dd>
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<dt>‘<samp><!-- /@w --></samp>’</dt>
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<dd><p>An empty response indicates that this operation is not recognized.
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</p>
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</dd>
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</dl>
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<p>These are the supported Host I/O operations:
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</p>
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<dl compact="compact">
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<dt>‘<samp>vFile:open: <var>filename</var>, <var>flags</var>, <var>mode</var></samp>’</dt>
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<dd><p>Open a file at <var>filename</var> and return a file descriptor for it, or
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return -1 if an error occurs. The <var>filename</var> is a string,
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<var>flags</var> is an integer indicating a mask of open flags
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(see <a href="Open-Flags.html#Open-Flags">Open Flags</a>), and <var>mode</var> is an integer indicating a mask
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of mode bits to use if the file is created (see <a href="mode_005ft-Values.html#mode_005ft-Values">mode_t Values</a>).
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See <a href="open.html#open">open</a>, for details of the open flags and mode values.
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</p>
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</dd>
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<dt>‘<samp>vFile:close: <var>fd</var></samp>’</dt>
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<dd><p>Close the open file corresponding to <var>fd</var> and return 0, or
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-1 if an error occurs.
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</p>
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</dd>
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<dt>‘<samp>vFile:pread: <var>fd</var>, <var>count</var>, <var>offset</var></samp>’</dt>
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<dd><p>Read data from the open file corresponding to <var>fd</var>. Up to
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<var>count</var> bytes will be read from the file, starting at <var>offset</var>
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relative to the start of the file. The target may read fewer bytes;
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common reasons include packet size limits and an end-of-file
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condition. The number of bytes read is returned. Zero should only be
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returned for a successful read at the end of the file, or if
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<var>count</var> was zero.
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</p>
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<p>The data read should be returned as a binary attachment on success.
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If zero bytes were read, the response should include an empty binary
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attachment (i.e. a trailing semicolon). The return value is the
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number of target bytes read; the binary attachment may be longer if
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some characters were escaped.
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</p>
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</dd>
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<dt>‘<samp>vFile:pwrite: <var>fd</var>, <var>offset</var>, <var>data</var></samp>’</dt>
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<dd><p>Write <var>data</var> (a binary buffer) to the open file corresponding
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to <var>fd</var>. Start the write at <var>offset</var> from the start of the
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file. Unlike many <code>write</code> system calls, there is no
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separate <var>count</var> argument; the length of <var>data</var> in the
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packet is used. ‘<samp>vFile:pwrite</samp>’ returns the number of bytes written,
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which may be shorter than the length of <var>data</var>, or -1 if an
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error occurred.
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</p>
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</dd>
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<dt>‘<samp>vFile:fstat: <var>fd</var></samp>’</dt>
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<dd><p>Get information about the open file corresponding to <var>fd</var>.
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On success the information is returned as a binary attachment
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and the return value is the size of this attachment in bytes.
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If an error occurs the return value is -1. The format of the
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returned binary attachment is as described in <a href="struct-stat.html#struct-stat">struct stat</a>.
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</p>
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</dd>
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<dt>‘<samp>vFile:unlink: <var>filename</var></samp>’</dt>
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<dd><p>Delete the file at <var>filename</var> on the target. Return 0,
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or -1 if an error occurs. The <var>filename</var> is a string.
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</p>
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</dd>
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<dt>‘<samp>vFile:readlink: <var>filename</var></samp>’</dt>
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<dd><p>Read value of symbolic link <var>filename</var> on the target. Return
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the number of bytes read, or -1 if an error occurs.
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</p>
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<p>The data read should be returned as a binary attachment on success.
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If zero bytes were read, the response should include an empty binary
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attachment (i.e. a trailing semicolon). The return value is the
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number of target bytes read; the binary attachment may be longer if
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some characters were escaped.
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</p>
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</dd>
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<dt>‘<samp>vFile:setfs: <var>pid</var></samp>’</dt>
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<dd><p>Select the filesystem on which <code>vFile</code> operations with
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<var>filename</var> arguments will operate. This is required for
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<small>GDB</small> to be able to access files on remote targets where
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the remote stub does not share a common filesystem with the
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inferior(s).
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</p>
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<p>If <var>pid</var> is nonzero, select the filesystem as seen by process
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<var>pid</var>. If <var>pid</var> is zero, select the filesystem as seen by
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the remote stub. Return 0 on success, or -1 if an error occurs.
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If <code>vFile:setfs:</code> indicates success, the selected filesystem
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remains selected until the next successful <code>vFile:setfs:</code>
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operation.
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</p>
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</dd>
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</dl>
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<div class="header">
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<p>
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Next: <a href="Interrupts.html#Interrupts" accesskey="n" rel="next">Interrupts</a>, Previous: <a href="Tracepoint-Packets.html#Tracepoint-Packets" accesskey="p" rel="previous">Tracepoint Packets</a>, Up: <a href="Remote-Protocol.html#Remote-Protocol" accesskey="u" rel="up">Remote Protocol</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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