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<a name="Protocol-Basics"></a>
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Next: <a href="The-F-Request-Packet.html#The-F-Request-Packet" accesskey="n" rel="next">The F Request Packet</a>, Previous: <a href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" accesskey="p" rel="prev">File-I/O Overview</a>, Up: <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" accesskey="u" rel="up">File-I/O Remote Protocol Extension</a> &nbsp; [<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="Protocol-Basics-1"></a>
<h4 class="subsection">E.13.2 Protocol Basics</h4>
<a name="index-protocol-basics_002c-file_002di_002fo"></a>

<p>The File-I/O protocol uses the <code>F</code> packet as the request as well
as reply packet.  Since a File-I/O system call can only occur when
<small>GDB</small> is waiting for a response from the continuing or stepping target, 
the File-I/O request is a reply that <small>GDB</small> has to expect as a result
of a previous &lsquo;<samp>C</samp>&rsquo;, &lsquo;<samp>c</samp>&rsquo;, &lsquo;<samp>S</samp>&rsquo; or &lsquo;<samp>s</samp>&rsquo; packet.
This <code>F</code> packet contains all information needed to allow <small>GDB</small>
to call the appropriate host system call:
</p>
<ul>
<li> A unique identifier for the requested system call.

</li><li> All parameters to the system call.  Pointers are given as addresses
in the target memory address space.  Pointers to strings are given as
pointer/length pair.  Numerical values are given as they are.
Numerical control flags are given in a protocol-specific representation.

</li></ul>

<p>At this point, <small>GDB</small> has to perform the following actions.
</p>
<ul>
<li> If the parameters include pointer values to data needed as input to a 
system call, <small>GDB</small> requests this data from the target with a
standard <code>m</code> packet request.  This additional communication has to be
expected by the target implementation and is handled as any other <code>m</code>
packet.

</li><li> <small>GDB</small> translates all value from protocol representation to host
representation as needed.  Datatypes are coerced into the host types.

</li><li> <small>GDB</small> calls the system call.

</li><li> It then coerces datatypes back to protocol representation.

</li><li> If the system call is expected to return data in buffer space specified
by pointer parameters to the call, the data is transmitted to the
target using a <code>M</code> or <code>X</code> packet.  This packet has to be expected
by the target implementation and is handled as any other <code>M</code> or <code>X</code>
packet.

</li></ul>

<p>Eventually <small>GDB</small> replies with another <code>F</code> packet which contains all
necessary information for the target to continue.  This at least contains
</p>
<ul>
<li> Return value.

</li><li> <code>errno</code>, if has been changed by the system call.

</li><li> &ldquo;Ctrl-C&rdquo; flag.

</li></ul>

<p>After having done the needed type and value coercion, the target continues
the latest continue or step action.
</p>
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Next: <a href="The-F-Request-Packet.html#The-F-Request-Packet" accesskey="n" rel="next">The F Request Packet</a>, Previous: <a href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" accesskey="p" rel="prev">File-I/O Overview</a>, Up: <a href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" accesskey="u" rel="up">File-I/O Remote Protocol Extension</a> &nbsp; [<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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