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 <a name="Protocol-Basics"></a>
 <div class="header">
 <p>
 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>
 </div>
 <hr>
 <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>
 <hr>
 <div class="header">
 <p>
 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"></a>
	<div class="header">
	<p>
	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>   [<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>
	</div>
	<hr>
	<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 ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’ or ‘<samp>s</samp>’ 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> “Ctrl-C” flag.

	</li></ul>

	<p>After having done the needed type and value coercion, the target continues
	the latest continue or step action.
	</p>
	<hr>
	<div class="header">
	<p>
	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>   [<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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