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<title>Debugging with GDB: Protocol Basics</title>
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<a name="Protocol-Basics"></a>
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<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="previous">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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<a name="Protocol-Basics-1"></a>
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<h4 class="subsection">E.13.2 Protocol Basics</h4>
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<a name="index-protocol-basics_002c-file_002di_002fo"></a>
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<p>The File-I/O protocol uses the <code>F</code> packet as the request as well
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as reply packet. Since a File-I/O system call can only occur when
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<small>GDB</small> is waiting for a response from the continuing or stepping target,
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the File-I/O request is a reply that <small>GDB</small> has to expect as a result
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of a previous ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’ or ‘<samp>s</samp>’ packet.
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This <code>F</code> packet contains all information needed to allow <small>GDB</small>
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to call the appropriate host system call:
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</p>
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<ul>
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<li> A unique identifier for the requested system call.
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</li><li> All parameters to the system call. Pointers are given as addresses
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in the target memory address space. Pointers to strings are given as
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pointer/length pair. Numerical values are given as they are.
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Numerical control flags are given in a protocol-specific representation.
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</li></ul>
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<p>At this point, <small>GDB</small> has to perform the following actions.
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</p>
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<ul>
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<li> If the parameters include pointer values to data needed as input to a
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system call, <small>GDB</small> requests this data from the target with a
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standard <code>m</code> packet request. This additional communication has to be
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expected by the target implementation and is handled as any other <code>m</code>
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packet.
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</li><li> <small>GDB</small> translates all value from protocol representation to host
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representation as needed. Datatypes are coerced into the host types.
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</li><li> <small>GDB</small> calls the system call.
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</li><li> It then coerces datatypes back to protocol representation.
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</li><li> If the system call is expected to return data in buffer space specified
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by pointer parameters to the call, the data is transmitted to the
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target using a <code>M</code> or <code>X</code> packet. This packet has to be expected
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by the target implementation and is handled as any other <code>M</code> or <code>X</code>
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packet.
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</li></ul>
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<p>Eventually <small>GDB</small> replies with another <code>F</code> packet which contains all
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necessary information for the target to continue. This at least contains
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</p>
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<ul>
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<li> Return value.
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</li><li> <code>errno</code>, if has been changed by the system call.
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</li><li> “Ctrl-C” flag.
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</li></ul>
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<p>After having done the needed type and value coercion, the target continues
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the latest continue or step action.
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</p>
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<div class="header">
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<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="previous">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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