111 lines
5.1 KiB
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111 lines
5.1 KiB
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<html lang="en">
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<title>Protocol Basics - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension" title="File-I/O Remote Protocol Extension">
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<link rel="prev" href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview" title="File-I/O Overview">
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<link rel="next" href="The-F-Request-Packet.html#The-F-Request-Packet" title="The F Request Packet">
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</head>
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<body>
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<div class="node">
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<a name="Protocol-Basics"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="The-F-Request-Packet.html#The-F-Request-Packet">The F Request Packet</a>,
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Previous: <a rel="previous" accesskey="p" href="File_002dI_002fO-Overview.html#File_002dI_002fO-Overview">File-I/O Overview</a>,
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Up: <a rel="up" accesskey="u" href="File_002dI_002fO-Remote-Protocol-Extension.html#File_002dI_002fO-Remote-Protocol-Extension">File-I/O Remote Protocol Extension</a>
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<hr>
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</div>
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<h4 class="subsection">E.13.2 Protocol Basics</h4>
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<p><a name="index-protocol-basics_002c-file_002di_002fo-3596"></a>
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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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<span class="sc">gdb</span> is waiting for a response from the continuing or stepping target,
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the File-I/O request is a reply that <span class="sc">gdb</span> has to expect as a result
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of a previous ‘<samp><span class="samp">C</span></samp>’, ‘<samp><span class="samp">c</span></samp>’, ‘<samp><span class="samp">S</span></samp>’ or ‘<samp><span class="samp">s</span></samp>’ packet.
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This <code>F</code> packet contains all information needed to allow <span class="sc">gdb</span>
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to call the appropriate host system call:
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<ul>
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<li>A unique identifier for the requested system call.
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<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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</ul>
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<p>At this point, <span class="sc">gdb</span> has to perform the following actions.
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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, <span class="sc">gdb</span> 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><span class="sc">gdb</span> 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><span class="sc">gdb</span> calls the system call.
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<li>It then coerces datatypes back to protocol representation.
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<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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</ul>
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<p>Eventually <span class="sc">gdb</span> 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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<ul>
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<li>Return value.
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<li><code>errno</code>, if has been changed by the system call.
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<li>“Ctrl-C” flag.
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</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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</body></html>
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