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94 lines
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<title>File-I/O Overview - Debugging with GDB</title>
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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="next" href="Protocol-Basics.html#Protocol-Basics" title="Protocol Basics">
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<div class="node">
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<a name="File-I%2fO-Overview"></a>
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<a name="File_002dI_002fO-Overview"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Protocol-Basics.html#Protocol-Basics">Protocol Basics</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.13.1 File-I/O Overview</h4>
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<p><a name="index-file_002di_002fo-overview-3595"></a>
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The <dfn>File I/O remote protocol extension</dfn> (short: File-I/O) allows the
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target to use the host's file system and console I/O to perform various
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system calls. System calls on the target system are translated into a
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remote protocol packet to the host system, which then performs the needed
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actions and returns a response packet to the target system.
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This simulates file system operations even on targets that lack file systems.
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<p>The protocol is defined to be independent of both the host and target systems.
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It uses its own internal representation of datatypes and values. Both
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<span class="sc">gdb</span> and the target's <span class="sc">gdb</span> stub are responsible for
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translating the system-dependent value representations into the internal
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protocol representations when data is transmitted.
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<p>The communication is synchronous. A system call is possible only when
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<span class="sc">gdb</span> is waiting for a response from the ‘<samp><span class="samp">C</span></samp>’, ‘<samp><span class="samp">c</span></samp>’, ‘<samp><span class="samp">S</span></samp>’
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or ‘<samp><span class="samp">s</span></samp>’ packets. While <span class="sc">gdb</span> handles the request for a system call,
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the target is stopped to allow deterministic access to the target's
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memory. Therefore File-I/O is not interruptible by target signals. On
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the other hand, it is possible to interrupt File-I/O by a user interrupt
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(‘<samp><span class="samp">Ctrl-C</span></samp>’) within <span class="sc">gdb</span>.
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<p>The target's request to perform a host system call does not finish
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the latest ‘<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>’ action. That means,
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after finishing the system call, the target returns to continuing the
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previous activity (continue, step). No additional continue or step
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request from <span class="sc">gdb</span> is required.
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<pre class="smallexample"> (gdb) continue
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<- target requests 'system call X'
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target is stopped, <span class="sc">gdb</span> executes system call
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-> <span class="sc">gdb</span> returns result
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... target continues, <span class="sc">gdb</span> returns to wait for the target
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<- target hits breakpoint and sends a Txx packet
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</pre>
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<p>The protocol only supports I/O on the console and to regular files on
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the host file system. Character or block special devices, pipes,
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named pipes, sockets or any other communication method on the host
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system are not supported by this protocol.
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<p>File I/O is not supported in non-stop mode.
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</body></html>
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