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| Next: <a href="Protocol-Basics.html#Protocol-Basics" accesskey="n" rel="next">Protocol Basics</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="File_002dI_002fO-Overview-1"></a> |
| <h4 class="subsection">E.13.1 File-I/O Overview</h4> |
| <a name="index-file_002di_002fo-overview"></a> |
| |
| <p>The <em>File I/O remote protocol extension</em> (short: File-I/O) allows the |
| target to use the host’s file system and console I/O to perform various |
| system calls. System calls on the target system are translated into a |
| remote protocol packet to the host system, which then performs the needed |
| actions and returns a response packet to the target system. |
| This simulates file system operations even on targets that lack file systems. |
| </p> |
| <p>The protocol is defined to be independent of both the host and target systems. |
| It uses its own internal representation of datatypes and values. Both |
| <small>GDB</small> and the target’s <small>GDB</small> stub are responsible for |
| translating the system-dependent value representations into the internal |
| protocol representations when data is transmitted. |
| </p> |
| <p>The communication is synchronous. A system call is possible only when |
| <small>GDB</small> is waiting for a response from the ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’ |
| or ‘<samp>s</samp>’ packets. While <small>GDB</small> handles the request for a system call, |
| the target is stopped to allow deterministic access to the target’s |
| memory. Therefore File-I/O is not interruptible by target signals. On |
| the other hand, it is possible to interrupt File-I/O by a user interrupt |
| (‘<samp>Ctrl-C</samp>’) within <small>GDB</small>. |
| </p> |
| <p>The target’s request to perform a host system call does not finish |
| the latest ‘<samp>C</samp>’, ‘<samp>c</samp>’, ‘<samp>S</samp>’ or ‘<samp>s</samp>’ action. That means, |
| after finishing the system call, the target returns to continuing the |
| previous activity (continue, step). No additional continue or step |
| request from <small>GDB</small> is required. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) continue |
| <- target requests 'system call X' |
| target is stopped, GDB executes system call |
| -> GDB returns result |
| ... target continues, GDB returns to wait for the target |
| <- target hits breakpoint and sends a Txx packet |
| </pre></div> |
| |
| <p>The protocol only supports I/O on the console and to regular files on |
| the host file system. Character or block special devices, pipes, |
| named pipes, sockets or any other communication method on the host |
| system are not supported by this protocol. |
| </p> |
| <p>File I/O is not supported in non-stop mode. |
| </p> |
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| Next: <a href="Protocol-Basics.html#Protocol-Basics" accesskey="n" rel="next">Protocol Basics</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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