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<h4 class="subsection">D.14.1 File-I/O Overview</h4>

<p><a name="index-file_002di_002fo-overview-2495"></a>
The <dfn>File I/O remote protocol extension</dfn> (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>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
<span class="sc">gdb</span> and the target's <span class="sc">gdb</span> stub are responsible for
translating the system-dependent value representations into the internal
protocol representations when data is transmitted.

   <p>The communication is synchronous.  A system call is possible only when
<span class="sc">gdb</span> is waiting for a response from the &lsquo;<samp><span class="samp">C</span></samp>&rsquo;, &lsquo;<samp><span class="samp">c</span></samp>&rsquo;, &lsquo;<samp><span class="samp">S</span></samp>&rsquo;
or &lsquo;<samp><span class="samp">s</span></samp>&rsquo; packets.  While <span class="sc">gdb</span> 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
(&lsquo;<samp><span class="samp">Ctrl-C</span></samp>&rsquo;) within <span class="sc">gdb</span>.

   <p>The target's request to perform a host system call does not finish
the latest &lsquo;<samp><span class="samp">C</span></samp>&rsquo;, &lsquo;<samp><span class="samp">c</span></samp>&rsquo;, &lsquo;<samp><span class="samp">S</span></samp>&rsquo; or &lsquo;<samp><span class="samp">s</span></samp>&rsquo; 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 <span class="sc">gdb</span> is required.

<pre class="smallexample">     (gdb) continue
       &lt;- target requests 'system call X'
       target is stopped, <span class="sc">gdb</span> executes system call
       -&gt; <span class="sc">gdb</span> returns result
       ... target continues, <span class="sc">gdb</span> returns to wait for the target
       &lt;- target hits breakpoint and sends a Txx packet
</pre>
   <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>File I/O is not supported in non-stop mode.

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