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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 ‘<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>’ 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 |
| (‘<samp><span class="samp">Ctrl-C</span></samp>’) within <span class="sc">gdb</span>. |
| |
| <p>The target's request to perform a host system call does not finish |
| 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, |
| 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 |
| <- target requests 'system call X' |
| target is stopped, <span class="sc">gdb</span> executes system call |
| -> <span class="sc">gdb</span> returns result |
| ... target continues, <span class="sc">gdb</span> returns to wait for the target |
| <- 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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