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| <h4 class="subsection">D.14.2 Protocol Basics</h4> |
| |
| <p><a name="index-protocol-basics_002c-file_002di_002fo-2496"></a> |
| The File-I/O protocol uses the <code>F</code> packet as the request as well |
| as reply packet. Since a File-I/O system call can only occur when |
| <span class="sc">gdb</span> is waiting for a response from the continuing or stepping target, |
| the File-I/O request is a reply that <span class="sc">gdb</span> has to expect as a result |
| 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. |
| This <code>F</code> packet contains all information needed to allow <span class="sc">gdb</span> |
| to call the appropriate host system call: |
| |
| <ul> |
| <li>A unique identifier for the requested system call. |
| |
| <li>All parameters to the system call. Pointers are given as addresses |
| in the target memory address space. Pointers to strings are given as |
| pointer/length pair. Numerical values are given as they are. |
| Numerical control flags are given in a protocol-specific representation. |
| |
| </ul> |
| |
| <p>At this point, <span class="sc">gdb</span> has to perform the following actions. |
| |
| <ul> |
| <li>If the parameters include pointer values to data needed as input to a |
| system call, <span class="sc">gdb</span> requests this data from the target with a |
| standard <code>m</code> packet request. This additional communication has to be |
| expected by the target implementation and is handled as any other <code>m</code> |
| packet. |
| |
| <li><span class="sc">gdb</span> translates all value from protocol representation to host |
| representation as needed. Datatypes are coerced into the host types. |
| |
| <li><span class="sc">gdb</span> calls the system call. |
| |
| <li>It then coerces datatypes back to protocol representation. |
| |
| <li>If the system call is expected to return data in buffer space specified |
| by pointer parameters to the call, the data is transmitted to the |
| target using a <code>M</code> or <code>X</code> packet. This packet has to be expected |
| by the target implementation and is handled as any other <code>M</code> or <code>X</code> |
| packet. |
| |
| </ul> |
| |
| <p>Eventually <span class="sc">gdb</span> replies with another <code>F</code> packet which contains all |
| necessary information for the target to continue. This at least contains |
| |
| <ul> |
| <li>Return value. |
| |
| <li><code>errno</code>, if has been changed by the system call. |
| |
| <li>“Ctrl-C” flag. |
| |
| </ul> |
| |
| <p>After having done the needed type and value coercion, the target continues |
| the latest continue or step action. |
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