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| <h4 class="subsection">28.3.2 The Interface of the Function in NSS Modules</h4> |
| |
| <p>Now we know about the functions contained in the modules. It is now |
| time to describe the types. When we mentioned the reentrant versions of |
| the functions above, this means there are some additional arguments |
| (compared with the standard, non-reentrant version). The prototypes for |
| the non-reentrant and reentrant versions of our function above are: |
| |
| <pre class="smallexample"> struct hostent *gethostbyname (const char *name) |
| |
| int gethostbyname_r (const char *name, struct hostent *result_buf, |
| char *buf, size_t buflen, struct hostent **result, |
| int *h_errnop) |
| </pre> |
| <p class="noindent">The actual prototype of the function in the NSS modules in this case is |
| |
| <pre class="smallexample"> enum nss_status _nss_files_gethostbyname_r (const char *name, |
| struct hostent *result_buf, |
| char *buf, size_t buflen, |
| int *errnop, int *h_errnop) |
| </pre> |
| <p>I.e., the interface function is in fact the reentrant function with the |
| change of the return value and the omission of the <var>result</var> |
| parameter. While the user-level function returns a pointer to the |
| result the reentrant function return an <code>enum nss_status</code> value: |
| |
| <dl> |
| <dt><code>NSS_STATUS_TRYAGAIN</code><a name="index-NSS_005fSTATUS_005fTRYAGAIN-3286"></a><dd>numeric value <code>-2</code> |
| |
| <br><dt><code>NSS_STATUS_UNAVAIL</code><a name="index-NSS_005fSTATUS_005fUNAVAIL-3287"></a><dd>numeric value <code>-1</code> |
| |
| <br><dt><code>NSS_STATUS_NOTFOUND</code><a name="index-NSS_005fSTATUS_005fNOTFOUND-3288"></a><dd>numeric value <code>0</code> |
| |
| <br><dt><code>NSS_STATUS_SUCCESS</code><a name="index-NSS_005fSTATUS_005fSUCCESS-3289"></a><dd>numeric value <code>1</code> |
| </dl> |
| |
| <p class="noindent">Now you see where the action items of the <samp><span class="file">/etc/nsswitch.conf</span></samp> file |
| are used. |
| |
| <p>If you study the source code you will find there is a fifth value: |
| <code>NSS_STATUS_RETURN</code>. This is an internal use only value, used by a |
| few functions in places where none of the above value can be used. If |
| necessary the source code should be examined to learn about the details. |
| |
| <p>In case the interface function has to return an error it is important |
| that the correct error code is stored in <code>*</code><var>errnop</var>. Some |
| return status value have only one associated error code, others have |
| more. |
| |
| <p><table summary=""><tr align="left"><td valign="top" width="30%"><code>NSS_STATUS_TRYAGAIN</code> </td><td valign="top" width="20%"> |
| <code>EAGAIN</code> </td><td valign="top" width="50%">One of the functions used ran temporarily out of |
| resources or a service is currently not available. |
| <br></td></tr><tr align="left"><td valign="top" width="30%"></td><td valign="top" width="20%"> |
| <code>ERANGE</code> </td><td valign="top" width="50%">The provided buffer is not large enough. |
| The function should be called again with a larger buffer. |
| <br></td></tr><tr align="left"><td valign="top" width="30%"><code>NSS_STATUS_UNAVAIL</code> </td><td valign="top" width="20%"> |
| <code>ENOENT</code> </td><td valign="top" width="50%">A necessary input file cannot be found. |
| <br></td></tr><tr align="left"><td valign="top" width="30%"><code>NSS_STATUS_NOTFOUND</code> </td><td valign="top" width="20%"> |
| <code>ENOENT</code> </td><td valign="top" width="50%">The requested entry is not available. |
| <br></td></tr></table> |
| |
| <p>These are proposed values. There can be other error codes and the |
| described error codes can have different meaning. <strong>With one |
| exception:</strong> when returning <code>NSS_STATUS_TRYAGAIN</code> the error code |
| <code>ERANGE</code> <em>must</em> mean that the user provided buffer is too |
| small. Everything is non-critical. |
| |
| <p>The above function has something special which is missing for almost all |
| the other module functions. There is an argument <var>h_errnop</var>. This |
| points to a variable which will be filled with the error code in case |
| the execution of the function fails for some reason. The reentrant |
| function cannot use the global variable <var>h_errno</var>; |
| <code>gethostbyname</code> calls <code>gethostbyname_r</code> with the last argument |
| set to <code>&h_errno</code>. |
| |
| <p>The <code>get</code><var>XXX</var><code>by</code><var>YYY</var> functions are the most important |
| functions in the NSS modules. But there are others which implement |
| the other ways to access system databases (say for the |
| password database, there are <code>setpwent</code>, <code>getpwent</code>, and |
| <code>endpwent</code>). These will be described in more detail later. |
| Here we give a general way to determine the |
| signature of the module function: |
| |
| <ul> |
| <li>the return value is <code>int</code>; |
| <li>the name is as explained in see <a href="NSS-Module-Names.html#NSS-Module-Names">NSS Module Names</a>; |
| <li>the first arguments are identical to the arguments of the non-reentrant |
| function; |
| <li>the next three arguments are: |
| |
| <dl> |
| <dt><code>STRUCT_TYPE *result_buf</code><dd>pointer to buffer where the result is stored. <code>STRUCT_TYPE</code> is |
| normally a struct which corresponds to the database. |
| <br><dt><code>char *buffer</code><dd>pointer to a buffer where the function can store additional data for |
| the result etc. |
| <br><dt><code>size_t buflen</code><dd>length of the buffer pointed to by <var>buffer</var>. |
| </dl> |
| |
| <li>possibly a last argument <var>h_errnop</var>, for the host name and network |
| name lookup functions. |
| </ul> |
| |
| <p class="noindent">This table is correct for all functions but the <code>set...ent</code> |
| and <code>end...ent</code> functions. |
| |
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