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| <h4 class="subsection">28.4.2 Internals of the NSS Module Functions</h4> |
| |
| <p>Until now we only provided the syntactic interface for the functions in |
| the NSS module. In fact there is not much more we can say since the |
| implementation obviously is different for each function. But a few |
| general rules must be followed by all functions. |
| |
| <p>In fact there are four kinds of different functions which may appear in |
| the interface. All derive from the traditional ones for system databases. |
| <var>db</var> in the following table is normally an abbreviation for the |
| database (e.g., it is <code>pw</code> for the password database). |
| |
| <dl> |
| <dt><code>enum nss_status _nss_</code><var>database</var><code>_set</code><var>db</var><code>ent (void)</code><dd>This function prepares the service for following operations. For a |
| simple file based lookup this means files could be opened, for other |
| services this function simply is a noop. |
| |
| <p>One special case for this function is that it takes an additional |
| argument for some <var>database</var>s (i.e., the interface is |
| <code>int set</code><var>db</var><code>ent (int)</code>). <a href="Host-Names.html#Host-Names">Host Names</a>, which describes the |
| <code>sethostent</code> function. |
| |
| <p>The return value should be <var>NSS_STATUS_SUCCESS</var> or according to the |
| table above in case of an error (see <a href="NSS-Modules-Interface.html#NSS-Modules-Interface">NSS Modules Interface</a>). |
| |
| <br><dt><code>enum nss_status _nss_</code><var>database</var><code>_end</code><var>db</var><code>ent (void)</code><dd>This function simply closes all files which are still open or removes |
| buffer caches. If there are no files or buffers to remove this is again |
| a simple noop. |
| |
| <p>There normally is no return value different to <var>NSS_STATUS_SUCCESS</var>. |
| |
| <br><dt><code>enum nss_status _nss_</code><var>database</var><code>_get</code><var>db</var><code>ent_r (</code><var>STRUCTURE</var><code> *result, char *buffer, size_t buflen, int *errnop)</code><dd>Since this function will be called several times in a row to retrieve |
| one entry after the other it must keep some kind of state. But this |
| also means the functions are not really reentrant. They are reentrant |
| only in that simultaneous calls to this function will not try to |
| write the retrieved data in the same place (as it would be the case for |
| the non-reentrant functions); instead, it writes to the structure |
| pointed to by the <var>result</var> parameter. But the calls share a common |
| state and in the case of a file access this means they return neighboring |
| entries in the file. |
| |
| <p>The buffer of length <var>buflen</var> pointed to by <var>buffer</var> can be used |
| for storing some additional data for the result. It is <em>not</em> |
| guaranteed that the same buffer will be passed for the next call of this |
| function. Therefore one must not misuse this buffer to save some state |
| information from one call to another. |
| |
| <p>Before the function returns the implementation should store the value of |
| the local <var>errno</var> variable in the variable pointed to be |
| <var>errnop</var>. This is important to guarantee the module working in |
| statically linked programs. |
| |
| <p>As explained above this function could also have an additional last |
| argument. This depends on the database used; it happens only for |
| <code>host</code> and <code>networks</code>. |
| |
| <p>The function shall return <code>NSS_STATUS_SUCCESS</code> as long as there are |
| more entries. When the last entry was read it should return |
| <code>NSS_STATUS_NOTFOUND</code>. When the buffer given as an argument is too |
| small for the data to be returned <code>NSS_STATUS_TRYAGAIN</code> should be |
| returned. When the service was not formerly initialized by a call to |
| <code>_nss_</code><var>DATABASE</var><code>_set</code><var>db</var><code>ent</code> all return value allowed for |
| this function can also be returned here. |
| |
| <br><dt><code>enum nss_status _nss_</code><var>DATABASE</var><code>_get</code><var>db</var><code>by</code><var>XX</var><code>_r (</code><var>PARAMS</var><code>, </code><var>STRUCTURE</var><code> *result, char *buffer, size_t buflen, int *errnop)</code><dd>This function shall return the entry from the database which is |
| addressed by the <var>PARAMS</var>. The type and number of these arguments |
| vary. It must be individually determined by looking to the user-level |
| interface functions. All arguments given to the non-reentrant version |
| are here described by <var>PARAMS</var>. |
| |
| <p>The result must be stored in the structure pointed to by <var>result</var>. |
| If there is additional data to return (say strings, where the |
| <var>result</var> structure only contains pointers) the function must use the |
| <var>buffer</var> or length <var>buflen</var>. There must not be any references |
| to non-constant global data. |
| |
| <p>The implementation of this function should honor the <var>stayopen</var> |
| flag set by the <code>set</code><var>DB</var><code>ent</code> function whenever this makes sense. |
| |
| <p>Before the function returns the implementation should store the value of |
| the local <var>errno</var> variable in the variable pointed to be |
| <var>errnop</var>. This is important to guarantee the module working in |
| statically linked programs. |
| |
| <p>Again, this function takes an additional last argument for the |
| <code>host</code> and <code>networks</code> database. |
| |
| <p>The return value should as always follow the rules given above |
| (see <a href="NSS-Modules-Interface.html#NSS-Modules-Interface">NSS Modules Interface</a>). |
| |
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