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1 | @c each section should have index entries corresponding to the section title |
2 | ||
3 | @node Name Service Switch | |
4 | @chapter System Databases and Name Service Switch | |
5 | ||
6 | Various functions in the C Library need to be configured to work | |
7 | correctly in the local environment. Traditionally, this was done by | |
8 | using files (e.g., @file{/etc/passwd}), but other nameservices (line the | |
9 | Network Information Service (NIS) and the Domain Name Service (DNS)) | |
10 | became popular, and were hacked into the C library, usually with a fixed | |
11 | search order @pxref{frobnicate, frobnicate, ,jargon}. | |
12 | ||
13 | The GNU C Library contains a cleaner solution of this problem. It is | |
14 | designed after a method used by Sun Microsystems in the C library of | |
15 | @w{Solaris 2}. GNU C Library follows their name and calls this | |
16 | scheme @dfn{Name Service Switch} (NSS). | |
17 | ||
18 | Though the interface might be similar to Sun's version there is no | |
19 | common code. We never saw any source code of Sun's implementation and | |
2de99474 | 20 | so the internal interface is incompatible. This is also manifests in the |
706074a5 UD |
21 | file names we use as we will see later. |
22 | ||
23 | ||
24 | @menu | |
25 | * NSS Basics:: What is this NSS good for. | |
26 | * NSS Configuration File:: Configuring NSS. | |
27 | * NSS Module Internals:: How does it work internally. | |
28 | * Extending NSS:: What to do to add services or databases. | |
29 | @end menu | |
30 | ||
31 | @node NSS Basics, NSS Configuration File, Name Service Switch, Name Service Switch | |
32 | @section NSS Basics | |
33 | ||
34 | The basic idea is to put the implementation of the different services | |
35 | offered to access the databases in separate modules. This has some | |
36 | advantages: | |
37 | ||
38 | @enumerate | |
39 | @item | |
40 | Contributors can add new services without adding them to GNU C Library. | |
41 | @item | |
42 | The modules can be updated separately. | |
43 | @item | |
44 | The C library image is smaller. | |
45 | @end enumerate | |
46 | ||
47 | To fulfill the first goal above the ABI of the modules will be described | |
48 | below. For getting the implementation of a new service right it is | |
49 | important to understand how the functions in the modules get called. | |
50 | They are in no way designed to be used by the programmer directly. | |
51 | Instead the programmer should only use the documented and standardized | |
52 | functions to access the databases. | |
53 | ||
54 | @noindent | |
55 | The databases available in the NSS are | |
56 | ||
57 | @cindex ethers | |
58 | @cindex group | |
59 | @cindex hosts | |
60 | @cindex network | |
61 | @cindex protocols | |
62 | @cindex passwd | |
63 | @cindex rpc | |
64 | @cindex services | |
65 | @cindex shadow | |
66 | @vtable @code | |
67 | @item ethers | |
68 | Ethernet numbers, | |
69 | @comment @pxref{Ethernet Numbers}. | |
70 | @item group | |
71 | Groups of users, @pxref{Group Database}. | |
72 | @item hosts | |
73 | Host names and numbers, @pxref{Host Names}. | |
74 | @item network | |
75 | Network names and numbers, @pxref{Networks Database}. | |
76 | @item protocols | |
77 | Network protocols, @pxref{Protocols Database}. | |
78 | @item passwd | |
79 | User passwords, @pxref{User Database}. | |
80 | @item rpc | |
81 | Remote procedure call names and numbers, | |
82 | @comment @pxref{RPC Database}. | |
83 | @item services | |
84 | Network services, @pxref{Services Database}. | |
85 | @item shadow | |
86 | Shadow user passwords, | |
87 | @comment @pxref{Shadow Password Database}. | |
88 | @end vtable | |
89 | ||
90 | @noindent | |
91 | There will be some more added later (@code{aliases}, @code{automount}, | |
92 | @code{bootparams}, @code{netgroup}, @code{netmasks}, and | |
93 | @code{publickey}). | |
94 | ||
95 | @node NSS Configuration File, NSS Module Internals, NSS Basics, Name Service Switch | |
96 | @section The NSS Configuration File | |
97 | ||
98 | @cindex @file{/etc/nsswitch.conf} | |
99 | @cindex @file{nsswitch.conf} | |
100 | Somehow the NSS code must be told about the wishes of the user. For | |
101 | this reason there is the file @file{/etc/nsswitch.conf}. For each | |
102 | database this file contain a specification how the lookup process should | |
103 | work. The file could look like this: | |
104 | ||
105 | @example | |
106 | @include nsswitch.texi | |
107 | @end example | |
108 | ||
109 | The first column is the database as you can guess from the table above. | |
110 | The rest of the line specifies how the lookup process works. Please | |
111 | note that you specify the way it works for each database individually. | |
112 | This cannot be done with the old way of a monolithic implementation. | |
113 | ||
114 | The configuration specification for each database can contain two | |
115 | different items: | |
116 | ||
117 | @itemize @bullet | |
118 | @item | |
119 | the service specification like @code{files}, @code{db}, or @code{nis}. | |
120 | @item | |
121 | the reaction on lookup result line @code{[NOTFOUND=return]}. | |
122 | @end itemize | |
123 | ||
124 | @menu | |
125 | * Services in the NSS configuration:: Service names in the NSS configuratin. | |
126 | * Actions in the NSS configuration:: React approprite on the lookup result. | |
127 | * Notes on NSS Configuration File:: Things to take care about while | |
128 | configuring NSS. | |
129 | @end menu | |
130 | ||
131 | @node Services in the NSS configuration, Actions in the NSS configuration, NSS Configuration File, NSS Configuration File | |
132 | @subsection Services in the NSS configuration File | |
133 | ||
134 | The above example file mentions four different services: @code{files}, | |
135 | @code{db}, @code{nis}, and @code{nisplus}. This does not mean these | |
136 | services are available on all sites and it does also not mean these are | |
137 | all the services which will ever be available. | |
138 | ||
139 | In fact, these names are simply strings which the NSS code uses to find | |
140 | the implicitly addressed functions. The internal interface will be | |
141 | described later. Visible to the user are the modules which implement an | |
142 | individual service. | |
143 | ||
144 | Assume the service @var{name} shall be used for a lookup. The code for | |
145 | this service is implemented in a module called @file{libnss_@var{name}}. | |
146 | On a system supporting shared libraries this is in fact a shared library | |
147 | with the name (for example) @file{libnss_@var{name}.so.1}. The number | |
148 | at the end is the currently used version of the interface which will not | |
149 | change frequently. Normally the user should not have to be cognizant of | |
150 | these files since they should be placed in a directory where they are | |
151 | found automatically. Only the names of all available services are | |
152 | important. | |
153 | ||
154 | @node Actions in the NSS configuration, Notes on NSS Configuration File, Services in the NSS configuration, NSS Configuration File | |
155 | @subsection Actions in the NSS configuration | |
156 | ||
157 | The second item in the specification gives the user much finer control | |
158 | on the lookup process. Action items are placed between two service | |
159 | names and are written within brackets. The general form is | |
160 | ||
2de99474 | 161 | @display |
57ba7bb4 | 162 | @code{[} ( @code{!}? @var{status} @code{=} @var{action} )+ @code{]} |
2de99474 | 163 | @end display |
706074a5 UD |
164 | |
165 | @noindent | |
166 | where | |
167 | ||
168 | @smallexample | |
169 | @var{status} @result{} success | notfound | unavail | tryagain | |
170 | @var{action} @result{} return | continue | |
171 | @end smallexample | |
172 | ||
173 | The case of the keywords is insignificant. The @var{status} | |
174 | values are the results of a call to a lookup function of a specific | |
175 | service. They mean | |
176 | ||
177 | @ftable @samp | |
178 | @item success | |
179 | No error occured an the wanted entry is returned. The default action | |
180 | for this is @code{return}. | |
181 | ||
182 | @item notfound | |
183 | The lookup process works ok but the needed value was not found. The | |
184 | default action is @code{continue}. | |
185 | ||
186 | @item unavail | |
187 | @cindex DNS server unavailable | |
188 | The service is permanently unavailable. This can either mean the needed | |
189 | file is not available, or, for DNS, the server is not available or does | |
190 | not allow queries. The default action is @code{continue}. | |
191 | ||
192 | @item tryagain | |
193 | The service is temporarily unavailable. This could mean a file is | |
194 | locked or a server currently cannot accept more connections. The | |
195 | default action is @code{continue}. | |
196 | @end ftable | |
197 | ||
198 | @noindent | |
199 | If we have a line like | |
200 | ||
201 | @smallexample | |
202 | ethers: nisplus [NOTFOUND=return] db files | |
203 | @end smallexample | |
204 | ||
205 | @noindent | |
206 | this is equivalent to | |
207 | ||
208 | @smallexample | |
209 | ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue | |
210 | TRYAGAIN=continue] | |
211 | db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue | |
212 | TRYAGAIN=continue] | |
213 | files | |
214 | @end smallexample | |
215 | ||
216 | @noindent | |
217 | (except that it would have to be written on one line). The default | |
218 | value for the actions are normally what you want, and only need to be | |
219 | changed in exceptional cases. | |
220 | ||
221 | If the optional @code{!} is placed before the @var{status} this means | |
222 | the following action is used for all statii but @var{status} itself. | |
223 | I.e., @code{!} is negation as in the C language (and others). | |
224 | ||
225 | Before we explain the exception which makes this action item necessary | |
226 | one more remark: obviously it makes no sense to add another action | |
227 | item after the @code{files} service. Since there is no other service | |
228 | following the action @emph{always} is @code{return}. | |
229 | ||
230 | @cindex nisplus, and completeness | |
231 | Now, why is this @code{[NOTFOUND=return]} action useful? To understand | |
232 | this we should know that the @code{nisplus} service is often | |
233 | complete; i.e., if an entry is not available in the NIS+ tables it is | |
234 | not available anywhere else. This is what is expressed by this action | |
235 | item: it is useless to examine further services since they will not give | |
236 | us a result. | |
237 | ||
238 | @cindex nisplus, and booting | |
239 | @cindex bootstrapping, and services | |
240 | The situation would be different if the NIS+ service is not available | |
241 | because the machine is booting. In this case the return value of the | |
242 | lookup function is not @code{notfound} but instead @code{unavail}. And | |
243 | as you can see in the complete form above: in this situation the | |
244 | @code{db} and @code{files} services are used. Neat, isn't it? The | |
245 | system administrator need not pay special care for the time the system | |
246 | is not completely ready to work (while booting or shutdown or | |
247 | network problems). | |
248 | ||
249 | ||
250 | @node Notes on NSS Configuration File, , Actions in the NSS configuration, NSS Configuration File | |
251 | @subsection Notes on the NSS Configuration File | |
252 | ||
253 | Finally a few more hints. The NSS implementation is not completely | |
254 | helpless if @file{/etc/nsswitch.conf} does not exist. For | |
255 | all supported databases there is a default value so it should normally | |
256 | be possible to get the system running even if the file is corrupted or | |
257 | missing. | |
258 | ||
259 | A second point is that the user should try to optimize the lookup | |
260 | process. The different service have different response times. A simple | |
261 | file look up on a local file could be fast, but if the file is long and the | |
262 | needed entry is near the end of the file this may take quite some time. | |
263 | In this case it might be better to use the @code{db} service which | |
264 | allows fast local access to large data sets. | |
265 | ||
266 | Often the situation is that some global information like NIS must be | |
267 | used. So it is unavoidable to use service entries like @code{nis} etc. | |
268 | But one should avoid slow services like this if possible. | |
269 | ||
270 | ||
271 | @node NSS Module Internals, Extending NSS, NSS Configuration File, Name Service Switch | |
272 | @section NSS Module Internals | |
273 | ||
274 | Now it is time to described how the modules look like. The functions | |
275 | contained in a module are identified by their names. I.e., there is no | |
276 | jump table or the like. How this is done is of no interest here; those | |
277 | interested in this topic should read about Dynamic Linking. | |
278 | @comment @ref{Dynamic Linking}. | |
279 | ||
280 | ||
281 | @menu | |
282 | * NSS Module Names:: Construction of the interface function of | |
283 | the NSS modules. | |
284 | * NSS Modules Interface:: Programming interface in the NSS module | |
285 | functions. | |
286 | @end menu | |
287 | ||
288 | @node NSS Module Names, NSS Modules Interface, NSS Module Internals, NSS Module Internals | |
289 | @subsection The Naming Scheme of the NSS Modules | |
290 | ||
291 | @noindent | |
292 | The name of each function consist of various parts: | |
293 | ||
294 | @quotation | |
295 | _nss_@var{service}_@var{function} | |
296 | @end quotation | |
297 | ||
298 | @var{service} of course corresponds to the name of the module this | |
299 | function is found in.@footnote{Now you might ask why to duplicate this | |
300 | information. The answer is that we want to keep the possibility to link | |
301 | directly with these shared objects.} The @var{function} part is derived | |
302 | from the interface function in the C library itself. If the user calls | |
303 | the function @code{gethostbyname} and the service used is @code{files} | |
304 | the function | |
305 | ||
306 | @smallexample | |
307 | _nss_files_gethostbyname_r | |
308 | @end smallexample | |
309 | ||
310 | @noindent | |
311 | in the module | |
312 | ||
313 | @smallexample | |
314 | libnss_files.so.1 | |
315 | @end smallexample | |
316 | ||
317 | @noindent | |
318 | @cindex reentrant NSS functions | |
319 | is used. You see, what is explained above in not the whole truth. In | |
320 | fact the NSS modules only contain reentrant versions of the lookup | |
321 | functions. I.e., if the user would call the @code{gethostbyname_r} | |
322 | function this also would end in the above function. For all user | |
323 | interface functions the C library maps this call to a call to the | |
324 | reentrant function. For reentrant functions this is trivial since the | |
325 | interface is (nearly) the same. For the non-reentrant version pointers | |
326 | to static buffers are used to replace the user supplied buffers. | |
327 | ||
328 | I.e., the reentrant functions @emph{can} have counterparts. No service | |
329 | module is forced to have functions for all databases and all kinds to | |
330 | access them. If a function is not available it is simply treated as if | |
331 | the function would return @code{unavail} | |
332 | (@pxref{Actions in the NSS configuration}). | |
333 | ||
2de99474 UD |
334 | The file name @file{libnss_files.so.1} would be on a @w{Solaris 2} |
335 | system @file{nss_files.so.1}. This is the difference mentioned above. | |
336 | Sun's NSS modules are usable as modules which get indirectly loaded | |
337 | only. | |
338 | ||
339 | The NSS modules in the GNU C Library are prepared to be used as normal | |
340 | libraries itself. | |
341 | @comment Fix me if necessary. | |
342 | This is @emph{not} true in the moment, though. But the different | |
343 | organization of the name space in the modules does not make it | |
344 | impossible like it is for Solaris. Now you can see why the modules are | |
345 | still libraries.@footnote{There is a second explanation: we were too | |
346 | lazy to change the Makefiles to allow the generation of shared objects | |
347 | not starting with @file{lib} but do not tell this anybody.} | |
348 | ||
706074a5 UD |
349 | |
350 | @node NSS Modules Interface, , NSS Module Names, NSS Module Internals | |
351 | @subsection The Interface of the Function in NSS Modules | |
352 | ||
353 | Now we know about the functions contained in the modules. It is now | |
354 | time to describe the types. When we mentioned the reentrant versions of | |
355 | the functions above, this means there are some additional arguments | |
356 | (compared with the standard, non-reentrant version). The prototypes for | |
357 | the non-reentrant and reentrant versions of our function above are: | |
358 | ||
359 | @smallexample | |
360 | struct hostent *gethostbyname (const char *name) | |
361 | ||
362 | struct hostent *gethostbyname_r (const char *name, | |
363 | struct hostent *result_buf, char *buf, | |
364 | int buflen, int *h_errnop) | |
365 | @end smallexample | |
366 | ||
367 | @noindent | |
368 | The actual prototype of the function is the NSS modules in this case is | |
369 | ||
370 | @smallexample | |
371 | int _nss_files_gethostbyname_r (const char *name, | |
372 | struct hostent *result_buf, char *buf, | |
373 | int buflen, int *h_errnop) | |
374 | @end smallexample | |
375 | ||
376 | I.e., the interface function is in fact the reentrant function with | |
377 | the change of the return value. While the user-level function returns a | |
378 | pointer to the result the reentrant function return an @code{int} value: | |
379 | ||
380 | @cindex NSS_STATUS_TRYAGAIN | |
381 | @cindex NSS_STATUS_UNAVAIL | |
382 | @cindex NSS_STATUS_NOTFOUND | |
383 | @cindex NSS_STATUS_SUCCESS | |
384 | @ftable @code | |
385 | @item NSS_STATUS_TRYAGAIN | |
386 | numeric value @code{-2} | |
387 | ||
388 | @item NSS_STATUS_UNAVAIL | |
389 | numeric value @code{-1} | |
390 | ||
391 | @item NSS_STATUS_NOTFOUND | |
392 | numeric value @code{0} | |
393 | ||
394 | @item NSS_STATUS_SUCCESS | |
395 | numeric value @code{1} | |
396 | @end ftable | |
397 | ||
398 | @noindent | |
399 | Now you see where the action items of the @file{/etc/nsswitch.conf} file | |
400 | are used. | |
401 | ||
402 | The above function has somthing special which is missing for almost all | |
403 | the other module functions. There is an argument @var{h_errnop}. This | |
404 | points to a variable which will be filled with the error code in case | |
405 | the execution of the function fails for some reason. The reentrant | |
406 | function cannot use the global variable @var{h_errno}; | |
407 | @code{gethostbyname} calls @code{gethostbyname_r} with the | |
408 | last argument set to @code{&h_errno}. | |
409 | ||
410 | The @code{get@var{XXX}by@var{YYY}} functions are the most important | |
411 | functions in the NSS modules. But there are others which implement | |
412 | the other ways to access system databases (say for the | |
413 | password database, there are @code{setpwent}, @code{getpwent}, and | |
414 | @code{endpwent}). These will be described in more detail later. | |
415 | Here we give a general way to determine the | |
416 | signature of the module function: | |
417 | ||
418 | @itemize @bullet | |
419 | @item | |
420 | the return value is @code{int}; | |
421 | @item | |
422 | the name is as explain in @pxref{NSS Module Names}; | |
423 | @item | |
424 | the first arguments are identical to the arguments of the non-reentrant | |
425 | function; | |
426 | @item | |
427 | the next three arguments are: | |
428 | ||
429 | @table @code | |
430 | @item STRUCT_TYPE result_buf | |
431 | pointer to buffer where the result is stored. @code{STRUCT_TYPE} is | |
432 | normally a struct which corresponds to the database. | |
433 | @item char *buffer | |
434 | pointer to a buffer where the function can store additional adata for | |
435 | the result etc. | |
436 | @item int buflen | |
437 | length of the buffer pointed to by @var{buffer}. | |
438 | @end table | |
439 | ||
440 | @item | |
441 | possibly a last argument @var{h_errnop}, for the host name and network | |
442 | name lookup functions. | |
443 | @end itemize | |
444 | ||
445 | @noindent | |
446 | This table is correct for all functions but the @code{set@dots{}ent} | |
447 | and @code{end@dots{}ent} functions. | |
448 | ||
449 | ||
450 | @node Extending NSS, , NSS Module Internals, Name Service Switch | |
451 | @section Extending NSS | |
452 | ||
453 | One of the advantages of NSS mentioned above is that it can be extended | |
454 | quite easily. There are two ways in which the extension can happen: | |
455 | adding another database or adding another service. The former is | |
456 | normally done only by the C library developers. It is | |
457 | here only important to remember that adding another database is | |
458 | independent from adding another service because a service need not | |
459 | support all databases or lookup functions. | |
460 | ||
461 | A designer/implementor of a new service is therefore free to choose the | |
462 | databases s/he is interested in and leave the rest for later (or | |
463 | completely aside). | |
464 | ||
465 | @menu | |
466 | * Adding another Service to NSS:: What is to do to add a new service. | |
467 | * NSS Module Function Internals:: Guidelines for writing new NSS | |
468 | service functions. | |
469 | @end menu | |
470 | ||
471 | @node Adding another Service to NSS, NSS Module Function Internals, Extending NSS, Extending NSS | |
472 | @subsection Adding another Service to NSS | |
473 | ||
474 | The sources for a new service need not (and should not) be part of the | |
475 | GNU C Library itself. The developer retains complete control over the | |
476 | sources and its development. The links between the C library and the | |
477 | new service module consists solely of the interface functions. | |
478 | ||
479 | Each module is designed following a specific interface specification. | |
480 | For now the version is 1 and this manifests in the version number of the | |
481 | shared library object of the NSS modules: they have the extension | |
482 | @code{.1}. If the interface ever changes in an incompatible way, | |
483 | this number will be increased---hopefully this will never be necessary. | |
484 | Modules using the old interface will still be usable. | |
485 | ||
486 | Developers of a new service will have to make sure that their module is | |
487 | created using the correct interface number. This means the file itself | |
488 | must have the correct name and on ElF systems the @dfn{soname} (Shared | |
489 | Object Name) must also have this number. Building a module from a bunch | |
490 | of object files on an ELF system using GNU CC could be done like this: | |
491 | ||
492 | @smallexample | |
493 | gcc -shared -o libnss_NAME.so.1 -Wl,-soname,libnss_NAME.so.1 OBJECTS | |
494 | @end smallexample | |
495 | ||
496 | @noindent | |
497 | @ref{Link Options, Options for Linking, , gcc, GNU CC}, to learn | |
498 | more about this command line. | |
499 | ||
500 | To use the new module the library must be able to find it. This can be | |
501 | achieved by using options for the dynamic linker so that it will search | |
502 | directory where the binary is placed. For an ELF system this could be | |
503 | done by adding the wanted directory to the value of | |
504 | @code{LD_LIBRARY_PATH}. | |
505 | ||
506 | But this is not always possible since some program (those which run | |
507 | under IDs which do not belong to the user) ignore this variable. | |
508 | Therefore the stable version of the module should be placed into a | |
509 | directory which is searched by the dynamic linker. Normally this should | |
510 | be the directory @file{$prefix/lib}, where @file{$prefix} corresponds to | |
511 | the value given to configure using the @code{--prefix} option. But be | |
512 | careful: this should only be done if it is clear the module does not | |
513 | cause any harm. System administrators should be careful. | |
514 | ||
515 | ||
516 | @node NSS Module Function Internals, , Adding another Service to NSS, Extending NSS | |
517 | @subsection Internals of the NSS Module Functions | |
518 | ||
519 | Until now we only provided the syntactic interface for the functions in | |
520 | the NSS module. In fact there is not more much we can tell since the | |
521 | implementation obviously is different for each function. But a few | |
522 | general rules must be followed by all functions. | |
523 | ||
524 | In fact there are four kinds of different functions which may appear in | |
525 | the interface. All derive from the traditional ones for system databases. | |
526 | @var{db} in the following table is normally an abbreviation for the | |
527 | database (e.g., it is @code{pw} for the password database). | |
528 | ||
529 | @table @code | |
530 | @item int _nss_@var{database}_set@var{db}ent (void) | |
531 | This function prepares the service for following operations. For a | |
532 | simple file based lookup this means files could be opened, for other | |
533 | services this function simply is a noop. | |
534 | ||
535 | One special case for this function is that it takes an additional | |
536 | argument for some @var{database}s (i.e., the interface is | |
537 | @code{int set@var{db}ent (int)}). @ref{Host Names}, which describes the | |
538 | @code{sethostent} function. | |
539 | ||
540 | The return value should be @var{NSS_STATUS_SUCCESS} or according to the | |
541 | table above in case of an error (@pxref{NSS Modules Interface}). | |
542 | ||
543 | @item int _nss_@var{database}_end@var{db}ent (void) | |
544 | This function simply closes all files which are still open or removes | |
545 | buffer caches. If there are no files or buffers to remove this is again | |
546 | a simple noop. | |
547 | ||
548 | There normally is no return value different to @var{NSS_STATUS_SUCCESS}. | |
549 | ||
550 | @item int _nss_@var{database}_get@var{db}ent_r (@var{STRUCTURE} *result, char *buffer, int buflen) | |
551 | Since this function will be called several times in a row to retrieve | |
552 | one entry after the other it must keep some kind of state. But this | |
553 | also means the functions are not really reentrant. They are reentrant | |
554 | only in that simultaneous calls to this function will not try to | |
555 | write the retrieved data in the same place (as it would be the case for | |
556 | the non-reentrant functions); instead, it writes to the structure | |
557 | pointed to by the @var{result} parameter. But the calls share a common | |
558 | state and in the case of a file access this means they return neighboring | |
559 | entries in the file. | |
560 | ||
561 | The buffer of length @var{buflen} pointed to by @var{buffer} can be used | |
562 | for storing some additional data for the result. It is @emph{not} | |
563 | guaranteed that the same buffer will be passed for the next call of this | |
564 | function. Therefore one must not misuse this buffer to save some state | |
565 | information from one call to another. | |
566 | ||
567 | As explained above this function could also have an additional last | |
568 | argument. This depends on the database used; it happens only for | |
569 | @code{host} and @code{network}. | |
570 | ||
571 | The function shall return @code{NSS_STATUS_SUCCESS} as long as their are | |
572 | more entries. When the last entry was read it should return | |
573 | @code{NSS_STATUS_NOTFOUND}. When the buffer given as an argument is too | |
574 | small for the data to be returned @code{NSS_STATUS_TRYAGAIN} should be | |
575 | returned. When the service was not formerly initialized by a call to | |
576 | @code{_nss_@var{DATABASE}_set@var{db}ent} all return value allowed for | |
577 | this function can also be returned here. | |
578 | ||
579 | @item int _nss_@var{DATABASE}_get@var{db}by@var{XX}_r (@var{PARAMS}, @var{STRUCTURE} *result, char *buffer, int buflen) | |
580 | This function shall return the entry from the database which is | |
581 | addressed by the @var{PARAMS}. The type and number of these arguments | |
582 | vary. It must be individually determined by looking to the user-level | |
583 | interface functions. All arguments given to the non-reentrant version | |
584 | are here described by @var{PARAMS}. | |
585 | ||
586 | The result must be stored in the structure pointed to by @var{result}. | |
587 | If there is additional data to return (say strings, where the | |
588 | @var{result} structure only contains pointers) the function must use the | |
589 | @var{buffer} or length @var{buflen}. There must not be any references | |
590 | to non-constant global data. | |
591 | ||
592 | The implementation of this function should honour the @var{stayopen} | |
593 | flag set by the @code{set@var{DB}ent} function whenever this makes sense. | |
594 | ||
595 | Again, this function takes an additional last argument for the | |
596 | @code{host} and @code{network} database. | |
597 | ||
598 | The return value should as always follow the rules given above | |
599 | (@pxref{NSS Modules Interface}). | |
600 | ||
601 | @end table |