NSS Module Function Internals - The GNU C Library

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#### 28.4.2 Internals of the NSS Module Functions

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.

In fact there are four kinds of different functions which may appear in the interface. All derive from the traditional ones for system databases. db in the following table is normally an abbreviation for the database (e.g., it is `pw` for the password database).

`enum nss_status _nss_`database`_set`db `ent (void)`
  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.

  One special case for this function is that it takes an additional argument for some databases (i.e., the interface is `int set`db `ent (int)`). Host Names, which describes the `sethostent` function.

  The return value should be NSS_STATUS_SUCCESS or according to the table above in case of an error (see NSS Modules Interface).

`enum nss_status _nss_`database`_end`db `ent (void)`
  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.

  There normally is no return value different to NSS_STATUS_SUCCESS.

`enum nss_status _nss_`database`_get`db `ent_r (`STRUCTURE`*result, char *buffer, size_t buflen, int *errnop)`
  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 result 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.

  The buffer of length buflen pointed to by buffer can be used for storing some additional data for the result. It is not 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.

  Before the function returns the implementation should store the value of the local errno variable in the variable pointed to be errnop. This is important to guarantee the module working in statically linked programs.

  As explained above this function could also have an additional last argument. This depends on the database used; it happens only for `host` and `networks`.

  The function shall return `NSS_STATUS_SUCCESS` as long as there are more entries. When the last entry was read it should return `NSS_STATUS_NOTFOUND`. When the buffer given as an argument is too small for the data to be returned `NSS_STATUS_TRYAGAIN` should be returned. When the service was not formerly initialized by a call to `_nss_`DATABASE`_set`db `ent` all return value allowed for this function can also be returned here.

`enum nss_status _nss_`DATABASE`_get`db `by`XX`_r (`PARAMS`,` STRUCTURE`*result, char *buffer, size_t buflen, int *errnop)`
  This function shall return the entry from the database which is addressed by the PARAMS. 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 PARAMS.

  The result must be stored in the structure pointed to by result. If there is additional data to return (say strings, where the result structure only contains pointers) the function must use the buffer or length buflen. There must not be any references to non-constant global data.

  The implementation of this function should honor the stayopen flag set by the `set`DB `ent` function whenever this makes sense.

  Before the function returns the implementation should store the value of the local errno variable in the variable pointed to be errnop. This is important to guarantee the module working in statically linked programs.

  Again, this function takes an additional last argument for the `host` and `networks` database.

  The return value should as always follow the rules given above (see NSS Modules Interface).