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 versions). The prototypes for the non-reentrant and reentrant versions of our function above are:
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)
The actual prototype of the function in the NSS modules in this case is
enum nss_status _nss_files_gethostbyname_r (const char *name, struct hostent *result_buf, char *buf, size_t buflen, int *errnop, int *h_errnop)
I.e., the interface function is in fact the reentrant function with the
change of the return value, the omission of the result parameter,
and the addition of the errnop parameter. While the user-level
function returns a pointer to the result the reentrant function return
enum nss_status value:
Now you see where the action items of the /etc/nsswitch.conf file are used.
If you study the source code you will find there is a fifth value:
NSS_STATUS_RETURN. 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.
In case the interface function has to return an error it is important
that the correct error code is stored in
return status values have only one associated error code, others have
|One of the functions used ran temporarily out of resources or a service is currently not available.|
|The provided buffer is not large enough. The function should be called again with a larger buffer.|
|A necessary input file cannot be found.|
|The requested entry is not available.|
|There are no entries. Use this to avoid returning errors for inactive services which may be enabled at a later time. This is not the same as the service being temporarily unavailable.|
These are proposed values. There can be other error codes and the
described error codes can have different meaning. With one
exception: when returning
NSS_STATUS_TRYAGAIN the error code
ERANGE must mean that the user provided buffer is too
small. Everything else is non-critical.
In statically linked programs, the main application and NSS modules do
not share the same thread-local variable
errno, which is the
reason why there is an explicit errnop function argument.
The above function has something special which is missing for almost all
the other module functions. There is an argument h_errnop. This
points to a variable which will be filled with the error code in case
the execution of the function fails for some reason. (In statically
linked programs, the thread-local variable
h_errno is not shared
with the main application.)
getXXXbyYYY 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
user database, there are
endpwent). These will be described in more detail later.
Here we give a general way to determine the
signature of the module function:
pointer to buffer where the result is stored.
normally a struct which corresponds to the database.
pointer to a buffer where the function can store additional data for the result etc.
length of the buffer pointed to by buffer.
the low-level error code to return to the application. If the return
value is not
*errnop needs to be
set to a non-zero value. An NSS module should never set
*errnop to zero. The value
ERANGE is special, as
*h_errnopneeds to be set to a non-zero value. A generic error code is
NETDB_INTERNAL, which instructs the caller to examine
*errnopfor further details. (This includes the
This table is correct for all functions but the