28.3.2 The Interface of the Function in NSS Modules
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:
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 and the omission of the result
parameter. While the user-level function returns a pointer to the
result the reentrant function return an enum nss_status value:
numeric value -2
numeric value -1
numeric value 0
numeric value 1
Now you see where the action items of the /etc/nsswitch.conf file
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 *errnop. Some
return status value 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.
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
ERANGEmust mean that the user provided buffer is too
small. Everything is non-critical.
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. The reentrant
function cannot use the global variable h_errno;
gethostbyname calls gethostbyname_r with the last argument
set to &h_errno.
The 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
password database, there are setpwent, getpwent, and
endpwent). These will be described in more detail later.
Here we give a general way to determine the
signature of the module function: